100 Amp Subpanel Feeder: Copper vs Aluminum
A 100 A, 240 V feeder at 80 ft, with copper and aluminum side by side.
Formula SSC-2026.1
K method · SSC-2026.1
Minimum size
3 AWG · 26.7 mm²
Single conductor set
Voltage drop
3.92 V
1.63% · end 236.1 V
Power loss
392 W
I²R at the operating current
Required ampacity
100.0 A
Each conductor 100 A
Material estimate
$991.20
3 conductors + PVC
Conductor weight
46.8 lb
21.2 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 | 80 ft one way | cuts excluded | 3 conductors | USD | $915.20 |
| PVC conduit | 80 ft | — | 80 ft | USD | $76.00 |
| Material | Minimum size | mm² | Drop | End voltage | Loss |
|---|---|---|---|---|---|
| copper | 3 AWG · 26.7 mm² | 26.7 | 1.63% | 236.1 V | 392 W |
| aluminum | 1 AWG · 42.4 mm² | 42.4 | 1.69% | 235.9 V | 405 W |
| Distance | Minimum size | Drop | Percent | End voltage |
|---|---|---|---|---|
| 80 ft | 3 AWG · 26.7 mm² | 3.92 V | 1.63% | 236.1 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: 100 Amp Subpanel Feeder: Copper vs Aluminum
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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
This feeder is not marked continuous, so the ampacity target is 100 A. The run is 80 ft at 240 V with a 3 percent limit, which matches the feeder informational note rather than the 5 percent combined note. The comparison table keeps every other input and swaps copper for aluminum. Aluminum is larger for the same ampacity and uses K = 21.2. Equipment grounding conductor size is estimated at 60 percent of the ungrounded conductor price, not from Table 250.122.
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
- Is aluminum allowed on a subpanel feeder?
- Yes when the conductor, lugs, and antioxidant practice are listed for aluminum. The table shows the larger aluminum size next to copper.
- Should the limit be 3 percent or 5 percent?
- This preset uses 3 percent for the feeder itself. The 5 percent note is the branch plus feeder total. Switch the control if you are budgeting the whole path.
- Does the cost include the panel?
- No. It is conductor length plus conduit. Labor, breakers, and the enclosure are separate.