Solar String Wire Size, 20A at 150 Feet
Twenty amps continuous at 150 ft shows how a modest current still grows the conductor when the one-way distance is long.
Formula SSC-2026.1
K method · SSC-2026.1
Minimum size
8 AWG · 8.37 mm²
Single conductor set
Voltage drop
4.69 V
1.95% · end 235.3 V
Power loss
94 W
I²R at the operating current
Required ampacity
25.0 A
Each conductor 50 A
Material estimate
$882.00
3 conductors + EMT
Conductor weight
28.8 lb
13.1 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 8 AWG · 8.37 mm² | 150 ft one way | cuts excluded | 3 conductors | USD | $604.50 |
| EMT conduit | 150 ft | — | 150 ft | USD | $277.50 |
| Material | Minimum size | mm² | Drop | End voltage | Loss |
|---|---|---|---|---|---|
| copper | 8 AWG · 8.37 mm² | 8.4 | 1.95% | 235.3 V | 94 W |
| aluminum | 6 AWG · 13.3 mm² | 13.3 | 2.02% | 235.2 V | 97 W |
| Distance | Minimum size | Drop | Percent | End voltage |
|---|---|---|---|---|
| 125 ft | 10 AWG · 5.26 mm² | 6.21 V | 2.59% | 233.8 V |
| 150 ft | 8 AWG · 8.37 mm² | 4.69 V | 1.95% | 235.3 V |
| 175 ft | 8 AWG · 8.37 mm² | 5.47 V | 2.28% | 234.5 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: Solar String Wire Size, 20A at 150 Feet
Prepared by
Date
Verified by (PE / electrician / inspector)
License number
Signature
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
Twenty amps continuous at 150 ft shows how a modest current still grows the conductor when the one-way distance is long. The run is 150 ft one way at 20 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 20 A voltage drop?
- Voltage drop uses the operating amps. Ampacity uses 125 percent because the load is 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.
Explore Related Engineering Calculations
- 50 Amp EV Charger Wire Size at 50, 75, and 100 Feet
- 100 Amp Subpanel Feeder: Copper vs Aluminum
- 30 Amp RV Outlet Wire Gauge and Voltage Drop
- 200 Amp Main Service Entrance Wire Size
- 240 V Mini-Split Heat Pump Wire Gauge
- 60 Amp Detached Garage Subpanel Wire Size
- 20×20 Garage Slab: 4 Inch vs 6 Inch
- Deck Stair Stringer Calculator, 36 to 96 Inch Rise
- 6/12 Roof Pitch, Rafter Length, and Shingle Bundles
- 20 ft dry, US 48×40 pallets: Pallet Count, CBM, and Payload