Wire Sizing

Revit MEP calculates the wire sizes for power circuits based on the size specified for circuit protection, voltage drop calculation, and correction factor. Wire sizes are defined in the Electrical Settings dialog.

The voltage drop for each circuit is reported as line-to-line for 2-pole (single phase) and 3-pole (three phase) circuits. The voltage drop is reported as line-to-neutral for 1-pole circuits.

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.
  1. 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.
  2. 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.
  3. 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.

  4. 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.

  5. 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.

  6. 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.
  7. 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.
  1. 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.
  2. 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.).

  3. 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.

  4. 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:

  5. 208V * 0.02 = 4.16V = maximum allowable voltage drop
  6. 4.16V = (150ft. * R * 66.6A)/1000
  7. 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.

  8. The neutral conductor is resized to #4 wire (1.5 * 0.0206119720 sq. in. = 0.030917958 sq. in.).

  9. 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 MEP 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 MEP 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 MEP 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
Parent topic: Electrical Sizing and Calculation Methods