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Selecting the proper wire size for circuits controlled by a 100 amp breaker is critical for safety and optimal electrical system performance. Factors like wire material, insulation type, and installation method all impact the wiring decisions. This article provides a comprehensive guide to choosing the right gauge wire for 100 amp breaker circuits.

Wire Size Selection for 100 Amp Breakers

Wire Size for 100 Amp Breaker

The National Electrical Code (NEC) minimum wiring size for a 100 amp breaker is #3 AWG copper or #1 AWG aluminum. This wire gauge is sized to safely handle the 100A current and prevent overload conditions from occurring. Smaller wires may overheat, melt or cause voltage drops. Oversizing beyond the code minimum is unnecessary and increases expense.

THHN Wire for 100 Amp Breakers

A common wire insulation type used with 100 amp breakers is THHN wire. This high heat-resistant thermoplastic wiring stands up to the elevated temperatures that can occur in a 100A circuit. THHN wire size for 100 amps would be the standard #3 AWG copper softened with THHN insulation. This allows it to be safely run through conduit.

Copper Wire Sizes

For a 100A breaker, the NEC recommends #3 AWG copper wire. This gauge handles 100A for installations including wiring in:

Conduit – #3 THHN wire
Underground – #3 USE-2 wire
Open air – #3 MTW wire
Optional larger sizes like #2 or #1 AWG can be used for less voltage drop.

Aluminum Wire Sizes

The code allows aluminum wires for 100A breakers following the NEC one size rule. This requires #1 AWG aluminum wire to match the capacity of #3 copper. Common insulated aluminum conductors would be #1 XHHW-2 or #1 RHH/RHW-2 for wet locations. Oversize aluminum to #1/0 AWG for better performance.

Underground Wiring

Underground wiring to a 100A breaker should use the proper underground rated wire like USE-2 or UF. For copper wire, the recommended size is #3 AWG based on the 75°C ampacity. For aluminum, increase to #1/0 AWG underground. A larger size helps compensate for heat buildup in the buried cable.

Conduit Fill Considerations

Running #3 AWG THHN wires through conduit needs to consider conduit fill requirements. Typically, a 1″ conduit can fit three #3 THHN wires for a 100A 240V circuit. For code compliance, calculate the conduit fill percentages and adjust conduit sizes as needed.

Wire Insulation Types

In addition to THHN, some other common insulations used with #3 copper wire for 100A breakers include:

XHHW-2 – rated for wet locations
THWN-2 – can be used for wet or dry locations
RHH/RHW-2 – flame retardant insulation
Look for the correct temperature, wet location, and other ratings needed for the installation.

Correct Lug Terminators

Use properly rated lugs to terminate the wires at the 100A breaker. Standard lugs may only be rated up to #4 AWG. For the larger #3 wires, look for 100A compatible mechanical lug connectors or compression lugs rated for the wire size.

Maintaining Ampacity

Size 100 amp breaker wires based on required ampacity and NEC guidelines to prevent undue heating. Ampacity denotes current carrying capacity. Correct wire sizes, insulation, and installation method all impact achieving proper ampacity for the 100A rating.

Minimizing Voltage Drop

For longer wire runs to a 100A breaker, consider upsizing wires one gauge to minimize voltage drop. Excessive drop can cause device problems. Larger wires reduce resistance and maintain voltage within acceptable levels at the outlet or equipment.


Choosing the optimal wire size and type for a 100 amp breaker involves numerous considerations for performance, protection, and code compliance. Following the #3 AWG copper recommendations ensures a properly wired 100A circuit. Careful attention should be given to insulation ratings, ampacity, voltage drop, conduit fill, and using properly rated connectors for a successful installation.

Published On: 2023年11月8日Categories: ,