100A Subpanel Feeder Wire Size at 50 Feet
Fifty feet is short for a 100 A feeder, so ampacity usually governs. The drop percentage is still reported.
Formula SSC-2026.1
K method · SSC-2026.1
Minimum size
3 AWG · 26.7 mm²
Single conductor set
Voltage drop
2.45 V
1.02% · end 237.5 V
Power loss
245 W
I²R at the operating current
Required ampacity
100.0 A
Each conductor 100 A
Material estimate
$664.50
3 conductors + EMT
Conductor weight
29.3 lb
13.3 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 | 50 ft one way | cuts excluded | 3 conductors | USD | $572.00 |
| EMT conduit | 50 ft | — | 50 ft | USD | $92.50 |
| Material | Minimum size | mm² | Drop | End voltage | Loss |
|---|---|---|---|---|---|
| copper | 3 AWG · 26.7 mm² | 26.7 | 1.02% | 237.5 V | 245 W |
| aluminum | 1 AWG · 42.4 mm² | 42.4 | 1.06% | 237.5 V | 253 W |
| Distance | Minimum size | Drop | Percent | End voltage |
|---|---|---|---|---|
| 25 ft | 3 AWG · 26.7 mm² | 1.23 V | 0.51% | 238.8 V |
| 50 ft | 3 AWG · 26.7 mm² | 2.45 V | 1.02% | 237.5 V |
| 75 ft | 3 AWG · 26.7 mm² | 3.68 V | 1.53% | 236.3 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 50 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
Fifty feet is short for a 100 A feeder, so ampacity usually governs. The drop percentage is still reported. The run is 50 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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