Wire Sizing Reference

Wire Sizing Examples

In this example, the following wire type settings are specified for two circuits created in the project, carried in steel conduit.

Material	Temperature Rating	Insulation	Max Size	Neutral Multiplier	Neutral Required	Neutral Size	Conduit Type
Copper	90 C	THHN	500 MCM	1.5	Yes	Hot Conductor	Steel
Copper	75 C	THWN	500 MCM	1.0	Yes	Hot Conductor	Steel

Feeder Circuit 1

Circuit	Power Factor	Voltage	Load	Poles	Rating	Wire Type	Ambient Temperature	Length of the Wiring Run
Feeder	0.8	240 V	12 kVA	2	50A	THHN	80 F	100 ft.

The circuit rating (50A) and wire type (THHN, Copper, 90C) determine the basic sizing for the hot conductor size. The basic wire size table, without considering the ambient temperature, calls for 2-#8 hot conductors for circuit 1.
The neutral conductor is sized as 1.5 times the cross sectional area of the hot conductors. A #8 hot conductor has a cross sectional area of 0.0129686799 sq. in. Applying the 1.5 multiplier (1.5 * 0.0129686799 = 0.01945301985 sq. in.), the basic wire size table calls for a #6 wire (area of 0.0206119720 sq. in.), which provides the minimum area that satisfies the neutral conductor requirement. The Neutral conductor is sized as 1-#6.
The ground conductor size table for a 50A copper conductor calls for a #10 ground conductor (#10 is adequate for up to 60A).
Preliminary sizing for the wiring package for circuit 1 is 2-#8, 1-#6, and 1-#10. However, the sizing must consider the correction factor (based on the ambient temperature) and support less than a 2 percent voltage drop at the furthest fixture from the source.
Assuming the #8 hot conductors at an ambient temperature of 80 degrees Fahrenheit (correction factor =1), the voltage drop calculation is VD = (L * Z * I)/1000:
- Length (L) = 100 ft.
- Impedance (Z) = 1.326 from Wire Impedance table for #8 in steel conduit at 80 degrees
- Load (I) = 50A
VD = (100 ft. *1.326 * 50A)/1000 = 6.63 V

6.63/240 = 0.027625 = 2.7625% which exceeds the 2 percent allowable voltage drop.
Assume #6 hot conductors (wiring package is adjusted to 2-#6, 1-#6, and 1-#10) and calculate the voltage drop again.
VD = (100 ft. * 0.860 * 50A)/1000 = .3

.3/240 = 0.017917 = 1.7917% within the 2 percent allowable voltage drop.
The neutral conductor is sized, 1.5 * 0.020611972 = 0.030917958 sq. in. The basic wire size table calls for a #4 wire (area of 0.0327813057 sq. in.), which provides the minimum area that satisfies the adjusted neutral conductor requirement. The Neutral conductor is resized as 1-#4.
The ground conductor must be changed in proportion to the change in the hot conductors:
- Cross sectional area of #8 = 0.029686799 sq. in.
- Cross sectional area of #6 = 0.0206119720 sq. in.
- Ratio = 1.5893
- 1.5893 * 0.0081552613 sq. in. = 0.01296115 sq. in. The ground conductor must be resized to #8.
The wiring package is adjusted to 2-#6, 1-#4, 1#8.

Feeder Circuit 2

Circuit	Power Factor	Voltage	Load	Poles	Rating	Wire Type	Ambient Temperature	Length of the Wiring Run
Branch	0.9	208 V	24 kVA	3	70A	THHN	80 F	150 ft.

The circuit rating (70A) and wire type (THHN, Copper, 90 C) determine the basic sizing for the hot conductors. The basic wire size table calls for #6 hot conductors.
The neutral conductor is sized as 1.5 times the cross sectional area of the hot conductors (1.5 * 0.0521172118 sq. in. = 0.0781758177 sq. in.).
The neutral conductor is sized as 1- 1/0 (0.0829065680 sq. in.).
The ground conductor size table for a 70A copper conductor calls for a #6 ground conductor.
Before considering voltage drop and ambient temperature, the wiring package consists of 3-#6, 1-1/0, and 1-#6.
The voltage drop calculation is (VD = (L * R * I)/1000:
- I = 24,000 kVA / 208 * sqrt(3) = 66.6 A
- VD = (150 ft. * 0.812 * 100A)/1000 = 8.11V
- 8.11/208 = 0.039 = 3.9% greater than the 2% allowable voltage drop
Working in reverse from the 2 percent allowable voltage drop and solving for impedance:
208V * 0.02 = 4.16V = maximum allowable voltage drop
4.16V = (150ft. * R * 66.6A)/1000
VD = 4.16V / ((150 ft. * 66.6A) / 1000) 0.416 which is approximately the impedance factor for #2 wire in steel conduit, in a 3-phase circuit.
The hot conductors are resized as 3-#1.
The neutral conductor is resized to #4 wire (1.5 * 0.0206119720 sq. in. = 0.030917958 sq. in.).
The #6 ground conductor (0.0206119720 sq. in.) must be changed in proportion to the change in the hot conductors:
- Cross sectional area of original #2 hot conductor = 0.0521172118 sq. in.
- Cross sectional area of new #1 hot conductor = 0.0657664432 sq. in.
- Ratio = 1.26
- 1.26 * 0.0206119720 sq. in. = 0.02597108472 sq. in. = #4 wire
  The ground conductor is resized to #4 wire.
The wiring package is adjusted to 3-#1, 1-#4, 1#4.

Wire Impedance Factors

For the purposes of Voltage Drop calculations, Revit always assumes Copper conductors. The volt loss tables below are consistent with the Cooper Bussmann® Selecting Protective Devices Handbook. The values indicate the line-to-line voltage drop per 1000 Amp Feet for three phase and single phase circuits. For line to neutral voltage drop on three phase systems, divide by 1.73; for single phase systems, divide by 2.

Conduit	Wire Size	Three Phase					Single Phase
Power Factor (%)		100	90	80	70	60	100	90	80	70	60
Steel	14	5.369	4.887	4.371	3.848	3.322	6.2	5.643	5.047	4.444	3.836
	12	3.464	3.169	2.841	2.508	2.172	4	3.659	3.281	2.897	2.508
	10	2.078	1.918	1.728	1.532	1.334	2.4	2.214	1.995	1.769	1.54
	8	1.35	1.264	1.148	1.026	0.9	1.56	1.46	1.326	1.184	1.04
	6	0.848	0.812	0.745	0.673	0.597	0.98	0.937	0.86	0.777	0.69
	4	0.536	0.528	0.491	0.45	0.405	0.62	0.61	0.568	0.519	0.468
	3	0.433	0.434	0.407	0.376	0.341	0.5	0.501	0.47	0.434	0.394
	2	0.346	0.354	0.336	0.312	0.286	0.4	0.409	0.388	0.361	0.331
	1	0.277	0.292	0.28	0.264	0.245	0.32	0.337	0.324	0.305	0.283
	1/0	0.207	0.228	0.223	0.213	0.2	0.24	0.263	0.258	0.246	0.232
	2/0	0.173	0.196	0.194	0.188	0.178	0.2	0.227	0.224	0.217	0.206
	3/0	0.136	0.162	0.163	0.16	0.154	0.158	0.187	0.188	0.184	0.178
	4/0	0.109	0.136	0.14	0.139	0.136	0.126	0.157	0.162	0.161	0.157
	250	0.093	0.132	0.128	0.129	0.128	0.108	0.142	0.148	0.149	0.148
	300	0.077	0.108	0.115	0.117	0.117	0.09	0.125	0.133	0.135	0.135
	350	0.067	0.098	0.106	0.109	0.109	0.078	0.113	0.122	0.126	0126
	400	0.06	0.091	0.099	0.103	0.104	0.07	0.105	0.114	0.118	0.12
	500	0.05	0.081	0.09	0.094	0.096	0.058	0.094	0.104	0.109	0.111
	600	0.043	0.075	0.084	0.089	0.092	0.05	0.086	0.097	0.103	0.106
	750	0.036	0.068	0.078	0.084	0.088	0.042	0.079	0.091	0.097	0.102
	1000	0.031	0.062	0.072	0.078	0.082	0.036	0.072	0.084	0.09	0.095
Non-Magnetic	14	5.369	5.876	4.355	3.83	3.301	6.2	5.63	5.029	4.422	3.812
	12	3.464	3.158	2.827	2.491	2.153	4	3.647	3.264	2.877	2.486
	10	2.078	1.908	1.714	1.516	1.316	2.4	2.203	1.98	1.751	1.52
	8	1.35	1.255	1.134	1.01	0.882	1.56	1.449	1.31	1.166	1.019
	6	0.848	0.802	0.731	0.657	0.579	0.98	0.926	0.845	0.758	0.669
	4	0.536	0.519	0.479	0.435	0.388	0.62	0.599	0.553	0.502	0.448
	3	0.433	0.425	0.395	0.361	0.324	0.5	0.49	0.456	0.417	375
	2	0.329	0.33	0.31	0.286	0.259	0.38	0.381	0.358	0.33	0.3
	1	0.259	0.268	0.255	0.238	0.219	0.3	0.31	0.295	0.275	0.253
	1/0	0.207	0.22	0.212	0.199	0.185	0.24	0.254	0.244	0.23	0.214
	2/0	0.173	0.188	0.183	0.174	0.163	0.2	0.217	0.211	0.201	0.188
	3/0	0.133	0.151	0.15	0.145	0.138	0.154	0.175	0.173	0.167	0.159
	4/0	0.107	0.127	0.128	0.125	0.121	0.124	0.147	0.148	0.145	0.14
	250	0.09	0.112	0.114	0.113	0.11	0.104	0.129	0.132	0.131	0.128
	300	0.076	0.099	0.103	0.104	0.102	0.088	0.114	0.119	0.131	0.128
	350	0.065	0.089	0.094	0.095	0.094	0.076	0.103	0.108	0.11	0.109
	400	0.057	0.081	0.087	0.089	0.089	0.066	0.094	0.1	0.103	0.103
	500	0.046	0.071	0.077	0.08	0.082	0.054	0.082	0.09	0.093	0.094
	600	0.039	0.065	0.072	0.076	0.077	0.046	0.075	0.083	0.087	0.09
	750	0.032	0.058	0.065	0.07	0.072	0.038	0.067	0.076	0.08	0.083
	1000	0.025	0.051	0.059	0.063	0.066	0.03	0.059	0.068	0.073	0.077

Wire Types

The wire types table lists wire types that are provided with Revit.

Copper			Aluminum
60 C	75 C	90 C	60 C	75 C	90 C
TW	FEPW	TA	TW	RH	TA
UF	RH	TBS	UF	RHW	TBS
	RHW	SA		THHW	SA
	THHW	SIS		THW	SIS
	THW	FEP		THWN	THHN
	THWN	FEPB		XHHW	THHW
	USE	MI		USE	THW-2
	ZW	RHH			THWN-2
		RHW-2			RHH
		THHN			RHW-2
		THHW			USE-2
		THW-2			XHH
		THWN-2			XHHW-2
		USE-2			ZW-2
		XHH
		XHHW
		XHHW-2
		ZW-2

Wire Sizes

Revit specifies hot wire sizes according to the following table of basic wire sizes.

Copper				Aluminum
Size	Temperature Rating			Temperature Rating			Size
	60 C	75 C	90 C	60 C	75 C	90 C
	TW, UF	FEPW, RH, RHW, THHW, THW, THWN, USE, ZW	TA, TBS, SA, SIS, FEP, FEPB, MI, RHH, RHW-2, THHN, THHW, THW-2, THWN-2, USE-2, XHH, XHHW, XHHW-2, ZW	TW, UF	FEPW, RH, RHW, THHW, THW, THWN, USE, ZW	TA, TBS, SA, SIS, FEP, FEPB, MI, RHH, RHW-2, THHN, THHW, THW-2, THWN-2, USE-2, XHH, XHHW, XHHW-2, ZW
18	--	--	14	--	--	--	18
16	--	--	18	--	--	--	16
14	15	15	15	--	--	--	14
12	20	20	20	15	15	15	12
10	30	30	30	25	25	30	10
8	40	50	55	30	40	45	8
6	55	65	75	40	50	60	6
4	70	85	95	55	65	75	4
3	85	110	110	65	75	85	3
2	95	115	130	75	90	100	2
1	110	130	150	85	100	115	1
0	125	150	170	100	120	135	0
00	145	175	195	115	135	150	00
000	165	200	225	130	155	175	000
0000	195	230	260	150	180	205	0000
250	215	255	290	170	205	230	250
300	240	285	320	190	230	255	300
350	260	310	350	210	250	280	350
400	280	335	380	225	270	305	400
500	320	380	430	260	310	350	500
600	355	420	475	285	340	385	600
700	385	460	520	310	375	420	700
750	400	475	535	320	385	435	750
800	410	490	555	330	395	450	800
900	435	520	585	355	425	480	900
1000	455	545	615	375	445	500	1000
1250	495	590	665	405	485	545	1250
1500	520	625	705	435	520	585	1500
1750	545	650	735	455	545	615	1750
2000	560	665	750	470	560	630	2000

Wire Sizing Correction Factors

The circuit rating is adjusted according to the correction factors below (circuit rating * correction factor). Then the correct size for the selected wire type is found in the Wire Sizes table.

Ambient Temperature C	Ambient Temperature F	60 C	75 C	90 C
21-25	70-77	1.08	1.05	1.04
26-30	78-86	1	1	1
31-35	87-95	0.91	0.94	0.96
36-40	96-104	0.82	0.88	0.91
41-45	105-113	0.71	0.82	0.87
46-50	114-122	0.58	0.75	0.82
51-55	123-132	0.41	0.67	0.76
56-60	132-140	--	0.58	0.71
61-70	141-158	--	0.33	0.58
71-80	159-176	--	--	0.41

Ground Wire Sizing

Ground conductors are sized according to the circuit rating.

Size	Copper	Aluminum	Size	Copper	Aluminum
14	15	--	250	2000	1200
12	20	15	300	2000	1200
10	60	20	350	2500	1600
8	100	60	400	3000	2000
6	200	100	500	4000	2000
4	300	200	600	5000	3000
3	400	200	700	5000	3000
2	500	300	750	5000	3000
1	600	400	800	6000	4000
1/0	800	500	900	6000	4000
2/0	1000	600	1000	6000	4000
3/0	1200	800	1250	6000	5000
4/0	1600	1000

Neutral Wire Sizing

When a Neutral Multiplier is specified, Revit calculates the neutral conductor size based on the cross sections listed in the following table:

Size (AWG/kcmil)	Diameter (in.)	Area (sq. in.)	Size (AWG/kcmil)	Diameter (in.)	Area (sq. in.)
14	0.064080	0.0032250357	300	0.5477226	0.2356194490
12	0.080810	0.0015288468	350	0.5916079	0.2748893572
10	0.101900	0.0081552613	400	0.6324555	0.3141592653
8	0.128500	0.0129686799	500	0.7071068	0.3929660816
6	0.162000	0.0206119720	600	0.7745967	0.4712388980
4	0.204300	0.0327813057	700	0.8366600	0.5497787144
3	0.229400	0.0413310408	750	0.8660254	0.5890486225
2	0.257600	0.0521172118	800	0.8944271	0.6283185307
1	0.289300	0.0657664432	900	0.9486833	0.7068583471
0	0.324900	0.0829065680	1000	1	0.785398163
00	0.364800	0.1045199453	1250	1.118034	0.981747704
000	0.409600	0.1317678350	1500	1.224745	1.178097245
0000	0.460000	0.1661901110	1750	1.322876	1.374446786
250	0.5	0.19634954085	2000	1.414214	1.570796327