Use the calculator tabs to help estimate electrical feeder (conductor) size. The Voltage Drop tab sizes conductors based on allowable voltage drop and run length, and the NEC Ampacity tab selects a minimum conductor size based on NEC Table 310.16 ampacities (without correction/adjustment factors). Final feeder sizing must also consider applicable code rules and installation conditions.

Feeder Size Calculator

Basic
Voltage Drop
NEC Ampacity

Enter any 3 values to calculate the missing variable (simplified resistance-based voltage drop sizing).

Feeder Size Formula

The following simplified formula is commonly used to estimate the minimum conductor area required to meet an allowable voltage drop (resistance-only approximation). 

FS = (m * K * I * L) / VD

Variables:

  • FS is the required conductor area (circular mils)
  • m is the phase multiplier: 2 for single-phase (two-wire) circuits, or √3 (≈ 1.732) for three-phase circuits
  • K is the conductor resistivity constant (Ω·cmil/ft), commonly taken as 12.9 for copper and 21.2 for aluminum for typical voltage-drop calculations
  • I is the load current (amperes)
  • L is the one-way conductor length (feet)
  • VD is the allowable voltage drop (volts). If you have an allowable percentage p and system voltage Vsys, then VD = Vsys · (p/100).

To calculate feeder size by voltage drop, multiply m · K · I · L and divide by the allowable voltage drop (VD). Then choose the next larger standard conductor size and confirm it also meets required ampacity per code.

What is Feeder Size?

Feeder size refers to the cross-sectional area of an electrical feeder wire, which is necessary to carry a certain amount of current without overheating or dropping too much voltage. It is typically measured in circular mils, which is a unit of area used specifically for wires. Proper feeder sizing is crucial for the safety and efficiency of an electrical system.

How to Calculate Feeder Size?

The following steps outline how to calculate the Feeder Size.

  1. Determine the load current (I) in amperes.
  2. Determine the one-way feeder length (L) in feet.
  3. Determine the allowable voltage drop (VD) in volts (or convert from an allowable percentage of system voltage).
  4. Select the conductor material (to choose K) and whether the circuit is single-phase or three-phase (to choose m).
  5. Use the formula from above: FS = (m · K · I · L) / VD.
  6. Choose the next larger standard conductor size and verify ampacity and any required correction/adjustment factors per the applicable electrical code.

Example Problem : 

Use the following variables as an example problem to test your knowledge.

current (I) = 80 amperes

system voltage (Vsys) = 240 volts

one-way length (L) = 100 ft

allowable voltage drop (p) = 3% ⇒ VD = 240 · 0.03 = 7.2 V

single-phase copper: m = 2, K = 12.9

FS = (2 · 12.9 · 80 · 100) / 7.2 ≈ 28,667 cmil (so you would select the next larger standard size and confirm ampacity; for example, 4 AWG is 41,740 cmil and is often checked against ampacity requirements).