20 amp copper run at 150 feet
Learn about the 3% voltage drop rule in the NEC for 240V copper branch circuits at 20A over 150 ft, and understand why derating is excluded.
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
20.0 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 | Source Spec Material / Tool |
|---|---|---|---|---|---|---|
| copper 8 AWG · 8.37 mm² | 150 ft one way | cuts excluded | 3 conductors | USD | $604.50 | Source Spec Material / Tool |
| EMT conduit | 150 ft | — | 150 ft | USD | $277.50 | Source Spec Material / Tool |
| 20 A breaker | 2-pole | — | 1 | Match the calculated load | — | Source Spec Material / Tool |
Source links open a product search on the Amazon storefront for the selected currency. SiteSpecCalc may earn a commission. The link does not change the quantities above.
| 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 |
|---|---|---|---|---|
| 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: 20 amp copper run at 150 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
The National Electrical Code (NEC) recommends a maximum voltage drop of 3% for branch circuits to ensure efficient operation of electrical equipment. For a 240V copper branch circuit at 20A over a distance of 150 ft, this means that the voltage drop should not exceed 7.2 volts (3% of 240V). This guideline helps maintain performance and safety in electrical installations.
It's important to note that derating due to ambient temperature or other factors is not considered in this calculation. The 3% rule is strictly about maintaining voltage levels to prevent equipment malfunction and ensure energy efficiency.
Code reference
Refer to NEC Article 210.19(A)(1) for branch circuits and the informational note regarding voltage drop.
Field questions
- What is the significance of the 3% voltage drop rule?
- The 3% voltage drop rule helps ensure that electrical equipment operates efficiently and safely, preventing performance issues due to insufficient voltage.
- Why is derating excluded from the 3% voltage drop calculation?
- Derating is excluded because the 3% voltage drop rule specifically addresses the maximum allowable drop for efficient operation, independent of other factors like temperature or conduit fill.
- How do I calculate the voltage drop for my circuit?
- To calculate voltage drop, you can use the formula: Voltage Drop (V) = (2 * Length of Circuit (ft) * Current (A) * Resistance of Wire (Ω/ft)). For copper wire, refer to standard resistance values based on wire gauge.