Informative Annex B Application Information for Ampacity Calculation

This informative annex is not a part of the requirements of this NFPA document but is included for informational purposes only.

Equation Application Information. This informative annex provides application information for ampacities calcula‐ ted under engineering supervision.
Typical Applications Covered by Tables. Typical ampaci‐ ties for conductors rated 0 through 2000 volts are shown in Table B.310.15(B)(2)(1) through Table B.310.15(B)(2)(10). Table B.310.15(B)(2)(11) provides the adjustment factors for more than three current-carrying conductors in a raceway or cable with load diversity. Underground electrical duct bank configurations, as detailed in Figure B.310.15(B)(2)(3), Figure B.310.15(B)(2)(4), and Figure B.310.15(B)(2)(5), are utilized for conductors rated 0 through 5000 volts. In Figure B.310.15(B)(2)(2) through Figure B.310.15(B)(2)(5), where adjacent duct banks are used, a separation of 1.5 m (5 ft) between the centerlines of the closest ducts in each bank or

1.2 m (4 ft) between the extremities of the concrete envelopes is sufficient to prevent derating of the conductors due to mutual heating. These ampacities were calculated as detailed in the basic ampacity paper, AIEE Paper 57-660, The Calculation of the Temperature Rise and Load Capability of Cable Systems, by

J. H. Neher and M. H. McGrath. For additional information concerning the application of these ampacities, see IEEE STD 835-1994, Standard Power Cable Ampacity Tables.

Typical values of thermal resistivity (Rho) are as follows: Average soil (90 percent of USA) = 90

Concrete = 55

Damp soil (coastal areas, high water table) = 60 Paper insulation = 550

Polyethylene (PE) = 450 Polyvinyl chloride (PVC) = 650 Rubber and rubber-like = 500

Very dry soil (rocky or sandy) = 120

Thermal resistivity, as used in this informative annex, refers to the heat transfer capability through a substance by conduction. It is the reciprocal of thermal conductivity and is normally expressed in the units°C-cm/watt. For additional information on determining soil thermal resistivity (Rho), see ANSI/IEEE STD 442-1996, Guide for Soil Thermal Resistivity Measurements.

Criteria Modifications. Where values of load factor and Rho are known for a particular electrical duct bank installation and they are different from those shown in a specific table or figure, the ampacities shown in the table or figure can be modi‐ fied by the application of factors derived from the use of Figure B.310.15(B)(2)(1).Where two different ampacities apply to adjacent portions of a circuit, the higher ampacity can be used beyond the point of transition, a distance equal to 3 m (10 ft) or 10 percent of the circuit length calculated at the higher ampacity, whichever is less.Where the burial depth of direct burial or electrical duct bank circuits are modified from the values shown in a figure or table, ampacities can be modified as shown in (a) and (b) as follows.
1. Where burial depths are increased in part(s) of an electrical duct run to avoid underground obstructions, no decrease in ampacity of the conductors is needed, provided the total length of parts of the duct run increased in depth to avoid obstructions is less than 25 percent of the total run length.
2. Where burial depths are deeper than shown in a specific underground ampacity table or figure, an ampacity derating factor of 6 percent per increased 300 mm (foot) of depth for all values of Rho can be utilized. No rating change is needed where the burial depth is decreased.
Electrical Ducts. The term electrical duct(s) is defined in 310.60.B.5 Table B.310.15(B)(2)(6) and Table B.310.15(B)(2)(7).
1. To obtain the ampacity of cables installed in two elec‐ trical ducts in one horizontal row with 190-mm (7.5-in.) center- to-center spacing between electrical ducts, similar to Figure B.310.15(B)(2)(2), Detail 1, multiply the ampacity shown for one duct in Table B.310.15(B)(2)(6) and Table B.310.15(B)(2)(7) by 0.88.
2. To obtain the ampacity of cables installed in four electrical ducts in one horizontal row with 190-mm (7.5-in.) center-to-center spacing between electrical ducts, similar to Figure B.310.15(B)(2)(2), Detail 2, multiply the ampacity shown for three electrical ducts in Table B.310.15(B)(2)(6) and Table B.310.15(B)(2)(7) by 0.94.

Electrical Ducts Used in Figure B.310.15(B)(2)(2). If spac‐ ing between electrical ducts, as shown in Figure B.310.15(B)(2)(2), is less than as specified where electrical ducts enter equipment enclosures from underground, the ampacity of conductors contained within such electrical ducts need not be reduced.
Examples Showing Use of Figure B.310.15(B)(2)(1) for Electrical Duct Bank Ampacity Modifications. Figure B.310.15(B)(2)(1) is used for interpolation or extrapolation for values of Rho and load factor for cables installed in electri‐ cal ducts. The upper family of curves shows the variation in ampacity and Rho at unity load factor in terms of I 1, the ampacity for Rho = 60, and 50 percent load factor. Each curve is designated for a particular ratio I 2/I 1, where I 2 is the ampac‐ ity at Rho = 120 and 100 percent load factor.

The lower family of curves shows the relationship between Rho and load factor that will give substantially the same ampac‐ ity as the indicated value of Rho at 100 percent load factor.

As an example, to find the ampacity of a 500-kcmil copper cable circuit for six electrical ducts as shown in Table B.310.15(B)(2)(5): At the Rho = 60, LF = 50, I 1 = 583; for Rho = 120 and LF = 100, I 2 = 400. The ratio I 2/I 1 = 0.686.

Locate Rho = 90 at the bottom of the chart and follow the 90 Rho line to the intersection with 100 percent load factor where the equivalent Rho = 90. Then follow the 90 Rho line to I2/I1 ratio of 0.686 where F = 0.74. The desired ampacity = 0.74 × 583

= 431, which agrees with the table for Rho = 90, LF = 100.

To determine the ampacity for the same circuit where Rho = 80 and LF = 75, using Figure B.310.15(B)(2)(1), the equivalent Rho = 43, F = 0.855, and the desired ampacity = 0.855 × 583 = 498 amperes. Values for using Figure B.310.15(B)(2)(1) are found in the electrical duct bank ampacity tables of this infor‐ mative annex.

Where the load factor is less than 100 percent and can be verified by measurement or calculation, the ampacity of electri‐ cal duct bank installations can be modified as shown. Different values of Rho can be accommodated in the same manner.

Informational Note: The ampacity limit for 10 through 85 current-carrying conductors is based on the following equation. For more than 85 conductors, special calculations are required that are beyond the scope of this table.

[B.310.15(B)(7)a]

Example 1

Calculate the ampacity limit for twelve 14 AWG THWN current-carrying conductors (75°C) in a raceway that contains 24 conductors that may, at different times, be current-carrying.

0.524

[B.310.15(B)(7)b]

A   200.7

 14 amperes i.e., 50 percent diversity

Example 2

Calculate the ampacity limit for eighteen 14 AWG THWN current-carrying conductors (75°C) in a raceway that contains 24 conductors that may, at different times, be current-carrying.

0.5N





A   A  or A , whichever is less

2 



A	=	ampacity from Table 310.15(B)(16),
		Table 310.15(B)(18), Table
		B.310.15(B)(2)(1), Table
		B.310.15(B)(2)(6), or Table
		B.310.15(B)(2)(7) multiplied by the
		appropriate adjustment factor from
		Table B.310.15(B)(2)(11).
N	=	total number of conductors used to
		select adjustment factor from Table
		B.310.15(B)(2)(11)
E	=	number of conductors carrying
		current simultaneously in the raceway
		or cable
A 2	=	ampacity limit for the current-carrying
		conductors in the raceway or cable

where:

1  1



[B.310.15(B)(7)c]

0.524

A   200.7  11.5 amperes

Table B.310.15(B)(2)(1) Ampacities of Two or Three Insulated Conductors, Rated 0 Through 2000 Volts, Within an Overall Covering (Multiconductor Cable), in Raceway in Free Air Based on Ambient Air Temperature of 30°C (86°F)*

Size (AWG or kcmil)	Temperature Rating of Conductor. [See Table 310.104(A).]	Size (AWG or kcmil)
60°C (140°F)	75°C (167°F)	90°C (194°F)	60°C (140°F)	75°C (167°F)	90°C (194°F)
Types TW, UF	Types RHW, THHW, THW, THWN, XHHW, ZW	Types THHN, THHW, THW-2, THWN-2,RHH, RWH-2, USE-2, XHHW, XHHW-2, ZW-2	Type TW	Types RHW, THHW, THW, THWN, XHHW	Types THHN, THHW, THW-2, THWN-2,RHH, RWH-2,USE-2,XHHW, XHHW-2, ZW-2
COPPER	ALUMINUM OR COPPER-CLAD ALUMINUM
141210**8	16202736	18243343	21273648	— 162128	— 182533	— 212837	1412108
64321	48667688102	587990105121	6589102119137	3851596980	4561708395	51697993106	64321
1/02/03/04/0	121138158187	145166189223	163186214253	94108124147	113129147176	127146167197	1/02/03/04/0
250300350400500	205234255274315	245281305328378	276317345371427	160185202218254	192221242261303	217250273295342	250300350400500
6007007508009001000	343376387397415448	413452466479500542	468514529543570617	279310321331350382	335371384397421460	378420435450477521	6007007508009001000

*Refer to 310.15(B)(2) for the ampacity correction factors where the ambient temperature is other than 30°C (86°F).

**Refer to 240.4(D) for conductor overcurrent protection limitations.

Table B.310.15(B)(2)(3) Ampacities of Multiconductor Cables with Not More Than Three Insulated Conductors, Rated 0 Through 2000 Volts, in Free Air Based on Ambient Air Temperature of 40°C (104°F) (for Types TC, MC, MI, UF, and USE Cables)*

Size (AWG or kcmil)	Temperature Rating of Conductor. [See Table 310.104(A).]	Size (AWG or kcmil)
60°C (140°F)	75°C (167°F)	85°C (185°F)	90°C (194°F)	60°C (140°F)	75°C (167°F)	85°C (185°F)	90°C (194°F)
COPPER	ALUMINUM OR COPPER-CLAD ALUMINUM
18	—	—	—	11	—	—	—	—	18
16	—	—	—	16	—	—	—	—	16
14**	18	21	24	25	—	—	—	—	14
12**	21	28	30	32	18	21	24	25	12
10**	28	36	41	43	21	28	30	32	10
8	39	50	56	59	30	39	44	46	8
6	52	68	75	79	41	53	59	61	6
4	69	89	100	104	54	70	78	81	4
3	81	104	116	121	63	81	91	95	3
2	92	118	132	138	72	92	103	108	2
1	107	138	154	161	84	108	120	126	1
1/0	124	160	178	186	97	125	139	145	1/0
2/0	143	184	206	215	111	144	160	168	2/0
3/0	165	213	238	249	129	166	185	194	3/0
4/0	190	245	274	287	149	192	214	224	4/0
250	212	274	305	320	166	214	239	250	250
300	237	306	341	357	186	240	268	280	300
350	261	337	377	394	205	265	296	309	350
400	281	363	406	425	222	287	317	334	400
500	321	416	465	487	255	330	368	385	500
600	354	459	513	538	284	368	410	429	600
700	387	502	562	589	306	405	462	473	700
750	404	523	586	615	328	424	473	495	750
800	415	539	604	633	339	439	490	513	800
900	438	570	639	670	362	469	514	548	900
1000	461	601	674	707	385	499	558	584	1000

*Refer to 310.15(B)(2) for the ampacity correction factors where the ambient temperature is other than 40°C (104°F).

**Refer to 240.4(D) for conductor overcurrent protection limitations.

Table B.310.15(B)(2)(5) Ampacities of Single Insulated Conductors, Rated 0 Through 2000 Volts, in Nonmagnetic Underground Electrical Ducts (One Conductor per Electrical Duct), Based on Ambient Earth Temperature of 20°C (68°F), Electrical Duct Arrangement in Accordance with Figure B.310.15(B)(2)(2), Conductor Temperature 75°C (167°F)

Size (kcmil)	3 Electrical Ducts [Fig.B.310.15(B)(2)(2),Detail 2]	6 Electrical Ducts [Fig.B.310.15(B)(2)(2),Detail 3]	9 Electrical Ducts [Fig.B.310.15(B)(2)(2),Detail 4]	3 Electrical Ducts [Fig.B.310.15(B)(2)(2),Detail 2]	6 Electrical Ducts [Fig.B.310.15(B)(2)(2),Detail 3]	9 Electrical Ducts [Fig.B.310.15(B)(2)(2),Detail 4]	Size (kcmil)
Types RHW, THHW, THW, THWN, XHHW, USE	Types RHW, THHW, THW, THWN, XHHW, USE	Types RHW, THHW, THW, THWN, XHHW, USE	Types RHW, THHW, THW, THWN, XHHW, USE	Types RHW, THHW, THW, THWN, XHHW, USE	Types RHW, THHW, THW, THWN, XHHW, USE
COPPER	ALUMINUM OR COPPER-CLAD ALUMINUM
	RHO 60 LF 50	RHO 90 LF 100	RHO 120 LF 100	RHO 60 LF 50	RHO 90 LF 100	RHO 120 LF 100	RHO 60 LF 50	RHO 90 LF 100	RHO 120 LF 100	RHO 60 LF 50	RHO 90 LF 100	RHO 120 LF 100	RHO 60 LF 50	RHO 90 LF 100	RHO 120 LF 100	RHO 60 LF 50	RHO 90 LF 100	RHO 120 LF 100
250350500750	410503624794	344418511640	327396484603	386472583736	295355431534	275330400494	369446545674	270322387469	252299360434	320393489626	269327401505	256310379475	302369457581	230277337421	214258313389	288350430538	211252305375	197235284347	250350500750
10001250150017502000	9361055116012501332	7458329079701027	700781849907959	864970106311421213	617686744793836	570632685729768	7768549189751030	533581619651683	493536571599628	74484894110261103	593668736796850	557627689745794	6877798639371005	491551604651693	453508556598636	629703767823877	432478517550581	399441477507535	10001250150017502000
Ambient Temp.(°C)	Correction Factors	Ambient Temp. (°F)
6–1011–1516–2021–2526–30	1.091.041.000.950.90	1.091.041.000.950.90	1.091.041.000.950.90	1.091.041.000.950.90	1.091.041.000.950.90	1.091.041.000.950.90	43–5052–5961–6870–7779–86

Table B.310.15(B)(2)(6) Ampacities of Three Insulated Conductors, Rated 0 Through 2000 Volts, Within an Overall Covering (Three-Conductor Cable) in Underground Electrical Ducts (One Cable per Electrical Duct) Based on Ambient Earth Temperature of 20°C (68°F), Electrical Duct Arrangement in Accordance with Figure B.310.15(B)(2)(2), Conductor Temperature 75°C (167°F)

Size (AWG or kcmil)	1 Electrical Duct [Fig.B.310.15(B)(2)(2),Detail 1]	3 Electrical Ducts [Fig.B.310.15(B)(2)(2),Detail 2]	6 Electrical Ducts [Fig.B.310.15(B)(2)(2),Detail 3]	1 Electrical Duct [Fig.B.310.15(B)(2)(2),Detail 1]	3 Electrical Ducts [Fig.B.310.15(B)(2)(2),Detail 2]	6 Electrical Ducts [Fig.B.310.15(B)(2)(2),Detail 3]	Size (AWG or kcmil)
Types RHW, THHW, THW, THWN, XHHW, USE	Types RHW, THHW, THW, THWN, XHHW, USE	Types RHW, THHW, THW, THWN, XHHW, USE	Types RHW, THHW, THW, THWN, XHHW, USE	Types RHW, THHW, THW, THWN, XHHW, USE	Types RHW, THHW, THW, THWN, XHHW, USE
COPPER	ALUMINUM OR COPPER-CLAD ALUMINUM
	RHO 60 LF 50	RHO 90 LF 100	RHO 120 LF 100	RHO 60 LF 50	RHO 90 LF 100	RHO 120 LF 100	RHO 60 LF 50	RHO 90 LF 100	RHO 120 LF 100	RHO 60 LF 50	RHO 90 LF 100	RHO 120 LF 100	RHO 60 LF 50	RHO 90 LF 100	RHO 120 LF 100	RHO 60 LF 50	RHO 90 LF 100	RHO 120 LF 100
86421	5877101132154	547193121140	536991118136	567496126146	486381105121	466077100114	537091119137	42546989102	3951658395	456078103120	42557294109	41547192106	43577598114	3749638294	3647607889	41547192107	3242547079	3039516574	86421
1/02/03/04/0	177203233268	160183210240	156178204232	168192221253	137156178202	130147158190	157179205234	116131148168	107121137155	138158182209	125143164187	122139159182	131150172198	107122139158	101115131149	122140160183	90102116131	8495107121	1/02/03/04/0
2503505007501000	297363444552628	265321389478539	256310375459518	280340414511579	222267320388435	209250299362405	258312377462522	184219261314351	169202240288321	233285352446521	207252308386447	201244297372430	219267328413480	174209254314361	163196237293336	202245299374433	144172207254291	132158190233266	2503505007501000
Ambient Temp.(°C)	Correction Factors	Ambient Temp.(°F)
6–1011–1516–2021–2526–30	1.091.041.000.950.90	1.091.041.000.950.90	1.091.041.000.950.90	1.091.041.000.950.90	1.091.041.000.950.90	1.091.041.000.950.90	43–5052–5961–6870–7779–86

Table B.310.15(B)(2)(7) Ampacities of Three Single Insulated Conductors, Rated 0 Through 2000 Volts, in Underground Electrical Ducts (Three Conductors per Electrical Duct) Based on Ambient Earth Temperature of 20°C (68°F), Electrical Duct Arrangement in Accordance with Figure B.310.15(B)(2)(2), Conductor Temperature 75°C (167°F)

Size (AWG or kcmil)	1 Electrical Duct [Fig.B.310.15(B)(2)(2),Detail 1]	3 Electrical Ducts [Fig.B.310.15(B)(2)(2),Detail 2]	6 Electrical Ducts [Fig.B.310.15(B)(2)(2),Detail 3]	1 Electrical Duct [Fig.B.310.15(B)(2)(2),Detail 1]	3 Electrical Ducts [Fig.B.310.15(B)(2)(2),Detail 2]	6 Electrical Ducts [Fig.B.310.15(B)(2)(2),Detail 3]	Size (AWG or kcmil)
Types RHW, THHW, THW, THWN, XHHW, USE	Types RHW, THHW, THW, THWN, XHHW, USE	Types RHW, THHW, THW, THWN, XHHW, USE	Types RHW, THHW, THW, THWN, XHHW, USE	Types RHW, THHW, THW, THWN, XHHW, USE	Types RHW, THHW, THW, THWN, XHHW, USE
COPPER	ALUMINUM OR COPPER-CLAD ALUMINUM
	RHO 60 LF 50	RHO 90 LF 100	RHO 120 LF 100	RHO 60 LF 50	RHO 90 LF 100	RHO 120 LF 100	RHO 60 LF 50	RHO 90 LF 100	RHO 120 LF 100	RHO 60 LF 50	RHO 90 LF 100	RHO 120 LF 100	RHO 60 LF 50	RHO 90 LF 100	RHO 120 LF 100	RHO 60 LF 50	RHO 90 LF 100	RHO 120 LF 100
864321	6384111129147171	5877100116132153	577598113128148	6180105122139161	51678699112128	49638194106121	577598113129149	4456738393106	415367778698	496686101115133	45607891103119	44587689100115	47637983108126	4052677787100	384963738294	45597784101116	344457657383	324152606777	864321
1/02/03/04/0	197226260301	175200228263	169193220253	185212243280	146166189215	137156177201	170194222255	121136154175	111126142161	153176203235	136156178205	132151172198	144165189219	114130147168	107121138157	133151173199	94106121137	8798111126	1/02/03/04/0
250300350400	334373409442	290321351376	279308337361	310344377394	236260283302	220242264280	281310340368	192210228243	176192209223	261293321349	227252276297	218242265284	242272296321	185204222238	172190207220	220245266288	150165179191	137151164174	250300350400
500600700750	503552602632	427468509529	409447486505	460511553574	341371402417	316343371385	412457492509	273296319330	249270291301	397446488508	338373408425	323356389405	364408443461	270296321334	250274297309	326365394409	216236255265	197215232241	500600700750
8009001000	654692730	544575605	520549576	597628659	428450472	395415435	527554581	338355372	308323338	530563597	439466494	418444471	481510538	344365385	318337355	427450475	273288304	247261276	8009001000
Ambient Temp.(°C)	Correction Factors	Ambient Temp.(°F)
6–1011–1516–2021–2526–30	1.091.041.000.950.90	1.091.041.000.950.90	1.091.041.000.950.90	1.091.041.000.950.90	1.091.041.000.950.90	1.091.041.000.950.90	43–5052–5961–6870–7779–86

Table B.310.15(B)(2)(8) Ampacities of Two or Three Insulated Conductors, Rated 0 Through 2000 Volts, Cabled Within an Overall (Two- or Three-Conductor) Covering, Directly Buried in Earth, Based on Ambient Earth Temperature of 20°C (68°F), Electrical Duct Arrangement in Accordance with Figure B.310.15(B)(2)(2), 100 Percent Load Factor, Thermal Resistance (Rho) of 90

Size (AWG or kcmil)	1 Cable[Fig. B.310.15(B)(2)(2),Detail 5]	2 Cables[Fig. B.310.15(B)(2)(2),Detail 6]	1 Cable[Fig. B.310.15(B)(2)(2),Detail 5]	2 Cables[Fig. B.310.15(B)(2)(2),Detail 6]	Size (AWG or kcmil)
60°C (140°F)	75°C (167°F)	60°C (140°F)	75°C (167°F)	60°C (140°F)	75°C (167°F)	60°C (140°F)	75°C (167°F)
TYPES	TYPES
UF	RHW, THHW, THW, THWN, XHHW, USE	UF	RHW, THHW, THW, THWN, XHHW, USE	UF	RHW, THHW, THW, THWN, XHHW, USE	UF	RHW, THHW, THW, THWN, XHHW, USE
COPPER	ALUMINUM OR COPPER-CLAD ALUMINUM
86421	6485107137155	75100125161182	6081100128145	7095117150170	516883107121	597597126142	476078110113	557091117132	86421
1/02/03/04/0	177201229259	208236269304	165188213241	193220250282	138157179203	162184210238	129146166188	151171195220	1/02/03/04/0
2503505007501000	—————	333401481585657	—————	308370442535600	—————	261315381473545	—————	241290350433497	2503505007501000
Ambient Temp. (°C)	Correction Factors	Ambient Temp. (°F)
6–1011–1516–2021–2526–30	1.121.061.000.940.87	1.091.041.000.950.90	1.121.061.000.940.87	1.091.041.000.950.90	1.121.061.000.940.87	1.091.041.000.950.90	1.121.061.000.940.87	1.091.041.000.950.90	43–5052–5961–6870–7779–86

Note: For ampacities of Type UF cable in underground electrical ducts, multiply the ampacities shown in the table by 0.74.

Table B.310.15(B)(2)(9) Ampacities of Three Triplexed Single Insulated Conductors, Rated 0 Through 2000 Volts, Directly Buried in Earth Based on Ambient Earth Temperature of 20°C (68°F), Electrical Duct Arrangement in Accordance with Figure B.310.15(B)(2) (2), 100 Percent Load Factor, Thermal Resistance (Rho) of 90

Size (AWG or kcmil)	See Fig. B.310.15(B)(2)(2),Detail 7	See Fig. B.310.15(B)(2)(2),Detail 8	See Fig. B.310.15(B)(2)(2),Detail 7	See Fig. B.310.15(B)(2)(2),Detail 8	Size (AWG or kcmil)
60°C (140°F)	75°C (167°F)	60°C (140°F)	75°C (167°F)	60°C (140°F)	75°C (167°F)	60°C (140°F)	75°C (167°F)
TYPES	TYPES
UF	USE	UF	USE	UF	USE	UF	USE
COPPER	ALUMINUM OR COPPER-CLAD ALUMINUM
86421	7291119153173	84107139179203	6684109140159	7799128164186	557292119135	6584108139158	516685109124	6077100128145	86421
1/02/03/04/0	197223254289	231262298339	181205232263	212240272308	154175199226	180205233265	141159181206	165187212241	1/02/03/04/0
2503505007501000	—————	370445536654744	—————	336403483587665	—————	289349424525608	—————	263316382471544	2503505007501000
Ambient Temp. (°C)	Correction Factors	Ambient Temp. (°F)
6–1011–1516–2021–2526–30	1.121.061.000.940.87	1.091.041.000.950.90	1.121.061.000.940.87	1.091.041.000.950.90	1.121.061.000.940.87	1.091.041.000.950.90	1.121.061.000.940.87	1.091.041.000.950.90	43–5052–5961–6870–7779–86

Table B.310.15(B)(2)(10) Ampacities of Three Single Insulated Conductors, Rated 0 Through 2000 Volts, Directly Buried in Earth Based on Ambient Earth Temperature of 20°C (68°F), Electrical Duct Arrangement in Accordance with Figure B.310.15(B)(2)(2), 100 Percent Load Factor, Thermal Resistance (Rho) of 90

Size (AWG or kcmil)	See Fig. B.310.15(B)(2)(2),Detail 9	See Fig. B.310.15(B)(2)(2),Detail 10	See Fig. B.310.15(B)(2)(2),Detail 9	See Fig. B.310.15(B)(2)(2),Detail 10	Size (AWG or kcmil)
60°C (140°F)	75°C (167°F)	60°C (140°F)	75°C (167°F)	60°C (140°F)	75°C (167°F)	60°C (140°F)	75°C (167°F)
TYPES	TYPES
UF	USE	UF	USE	UF	USE	UF	USE
COPPER	ALUMINUM OR COPPER-CLAD ALUMINUM
86421	84107139178201	98126163209236	78101130165187	92118152194219	6684108139157	7798127163184	6178101129146	7292118151171	86421
1/02/03/04/0	230261297336	270306348394	212241274309	249283321362	179204232262	210239272307	165188213241	194220250283	1/02/03/04/0
250350500750	————	429516626767	————	394474572700	————	335403490605	————	308370448552	250350500750
10001250150017502000	—————	887979106311331195	—————	80889196510271082	—————	706787862930990	—————	642716783843897	10001250150017502000
Ambient Temp. (°C)	Correction Factors	Ambient Temp. (°F)
6–1011–1516–2021–2526–30	1.121.061.000.940.87	1.091.041.000.950.90	1.121.061.000.940.87	1.091.041.000.950.90	1.121.061.000.940.87	1.091.041.000.950.90	1.121.061.000.940.87	1.091.041.000.950.90	43–5052–5961–6870–7779–86

Table B.310.15(B)(2)(11) Adjustment Factors for More Than Three Current-Carrying Conductors in a Raceway or Cable with Load Diversity

Number of Conductors*

Percent of Values in Tables as Adjusted for Ambient Temperature if Necessary

4–6 80

7–9 70

10–24 70**

25–42 60**

43–85 50**

*Number of conductors is the total number of conductors in the raceway or cable adjusted in accordance with 310.15(B)(4) and (5).

**These factors include the effects of a load diversity of 50 percent.

10 20 30 40 50 60 70 80 90 100 110 120 130140 150 160 170 180 190 200

1.15

1.10

1.05

1.00

0.95

0.90

0.85

I 2

Ratio — I

0.85

0.80

0.75

0.70

0.65

0.60

0.55

0.50

0.45

0.40

100

Load Factor

0 10 20 30 40 50 60 70 80 90 100 110 120 130140 150 160 170 180 190 200

RHO

FIGURE B.310.15(B)(2)(1) Interpolation Chart for Cables in a Duct Bank. I 1 = ampacity for Rho = 60, 50 LF; I 2 = ampacity for Rho = 120, 100 LF (load factor); desired ampacity

= F × I 1.

190 mm

(7.5 in.)

Detail 1

290 mm  290 mm

(11.5 in.  11.5 in.)

Electrical duct bank One electrical duct

190 mm (7.5in.)

190 mm (7.5 in.)

Detail 2

475 mm  475 mm

(19 in.  19 in.) Electrical duct bank Three electrical ducts

190 mm (7.5 in.)

Detail 3

475 mm  675 mm

190 mm

(7.5 in.)

190 mm 190 mm

(7.5 in.) (7.5 in.)

190 mm

(7.5 in.)

Detail 4

190 mm

(7.5 in.)

190 mm

(7.5 in.)

190 mm

(7.5 in.)

(19 in.  27 in.) Electrical duct bank Six electrical ducts

675 mm  675 mm

190 mm

(7.5 in.)

(27 in.  27 in.) Electrical duct bank Nine electrical ducts

675 mm  290 mm

(27 in.  11.5 in.) Electrical duct bank Three electrical ducts

190 mm

(7.5 in.)

190 mm

(7.5 in.)

600 mm (24 in.)

675 mm  475 mm

(27 in.  19 in.) Electrical duct bank

600 mm (24 in.)

Detail 5

Buried 3 conductor cable

Detail 6

Buried 3 conductor cables

Detail 7

Buried triplexed cables (1 circuit)

Detail 8

Buried triplexed cables (2 circuits)

190 mm

(7.5 in.)

190 mm

(7.5 in.)

190 mm

(7.5 in.)

190 mm

(7.5 in.)

(24 in.)

600 mm

190 mm

(7.5 in.)

190 mm

(7.5 in.)

Detail 9

Buried single-conductor cables (1 circuit)

Detail 10

Buried single-conductor cables (2 circuits)

Note 1: Minimum burial depths to top electrical ducts or cables shall be in accordance with 300.5. Maximum depth to the top of electrical duct banks shall be 750 mm (30 in.) and maximum depth to the top of direct-buried cables shall be 900 mm (36 in.)

Note 2: For two and four electrical duct installations with electrical ducts installed in a single row, see B.310.15(B)(5).

Legend

Backfill

(earth or concrete) Electrical duct

Cable or cables

FIGURE B.310.15(B)(2)(2) Cable Installation Dimensions for Use with Table B.310.15(B)(2)(5) Through Table B.310.15(B)(2)(10).

Surface

150 mm (6 in.)

N and EGC

150 mm (6 in.)

600 mm (24 in.)

190 mm (7.5 in.)

750 mm (30 in.)

190 mm (7.5 in.)

190 mm 190 mm

(7.5 in.) (7.5 in.)

675 mm (27 in.)

(Typical for all)

N and EGC

Surface 150 mm (6 in.)

150 mm (6 in.)

AØ

250 mm (10 in.) BØ

250 mm (10 in.) CØ

600 mm (24 in.)

950 mm (38 in.)

250 mm

(10 in.)

250 mm

(10 in.)

250 mm

(10 in.)

Design Criteria

Neutral and Equipment Number of Cables per Phase = 9

Grounding conductor (EGC) Rho Concrete = Rho Earth – 5

Duct = 150 mm (6 in.) Rho PVC Duct = 650 Phase Ducts = 75 to 125 mm (3 to 5 in.) Rho Cable Insulation = 500 Conductor Material = Copper Rho Cable Jacket = 650 Number of Cables per Duct = 3

Notes:

Neutral configuration per 300.5(I), Exception No. 2, for isolated phase installations in
Design CriteriaNeutral and Equipment Grounding conductor (EGC) Duct = 150 mm (6 in.)Phase Ducts = 75 mm (3 in.) Conductor Material = Copper Number of Cables per Duct = 1
Notes:1.1 m (44 in.)
Number of Cables per Phase = 4 Rho Concrete = Rho Earth – 5 Rho PVC Duct = 650Rho Cable Insulation = 500Rho Cable Jacket = 650nonmagnetic ducts.
Phasing is A, B, C in rows or columns. Where magnetic electrical ducts are used, conductors are installed A, B, C per electrical duct with the neutral and all equipment grounding conductors in the same electrical duct. In this case, the 6-in. trade size neutral duct is eliminated.
Maximum harmonic loading on the neutral conductor cannot exceed 50 percent of the phase current for the ampacities shown in the table below.
Metallic shields of Type MV-90 cable shall be grounded at one point only where using A, B, C phasing in rows or columns.

Size kcmil	TYPESRHW, THHW, THW, THWN, XHHW, USE, OR MV-90*	Size kcmil
Total per Phase Ampere Rating
RHO EARTH 60LF 50	RHO EARTH 90LF 100	RHO EARTH 120LF 100
250	2340(260A/Cable)	1530(170A/Cable)	1395(155A/Cable)	250
350	2790(310A/Cable)	1800(200A/Cable)	1665(185A/Cable)	350
500	3375(375A/Cable)	2160(240A/Cable)	1980(220A/Cable)	500

Ambient Temp. (C)	For ambient temperatures other than 20C (68F), multiply the ampacities shown above by the appropriate factor shown below.	Ambient Temp. (F)
6–10	1.09	1.09	1.09	1.09	1.09	43–50
11–15	1.04	1.04	1.04	1.04	1.04	52–59
16–20	1.00	1.00	1.00	1.00	1.00	61–68
21–25	0.95	0.95	0.95	0.95	0.95	70–77
26–30	0.90	0.90	0.90	0.90	0.90	79–86

*Limited to 75C conductor temperature.

Informational Note Figure B.310.15(B)(2)(3) Ampacities of Single Insulated Conductors Rated 0 Through 5000 Volts in Underground Electrical Ducts (Three Conductors per Electrical Duct), Nine Single-Conductor Cables per Phase Based on Ambient Earth Temperature of 20°C (68°F), Conductor Temperature 75°C (167°F).

Neutral configuration per 300.5(I), Exception No 2.
Maximum harmonic loading on the neutral conductor cannot exceed 50 percent of the phase current for the ampacities shown in the table below.
Metallic shields of Type MV-90 cable shall be grounded at one point only.

Size kcmil	TYPES RHW,THHW,THW,THWN, XHHW,USE,OR MV-90*	Size kcmil
Total per Phase Ampere Rating
RHO EARTH 60LF 50	RHO EARTH 90LF 100	RHO EARTH 120LF 100
	2820	1860	1680
750	(705A/Cable)	(465A/Cable)	(420A/Cable)	750
	3300	2140	1920
1000	(825A/Cable)	(535A/Cable)	(480A/Cable)	1000
1250	3700(925A/Cable)	2380(595A/Cable)	2120(530A/Cable)	1250
	4060	2580	2300
1500	(1015A/Cable)	(645A/Cable)	(575A/Cable)	1500
1750	4360(1090A/Cable)	2740(685A/Cable)	2460(615A/Cable)	1750

Ambient Temp. (C)	For ambient temperatures other than 20C (68F), multiply the ampacities shown above by the appropriate factor shown below.	Ambient Temp. (F)
6–10	1.09	1.09	1.09	1.09	1.09	43–50
11–15	1.04	1.04	1.04	1.04	1.04	52–59
16–20	1.00	1.00	1.00	1.00	1.00	61–68
21–25	0.95	0.95	0.95	0.95	0.95	70–77
26–30	0.90	0.90	0.90	0.90	0.90	79–86

*Limited to 75C conductor temperature.

Informational Note Figure B.310.15(B)(2)(4) Ampacities of Single Insulated Conductors Rated 0 Through 5000 Volts in Nonmagnetic Underground Electrical Ducts (One Conductor per Electrical Duct), Four Single-Conductor Cables per Phase Based on Ambient Earth Temperature of 20°C (68°F), Conductor Temperature 75°C (167°F).

Surface

N and EGC

150 mm (6 in.)

AØ

250 mm (10 in.) BØ

250 mm (10 in.) CØ

600 mm (24 in.)

950 mm (38 in.)

250 mm 250 mm 250 mm 250 mm

(10 in.)

1.1 m (44 in.)

Design Criteria

Neutral and Equipment Number of Cables per Phase = 5 Grounding conductor (EGC) Rho Concrete = Rho Earth – 5 Duct = 150 mm (6 in.) Rho PVC Duct = 650

Phase Ducts = 75 mm (3 in.) Rho Cable Insulation = 500 Conductor Material = Copper Rho Cable Jacket = 650 Number of Cables per Duct = 1

Notes:

Neutral configuration per 300.5(I), Exception No. 2.
Maximum harmonic loading on the neutral conductor cannot exceed 50 percent of the phase current for the ampacities shown in the table below.
Metallic shields of Type MV-90 cable shall be grounded at one point only.

Size kcmil	TYPESRHW, THHW, THW, THWN, XHHW, USE, OR MV-90*	Size kcmil
Total per Phase Ampere Rating
RHO EARTH 60LF 50	RHO EARTH 90LF 100	RHO EARTH 120LF 100
2000	5575(1115A/Cable)	3375(675A/Cable)	3000(600A/Cable)	2000

Ambient Temp. (C)	For ambient temperatures other than 20C (68F), multiply the ampacities shown above by the appropriate factor shown below.	Ambient Temp. (F)
6–10	1.09	1.09	1.09	1.09	1.09	43–50
11–15	1.04	1.04	1.04	1.04	1.04	52–59
16–20	1.00	1.00	1.00	1.00	1.00	61–68
21–25	0.95	0.95	0.95	0.95	0.95	70–77
26–30	0.90	0.90	0.90	0.90	0.90	79–86

*Limited to 75C conductor temperature.

Informational Note Figure B.310.15(B)(2)(5) Ampacities of Single Insulated Conductors Rated 0 Through 5000 Volts in Nonmagnetic Underground Electrical Ducts (One Conductor per Electrical Duct), Five Single-Conductor Cables per Phase Based on Ambient Earth Temperature of 20°C (68°F), Conductor Temperature 75°C (167°F).

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