What Gauge Wire for 100 Amp Subpanel Feeder: Sizing Guide

For a 100 amp feeder, use 3 AWG copper or 1 AWG aluminum if terminals are rated 75°C. If the load is continuous (3+ hours) or terminals are unrated (defaulting to 60°C), upsize to 1 AWG copper or 1/0 AWG aluminum, protected by a 100A breaker.

The Baseline: Sizing for a 100 Amp Feeder

Before pulling any wire through conduit, we have to establish the baseline conditions. Wire ampacity is not a fixed number; it changes based on insulation type, ambient temperature, and how many wires are sharing a raceway. The direct answer above relies on a specific set of assumptions that match 90% of residential and light commercial subpanel installs.

To understand why we select these specific gauges, we need to look at the National Electrical Code (NEC) Table 310.16. This table is the bible for conductor ampacity. Below is the exact excerpt relevant to a 100A circuit.

Wire Size (AWG/kcmil)	Copper 60°C Column	Copper 75°C Column	Copper 90°C Column	Aluminum 75°C Column
3 AWG	85A	100A	115A	75A
2 AWG	95A	115A	130A	90A
1 AWG	110A	130A	145A	100A
1/0 AWG	125A	150A	170A	120A

Source: NEC Table 310.16 (formerly 310.15(B)(16)). Always verify against the latest adopted code cycle in your jurisdiction.

Why 1 AWG Copper (and Not 3 AWG or the 90°C Column)?

Looking at the table above, a common DIY mistake is pointing to the 90°C column and claiming 3 AWG THHN copper (rated 115A at 90°C) is sufficient for a 100A breaker. This is incorrect and violates NEC 110.14(C).

Here is the reality of terminal temperature limitations: while your THHN wire insulation can handle 90°C, the brass or aluminum lugs inside your breaker and subpanel are almost universally rated for a maximum of 75°C. The NEC requires you to size the wire based on the lowest temperature rating of any connected component. Therefore, you must use the 75°C column for your final ampacity check.

So why do I recommend 1 AWG copper over 3 AWG copper (which hits exactly 100A in the 75°C column)? Two reasons:

1. Continuous Loads: If your 100A load is considered 'continuous' (expected to run for 3 hours or more, like EV chargers, heavy HVAC, or continuous workshop machinery), NEC 210.19(A)(1) requires the conductors to be sized at 125% of the load. 100A × 1.25 = 125A. 3 AWG (100A) will fail this check. 1 AWG (130A at 75°C) passes easily.
2. The 60°C Default Rule: NEC 110.14(C)(1)(a) states that for circuits rated 100A or less, you must default to the 60°C column unless the equipment is explicitly marked otherwise. If you are using an older panel or a breaker where the 75°C marking is obscured or missing, 3 AWG drops to 85A in the 60°C column, which is insufficient. 1 AWG in the 60°C column is rated 110A, keeping you safe and code-compliant regardless of the terminal marking.

Variables That Force a Wire Size Upgrade

The baseline assumptions only hold true in ideal conditions. On the jobsite, three main variables will force you to buy thicker wire.

1. Voltage Drop Over Distance

The NEC recommends a maximum 3% voltage drop on feeders. If you push 100A through 3 AWG copper over a long distance, the voltage at the subpanel will sag, causing motors to overheat and electronics to brown out.

• 3 AWG Copper: Good for up to 146 feet at 240V before exceeding a 3% drop (7.2V).
• 1 AWG Copper: Good for up to 233 feet at 240V.
• 1/0 AWG Aluminum: Good for up to 188 feet at 240V.

If your trench is 250 feet long, you must upsize to 2/0 AWG copper or 3/0 AWG aluminum, regardless of what the ampacity table says. Use a dedicated voltage drop calculator to verify your exact run.

2. Conduit Bundling and Derating

Table 310.16 assumes you have no more than three current-carrying conductors in a single conduit. If you are pulling two separate 120/240V circuits through the same PVC pipe, you now have four current-carrying conductors (the equipment grounding conductor does not count). According to NEC Table 310.15(C)(1), 4 to 6 conductors require an 80% derating factor applied to the 90°C column.

For 1 AWG copper: 145A (90°C column) × 0.80 = 116A. This still covers a 100A breaker. But if you tried to use 2 AWG (130A × 0.80 = 104A), you are cutting it dangerously close, and if ambient temps rise, you will trip the thermal limits.

3. High Ambient Temperatures

If your conduit runs through an unventilated attic in a southern climate where ambient temperatures routinely hit 110°F (43°C), you must apply a temperature correction factor. At 110°F, the 75°C column requires a 0.82 multiplier. 1 AWG copper (130A) × 0.82 = 106.6A. You are still safe for 100A, but 3 AWG (100A × 0.82 = 82A) would be a severe code violation and a fire hazard.

When to Defer to an Engineer or the AHJ

While the guidelines above cover standard residential and light commercial subpanels, certain scenarios require a stamped engineering drawing or a direct consultation with your local Authority Having Jurisdiction (AHJ).

Scenario	Action Required
Run exceeds 300 feet	Voltage drop calculations become complex; parallel conductors may be required. Consult an electrical engineer.
Ambient temp exceeds 122°F (50°C)	Standard THHN derating curves bottom out. Specialized high-temp insulation (like XHHW-2 in specific configurations) or larger raceways needed.
Utility Interconnection (Solar/BESS)	Utility companies often have strict, non-NEC-specific feeder requirements for bidirectional current and fault current availability.
Aluminum to Copper Splices	If transitioning from 1/0 Aluminum service entrance to 1 AWG Copper subpanel feeder, AHJ must approve the specific antioxidant compound and lug type used.

Always remember that the NEC is a minimum safety standard, not a design manual. Sizing up to 1 AWG copper for a 100A breaker provides a robust safety margin, accommodates continuous loads, and future-proofs your subpanel against minor voltage drop issues. When in doubt, pull the larger wire—the cost difference in copper is negligible compared to the cost of tearing out conduit and repulling wire three years down the road.