The percentages need context
Context matters. Public NFPA 70 informational notes point to 3% on a branch circuit and 5% total across the feeder plus branch circuit as reasonable efficiency benchmarks, so you should treat those two percentages as connected parts of the same wire path rather than interchangeable rules.
Suppose an upstream subpanel feeder already drops 2.2% under load. That leaves only 2.8% for the downstream branch circuit if you want to stay within a 5% combined budget, and sensitive electronics or long motor runs may require an even tighter limit.
How do you calculate wire size?
The size of an electrical conductor refers to its cross-sectional area (measured in circular mils (CM) or AWG in the US). The larger the cross-sectional area, the more current the wire can transmit with lower resistance—very similar to water flowing through a wider pipe with less pressure loss. We derive the required conductor area from Ohm's law and conductor resistivity:
where:
- V_drop – Voltage drop calculated between the supply source and the farthest end of the wire, measured in volts (V_drop = V_source × (% drop ÷ 100));
- I – Maximum current running through the conductor, measured in amps (convert power using the watts to amps calculator);
- K – Specific resistance constant of the conductor material at 75°C (12.9 Ω·CM/ft for copper and 21.2 Ω·CM/ft for aluminum);
- L – One-way length of the wire run from panel to load, measured in feet; and
- CM – Cross-sectional area of the conductor, measured in circular mils (where 1,000 CM = 1 kcmil).
Calculating wire sizes for three-phase AC
For a balanced three-phase AC circuit, three current-carrying phase conductors are used instead of a two-wire loop. The calculator accepts line-to-line voltage and line current, modifying the required area of a single conductor to:
The factor of √3 ≈ 1.732 converts between phase and line quantities in a balanced three-phase system, replacing the factor of 2. The calculator automatically selects the next AWG size up from its listed 14 AWG – 4/0 table so the voltage-drop limit is never exceeded. To check apparent power feeding that circuit, use the kVA to amps calculator.
Same run, two planning targets
| Target | Allowed volts | Required circular mils | Formula candidate |
|---|---|---|---|
| 3% | 7.2 V | 21,500 | 6 AWG |
| 5% | 12.0 V | 12,900 | 8 AWG |
Allocate the path visibly
- 01
Identify feeder drop
Estimate or obtain voltage drop to the branch-circuit source.
- 02
Identify branch drop
Calculate to the farthest relevant load.
- 03
Add like-for-like percentages
Use the same voltage basis and operating condition.
- 04
Check equipment voltage
Confirm estimated delivered voltage remains suitable for the actual equipment.
Common misuse
- Calling 5% an automatic branch-circuit allowance
- Ignoring feeder drop because the branch result is below 3%
- Using nominal voltage instead of relevant operating conditions
- Changing the target only to keep a smaller conductor
- Treating an informational design value as complete code approval