Enter your Feeder Load, System Voltage, System Type, One-Way Distance, Insulation Type, Conductor Material, and Wire Counts into the Feeder Sizing Calculator to find the Required Wire Size, Minimum Conduit Size, and Voltage Drop at 2% and 3% thresholds.
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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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.
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.
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.
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.
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.
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.
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.
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.