100A Subpanel Feeder Wire Size at 100 Feet
A 100 A feeder at 100 ft one way is the case where copper often steps up for voltage drop before the 75°C ampacity is exhausted.
Formula SSC-2026.1
K method · SSC-2026.1
Minimum size
3 AWG · 26.7 mm²
Single conductor set
Voltage drop
4.90 V
2.04% · end 235.1 V
Power loss
490 W
I²R at the operating current
Required ampacity
100.0 A
Each conductor 100 A
Material estimate
$1,329.00
3 conductors + EMT
Conductor weight
58.5 lb
26.5 kg · approximate insulated weight
Circle diameters track circular mils. The drawing is a schematic, not a conduit-fill layout.
| Item | Net | Waste | Order | Unit price | Line total |
|---|---|---|---|---|---|
| copper 3 AWG · 26.7 mm² | 100 ft one way | cuts excluded | 3 conductors | USD | $1,144.00 |
| EMT conduit | 100 ft | — | 100 ft | USD | $185.00 |
| Material | Minimum size | mm² | Drop | End voltage | Loss |
|---|---|---|---|---|---|
| copper | 3 AWG · 26.7 mm² | 26.7 | 2.04% | 235.1 V | 490 W |
| aluminum | 1 AWG · 42.4 mm² | 42.4 | 2.11% | 234.9 V | 507 W |
| Distance | Minimum size | Drop | Percent | End voltage |
|---|---|---|---|---|
| 75 ft | 3 AWG · 26.7 mm² | 3.68 V | 1.53% | 236.3 V |
| 100 ft | 3 AWG · 26.7 mm² | 4.90 V | 2.04% | 235.1 V |
| 125 ft | 3 AWG · 26.7 mm² | 6.13 V | 2.55% | 233.9 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.
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Project: 100A Subpanel Feeder Wire Size at 100 Feet
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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
A 100 A feeder at 100 ft one way is the case where copper often steps up for voltage drop before the 75°C ampacity is exhausted. The run is 100 ft one way at 100 A on 1ph-240. K is 12.9 for copper and 21.2 for aluminum. Ambient temperature, bundling, and rooftop adders are not applied.
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
- What current is used for the 100 A voltage drop?
- Both the drop and the ampacity check use the entered amps, because this preset is not marked continuous.
- Does this replace the equipment installation manual?
- No. The nameplate, the breaker, and the adopted NEC edition still control. This page is the K-method size and the percent drop.
- Can I switch copper and aluminum?
- Yes. The conductor control and the comparison table both recompute size, drop, and the planning cost.
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