Feeder Sizing Calculator

A feeder is the wiring that runs from your electrical panel to a subpanel or large load — sizing it correctly prevents overheating, tripped breakers, and excessive voltage drop. Enter your feeder load (amperes), system voltage, system type (single or three phase), and one-way distance, then select your wire insulation type, conductor material, conduit type, and neutral/ground preferences into the Feeder Sizing Calculator to get the required wire size (AWG) and minimum conduit size. Secondary outputs include voltage drop at 2% and 3% thresholds, plus the maximum allowable run length at 3% voltage drop.

A

Enter the calculated load current in amperes

V

Enter system voltage (120V, 240V, 208V, 480V, etc.)

ft

Distance from panel to load

Results

Required Wire Size

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Minimum Conduit Size

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Voltage Drop (2%)

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Voltage Drop (3%)

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Maximum Length (3% VD)

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Results Table

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Frequently Asked Questions

What wire size do I need for a 100 amp feeder?

For a 100 amp feeder, you typically need #3 AWG copper or #1 AWG aluminum wire, but the exact size depends on the distance, voltage, and acceptable voltage drop percentage.

How do I calculate voltage drop for electrical feeders?

Voltage drop is calculated using the formula: VD = (2 × K × I × L) / CM, where K is the conductor resistance constant, I is current, L is distance, and CM is circular mils. Most applications require voltage drop to be 3% or less.

What's the difference between copper and aluminum conductors?

Copper conductors have lower resistance and higher ampacity than aluminum for the same size. Aluminum is lighter and less expensive but requires larger wire sizes to carry the same current safely.

Do I need a neutral wire for three-phase feeders?

Three-phase feeders may or may not require a neutral depending on the load type. Motor loads typically don't need neutrals, while mixed lighting and receptacle loads usually do require a neutral conductor.

What conduit size is required for my feeder?

Conduit size depends on the total cross-sectional area of all conductors (phase, neutral, and ground wires). NEC Table 1 provides conduit fill percentages, typically allowing 40% fill for 3+ conductors.

When do I need to upsize the ground wire?

Ground wire sizing is based on the overcurrent protection device rating according to NEC Table 250.122. If you upsize phase conductors for voltage drop, you may also need to proportionally upsize the ground conductor.

What's the maximum voltage drop allowed by electrical codes?

The NEC recommends maximum voltage drop of 3% for feeders and 5% total for the entire electrical system (feeders plus branch circuits). Some local codes may have stricter requirements.

How does wire insulation type affect feeder sizing?

Different insulation types have different temperature ratings and ampacities. THHN/THWN is rated for 90°C and has higher ampacity than THW (75°C rated), allowing for smaller wire sizes in many applications.