Calculate feeder size in copper or aluminum from load amps, voltage, run length and a 3% voltage drop, or find the maximum run for a chosen wire size.

Load running 3 hours or more; counted at 125%. Leave blank if none.

Leave blank if none.

Line-to-line for balanced three-phase; two-wire supply voltage otherwise.

3% is the common NEC informational-note recommendation for feeders, not a code requirement. Change it to your project limit.

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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 ร— K ร— I ร— L) / VD

Variables:

  • FS is the required conductor area (circular mils)
  • m is the phase multiplier: 2 for single-phase or DC two-wire circuits, or โˆš3 (โ‰ˆ 1.732) for balanced three-phase circuits
  • K is the approximate 75 ยฐC resistance constant (ฮฉยทcmil/ft), 12.9 for copper or 21.2 for aluminum. This resistance-only approximation omits AC reactance and power factor and can differ from an actual conductor impedance calculation
  • I is the actual load current (continuous plus noncontinuous amperes), separate from the 125%-continuous current screen
  • 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 ยท (p/100).

To calculate a voltage-drop area estimate, multiply m ยท K ยท I ยท L and divide by VD, then round up to a listed conductor area. The calculator also screens 125% of continuous load plus noncontinuous load against a 75 ยฐC reference table with general small-conductor protection caps. Assumptions are 30 ยฐC ambient, no more than three current-carrying conductors and suitable 75 ยฐC terminals. Other corrections, demand factors, protection and equipment rules are outside the screen; a qualified person must apply the adopted code. See Southwireโ€™s published ampacity reference and the voltage-drop derivation.

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 ยท K ยท I ยท 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% โ‡’ VD = 240 ยท 0.03 = 7.2 V

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

FS = (2 ยท 12.9 ยท 80 ยท 100) / 7.2 โ‰ˆ 28,667 cmil, so 4 AWG copper at 41,740 cmil passes the voltage-drop area screen. If the entire 80 A is continuous, the simplified current screen is 100 A and requires at least 3 AWG copper (100 A in the reference table). If it is entirely noncontinuous, the screen is 80 A and 4 AWG copper (85 A) passes both checks. Neither example establishes complete installation compliance.