Why is the three-phase factor 1.732?

It is the square root of 3, used to convert the line current and conductor resistance into a line-to-line voltage drop.

Does 480 volts always allow a smaller wire?

Often, because percent drop divides by 480. Ampacity can still force a minimum size. Both checks are required.

Can I switch this to 208 or 400 volts?

Yes. The system control includes 208 V and 400 V three-phase. The same current and distance will show a larger percent drop at the lower voltage.

480 V Three-Phase 100 Amp Shop Feeder

A 100 A, 480 V, three-phase feeder, 120 ft, copper in EMT.

Formula SSC-2026.1

K method · SSC-2026.1

Minimum size

3 AWG · 26.7 mm²

Single conductor set

Voltage drop

5.10 V

1.06% · end 474.9 V

Power loss

883 W

I²R at the operating current

Required ampacity

100.0 A

Each conductor 100 A

Material estimate

$2,122.80

4 conductors + EMT

Conductor weight

93.6 lb

42.5 kg · approximate insulated weight

Single-line and conductor gaugeLive scaled blueprint

Circle diameters track circular mils. The drawing is a schematic, not a conduit-fill layout.

Bill of materials and shopping list
Item	Net	Waste	Order	Unit price	Line total
copper 3 AWG · 26.7 mm²	120 ft one way	cuts excluded	4 conductors	USD	$1,900.80
EMT conduit	120 ft	—	120 ft	USD	$222.00

Copper versus aluminum at this run
Material	Minimum size	mm²	Drop	End voltage	Loss
copper	3 AWG · 26.7 mm²	26.7	1.06%	474.9 V	883 W
aluminum	1 AWG · 42.4 mm²	42.4	1.10%	474.7 V	912 W

Same load at other one-way distances
Distance	Minimum size	Drop	Percent	End voltage
60 ft	3 AWG · 26.7 mm²	2.55 V	0.53%	477.5 V
120 ft	3 AWG · 26.7 mm²	5.10 V	1.06%	474.9 V
200 ft	3 AWG · 26.7 mm²	8.49 V	1.77%	471.5 V

Show the deterministic formula

Single phase: Vdrop = 2 × K × I × D / cmil. Three phase: Vdrop = 1.732 × K × I × D / cmil. K is 12.9 for copper and 21.2 for aluminum. D is one-way feet.

Percent drop = Vdrop ÷ source voltage × 100. Green is 3% or less, amber is 3 to 5%, and red is over 5%. Power loss is I²R on the current-carrying conductors.

The smallest conductor that meets both the ampacity (125% when continuous) and the drop limit is selected. Ampacity does not include bundling or temperature correction.

Contractor sign-off

Project: 480 V Three-Phase 100 Amp Shop Feeder

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Verified by (PE / electrician / inspector)

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This sheet is an estimate for ordering and layout discussion. It is not a sealed design. Confirm the quantities with a licensed local professional and the authority having jurisdiction before you buy or build.

Engineering notes

Three-phase drop uses 1.732 × K × I × D / cmil instead of the single-phase factor of 2. The higher source voltage makes the same volt drop a smaller percentage, so a 480 V feeder can stay smaller than a 240 V feeder of the same amps and length. The preset is 100 A, 120 ft, copper, 3 percent, not continuous. Power loss uses three conductors. Confirm the motor load, the starter, and any required equipment grounding conductor separately.

Code reference

NEC 210.19(A) and 215.2(A) informational notes recommend 3 percent on a branch circuit and 5 percent total with the feeder. Ampacity follows the 75°C column of Table 310.16 plus the small-conductor limits. Confirm the adopted edition.

Field questions

Why is the three-phase factor 1.732?: It is the square root of 3, used to convert the line current and conductor resistance into a line-to-line voltage drop.
Does 480 volts always allow a smaller wire?: Often, because percent drop divides by 480. Ampacity can still force a minimum size. Both checks are required.
Can I switch this to 208 or 400 volts?: Yes. The system control includes 208 V and 400 V three-phase. The same current and distance will show a larger percent drop at the lower voltage.