30 amp copper run at 100 feet
Learn about the 3% NEC informational note for 240V copper branch circuits at 30A over 100 ft. Understand derating exclusions and ensure code compliance.
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
141 W
I²R at the operating current
Required ampacity
30.0 A
Each conductor 50 A
Material estimate
$588.00
3 conductors + EMT
Conductor weight
19.2 lb
8.7 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 | 100 ft one way | cuts excluded | 3 conductors | USD | $403.00 | Source Spec Material / Tool |
| EMT conduit | 100 ft | — | 100 ft | USD | $185.00 | Source Spec Material / Tool |
| 30 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 | 141 W |
| aluminum | 6 AWG · 13.3 mm² | 13.3 | 2.02% | 235.2 V | 145 W |
| Distance | Minimum size | Drop | Percent | End voltage |
|---|---|---|---|---|
| 100 ft | 8 AWG · 8.37 mm² | 4.69 V | 1.95% | 235.3 V |
| 125 ft | 8 AWG · 8.37 mm² | 5.86 V | 2.44% | 234.1 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.
Contractor sign-off
Project: 30 amp copper run at 100 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
The National Electrical Code (NEC) provides guidelines to ensure safe and efficient electrical installations. For a 240V copper branch circuit operating at 30A over a distance of 100 feet, the 3% voltage drop rule is an important consideration. This rule suggests that the voltage drop should not exceed 3% of the supply voltage to maintain efficiency and performance.
In this case, a 3% voltage drop would equate to 7.2 volts (0.03 x 240V), meaning the voltage at the load should not fall below 232.8 volts. It's also crucial to note that derating for conductor ampacity due to ambient temperature or other factors is excluded when calculating this voltage drop, ensuring that the circuit remains compliant with NEC standards.
Code reference
Refer to NEC Article 310 for conductor ampacity and Article 215 for branch circuits.
Field questions
- What is the significance of the 3% voltage drop rule?
- The 3% voltage drop rule is significant as it helps ensure that electrical devices receive adequate voltage for optimal performance, reducing the risk of overheating and equipment failure.
- Why is derating excluded from the 3% voltage drop calculation?
- Derating is excluded from the voltage drop calculation to provide a clear guideline for maintaining efficiency without complicating the assessment with additional factors that may affect conductor performance.
- How do I calculate the voltage drop for my circuit?
- To calculate voltage drop, you can use the formula: Voltage Drop (V) = (2 x Length of Circuit (ft) x Current (A) x Resistance of Conductor (ohms/ft)). Ensure you use the appropriate resistance value for copper conductors.