100 Amp Sub Panel Wire Diagram: Wire Sizing and Voltage Drop Guide

For a standard 100 amp sub panel wire diagram, you need 3 AWG copper or 1/0 AWG aluminum conductors for the hot and neutral legs, paired with a 100A double-pole breaker at the main panel. This assumes 75°C terminations, 30°C ambient temperature, and a run under 100 feet.

Core Assumptions and the 100 Amp Sub Panel Wire Diagram

Before pulling any wire or torqueing any lugs, you must establish the baseline conditions for your installation. Wire ampacity is not a fixed number; it changes based on insulation type, ambient heat, and terminal ratings. The sizing recommendations in this guide rely on the following strict parameters:

With those assumptions locked in, here is the exact conductor sizing data derived from NFPA 70 (NEC) Table 310.16 and NEC 250.122. This table covers the minimum allowable sizes for a 100A feeder, alongside common upsized alternatives for comparison.

*Note: 1/0 AL is rated 120A, which is the next standard breaker size up for specific motor/continuous load exceptions, but for a standard 125A residential feeder, you must use 1 AWG AL (135A).
**Note: NEC 310.12 allows 2/0 Cu or 4/0 Al for 200A residential dwelling services, but standard Table 310.16 ampacity for 2/0 Cu is 175A.

Why 3 AWG Copper? (And Why 4 AWG Fails)

A common mistake when reading a 100 amp sub panel wire diagram is assuming 4 AWG copper is sufficient. After all, 4 AWG THHN wire is rated for 95A in the 90°C column of Table 310.16. However, you cannot use the 90°C column for sizing your breaker and lugs.

Under NEC 110.14(C), the ampacity of a circuit is limited by the lowest temperature rating of any connected termination, conductor, or device. While your THHN wire insulation can withstand 90°C, the lugs inside standard residential breakers (like the Square D HOM1100 or Eaton BR2100) and the bus bars in your subpanel are almost universally rated for 75°C.

When you look at the 75°C column for copper, 4 AWG drops to an ampacity of 85A. Because 85A is not a standard breaker size listed in NEC 240.6, and because your load calculation demands a full 100A feed, 4 AWG is illegal and unsafe for a 100A breaker. You must step up to 3 AWG copper, which is rated exactly 100A in the 75°C column.

Variables That Change Your Wire Size

The 3 AWG copper / 1/0 AWG aluminum baseline holds true for short runs in standard conditions. However, real-world jobsite conditions frequently force you to upsize. Here is the decision tree for when to abandon the baseline sizes.

The Voltage Drop Check

The NEC recommends (Informational Note to 210.19) that voltage drop should not exceed 3% for branch circuits and feeders to ensure efficient operation. Let us run the math for a 100-foot and a 200-foot run using 3 AWG copper at an 80A continuous load (the realistic maximum continuous draw on a 100A breaker).

Formula: VD = (2 × K × I × D) / CM
K (Copper) = 12.9 | I (Current) = 80A | CM (Circular Mills for 3 AWG) = 52,620

• At 100 feet: VD = (2 × 12.9 × 80 × 100) / 52,620 = 3.92V. (3.92V / 240V = 1.63%). This is well within the 3% limit. 3 AWG is perfectly safe.
• At 200 feet: VD = (2 × 12.9 × 80 × 200) / 52,620 = 7.84V. (7.84V / 240V = 3.26%). This exceeds the 3% recommendation. For a 200-foot run, you must upsize to 2 AWG copper (CM = 66,360) to drop the VD down to 2.48%.

For exact calculations on your specific site, use the Southwire Voltage Drop Calculator to account for exact conduit types and AC reactance.

Physical Installation Realities

Pulling 3 AWG THHN through conduit requires physical effort and specific techniques. Unlike 12 AWG or 10 AWG Romex, 3 AWG wire is incredibly stiff. If you are pulling through 1.5-inch or 2-inch PVC with multiple sweeps, use a high-quality wire pulling lubricant (like Polywater J) to prevent damaging the insulation.

When terminating the wires at the breaker and the subpanel lugs, do not guess the tightness. Use a calibrated torque screwdriver or torque wrench set to the exact inch-pound specification printed on the breaker label (typically between 40 and 50 in-lbs for 3 AWG in standard 100A lugs). Loose connections on high-amperage feeders cause arcing, heat buildup, and eventual fires.

When to Call an Engineer or the AHJ

While the 100 amp sub panel wire diagram and sizing rules outlined here cover 95% of residential and light-commercial garage or workshop upgrades, there are scenarios where you must defer to a licensed Professional Engineer (PE) or your local Authority Having Jurisdiction (AHJ).

First, if your calculated continuous load approaches 80A and includes large motor loads (like a 5HP air compressor or a commercial welder), you must account for motor starting inrush currents and NEC Article 430 voltage drop requirements. A PE can model the transient voltage dip to ensure your sensitive electronics do not brownout when the compressor kicks on.

Second, local amendments frequently override the baseline NEC. Some municipalities in the US mandate that all residential feeders must be sized for a maximum 2% voltage drop rather than the NEC's 3% recommendation. Others may strictly prohibit aluminum SER cable for indoor subpanel feeds due to historical oxidation concerns, forcing you to use copper regardless of cost.

Finally, if your conduit run passes through high-heat environments—such as running directly above a wood-fired boiler or through an uninsulated attic space in a desert climate where ambient temperatures routinely exceed 110°F (43°C)—the temperature correction factors in Table 310.15(B)(1) will severely derate your wire. In a 50°C ambient attic, 3 AWG copper at 75°C is derated by a factor of 0.75, dropping its ampacity to just 75A. In this scenario, you would need to upsize to 1 AWG copper just to maintain your 100A rating. Always verify ambient conditions and consult your local electrical inspector before purchasing your wire.