What is the Correct Wire Size for 100A Breaker and Load?

Electrical Technology

How to Determine the Right Wire Size for 100A Breaker and Load Circuits Based on NEC?

A 100-amp breaker is typically used for large electrical loads such as main panels, subpanels, heavy appliances, and high power circuits in both residential and commercial applications. Selecting the right wire size for this breaker is crucial to ensure safety, efficiency, and NEC compliance. This article explains how to choose the appropriate wire size for a 100A breaker, based on the National Electrical Code (NEC).

According to the NEC, the recommended wire size for a 100-amp circuit is #1 AWG copper or 1/0 AWG aluminum. The final choice depends on factors like voltage drop, distance, temperature, and wire insulation. Always consult a licensed electrician to verify that your installation complies with NEC and local codes.

Key Factors to Consider When Selecting Wire for a 100A Breaker

Several factors influence the wire size selection for a 100-amp breaker:

Types of Circuit: The required wire size for 100A service and 100A circuit would be different. For example, the wire size for 100A service / feeder for single phase dwelling unit is #4 AWG for copper and #2 AWG for aluminum – NEC – Table 310.12(A). On the other hand, the wire size for 100A load circuit is #1 AWG for copper and #1/0 for aluminum – NEC Table 310.15(B)(16).
Voltage Drop: Over long distances, voltage drop becomes an issue. For runs over 50 feet, increasing the wire size can help maintain power efficiency. Consider a larger gauge to minimize voltage drop.
Temperature Rating: Wire ampacity changes based on the insulation temperature rating. For example:
- At 60°C (140°F), #1 AWG copper wire can carry 110 amps.
- At 75°C (167°F), it can handle 130 amps.
Wire Insulation Type: The insulation type, such as THHN or THWN, affects performance. These types resist heat and moisture, making them suitable for different environmental conditions.

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Correct Wire Size for a 100A Circuit

Based on NEC Table 310.15(B)(16), the recommended wire sizes for a 100A breaker are:

#1 AWG copper
1/0 AWG aluminum

These sizes are designed to safely handle 100 amps. Copper is often preferred due to its superior conductivity and durability compared to aluminum. However, the decision should also factor in temperature, insulation type, and the length of the run.

At various ambient temperatures, the ampacity of #1 gauge copper and 1/0 gauge aluminum wires are as follows:

#1 AWG Copper:

110 Amps at 60°C (140°F)
130 Amps at 75°C (167°F)
145 Amps at 90°C (194°F)

1/0 AWG Aluminum:

100 Amps at 60°C (140°F)
125 Amps at 75°C (167°F)
132 Amps at 90°C (194°F)

These ampacity values illustrate the importance of considering both temperature and wire material when choosing the correct wire size for a 100A breaker.

Good to Know:

According NEC 310.15(B)(16), the recommended wire size for a 100A breaker and load circuit is #1 AWG copper or 1/0 AWG aluminum.
According to NEC Table 310.12(A), the recommended wire size for a 100A service is #4 AWG copper or #2 AWG aluminum.
A 100A breaker should be used as dedicated circuit for single high power appliance which needs high amp to operate (NEC 210.23). It may also used in subpanel to feed multiple load appliances.
The wire size for 100A breaker may be differ when use for subpanel or dedicated circuit.
For longer wire runs (over 50 feet or 15.25 meters), consider using larger wire to compensate for voltage drop. NEC suggests adding 20% more ampacity for every 100 feet (30.50 meters) – NEC – 310.16.
Ensure the wire’s temperature rating matches the ambient conditions to avoid overheating.

Selecting the Correct Wire Size for a 100A Breaker

A 100-amp breaker can be used in three different scenarios, each requiring different wire sizes, for example:

100A breaker in the main or subpanel
100A breaker for continuous load
100A breaker for non-continuous load

Wire Size for 100-Amp Breaker in Main or Subpanel

One of the main differences is that the wire size in AWG needed for a 100A breaker in a subpanel or main panel is typically smaller than for a 100A breaker dedicated to a specific circuit. This is due to the demand factor, which indicates that not all the circuits in the subpanel or main panel operate simultaneously. Therefore, less current flows through the cable connected to the 100A breaker.

NEC Tables 220.42 and 220.45 state that demand factors (diversity factors) for general lighting loads rate the first 3 kVA at 100% and the remaining load at 35%.

A dedicated circuit operates at full load current. Therefore, the breaker and wire size must be rated for the exact 100-amp current, especially for continuous load circuits that operate for three or more hours.

Related Post: What is the Right Wire Size for 60A Breaker and Outlet?

Example 1 – Based on Load Calculation:

A 100A circuit in 240V supply handles maximum of 24kVA. Since not all loads operate simultaneously (e.g., AC runs in summer, while heaters run in winter), the NEC allows a demand factor in 220.42 and 220.45. Let’s calculate the required load for a 100A subpanel designed for general lighting.

First 3kVA at 100% = 3kVA
Remaining 21kVA at 35% = 7.35kVA
Total Load = 3kVA + 7.35kVA = 10.35kVA

To calculate the current:

To account for an 80% safety factor:

Now, accounting for voltage drop over a distance of 100 feet:

For these amperes, the correct wire size for 120/240V main or subpanel service is #4 AWG Copper and #2 AWG Aluminum According to NEC Table 310.15(B)(16).

Related Post: What is the Correct Wire Size for 55A Breaker and Load?

Example 2 – Based on 83% Rule:

Another approach is to use Table 310.12(A) in NEC 310.12 which states that the ampacity of the feeder or service should not less than 83% of its rated capacity. It should be noted that the 83% rule for sizing cable for 100A to 400A is only applicable for the first feeder or service coming in to a dwelling when the service provides electrical supply to the whole dwelling unit.

This way

100A x 83% = 83 A

It clearly shows that the cable size for 100A main or subpanel is #4 AWG Copper and #2 AWG Aluminum According to NEC Table 310.12.

Good to Know:

This calculation are based on the NEC guidelines. For more details, refer to NEC 210.21, 210.24, 220.110, 220.14, 220.42, 220.45, 220.53, 220.55, 240.4, 310.12, 310.12, 310-14 and 517.22
Always consult with a licensed electrician for proper wiring installation to ensure compliance with local codes. It is also recommended to use copper wires with THHN or XHHW insulation for durability and performance in high-temperature environments.

Wire Size for 100A Breaker for Continuous Circuit

For continuous loads such as dedicated circuits, the NEC states that the load should not exceed 80% of the breaker’s capacity (NEC 210.20(A). For a 100A breaker:

100A × 80% = 80A

If a device has a Minimum Circuit Ampacity (MCA) of 80A, and you need to select the correct size of breaker for this 80-Amp continuous load circuit, the breaker should handle 125% of the rated load current amps.

80A × 125% = 100A

This means a 100-amp breaker can handle up to 80 amps of continuous load. In such cases, #1 AWG copper or 1/0 AWG aluminum wire is needed to handle up to 80 amps safely.

Related Post: What is the Suitable Wire Size for 50A Breaker and Outlet?

Wire Size for 100A Breaker for Non-Continuous Circuit

For non-continuous loads, the breaker can support the full 100 amps. Both #1 AWG copper and 1/0 AWG aluminum wires are sufficient for this purpose.

Good to Know:

Keep in mind the right wire size may be differ for dedicated circuit (continuous and non-continuous) and sub-panel.
A 100A deducted circuit breaker needs larger size than the 100A circuit breaker used in main or subpanel.
Always consider in voltage drop in the cables especially for long run circuits and ambient temperature when selecting the wire size for any load.

How Many Amps Can a 100A Breaker Safely Handle?

As per NEC standards, a 100A breaker can handle:

80A for continuous loads (100A × 80% = 80A)
100A for non-continuous loads

These limits align with NEC sections 210.19(A) and 215.2, ensuring safe operation of the breaker.

Good to know:

Use a 100A breaker for 80A continuous load circuits or maximum 100A non-continuous load circuits.
If the load circuit is greater than 60A, it will require a disconnect switch due to the absence of outlets and switches rated for 100A. (NEC 625.23 – 2011).

How Many Watts Can a 100A Breaker Hold?

To calculate the wattage capacity of a 100-amp breaker, use the formula:

Watts = Volts × Amps

120V Circuit:

A 1-P, 100A breaker on a 120V circuit can hold:

Maximum Watts = 100A × 120V = 12,000W
Continuous Load = 100A × 80% = 80A
Safe Wattage = 80A × 120V = 9,600W

For a 240V Circuit:

Maximum Watts = 100A × 240V = 24,000W
Continuous Load = 100A × 80% = 80A
Safe Wattage = 80A × 240V = 19,200W

This calculation shows that a 2-P, 100A breaker can safely support up to 19,200 watts on a 240V circuit for continuous loads. Hence, you may wire a single appliance up to 100-amps or multiple breakers and circuits in a subpanel but the total amps should not exceed the total 100-amp rated breaker.

Good to Know:

A 90-amp breaker can handle a safe load of 9,600 watts in a 120V circuit and 19,200 watts in a 240V circuit.
Similarly, a 90-amp breaker are suitable for maximum 12,000 watts in a 120V circuit and 24,000 watts in a 240V circuit.

Related Post: What is the Correct Wire Size for a 40A Breaker and Load?

Good to Know:

A 100A circuit needs #1 AWG copper wire or 1/0 AWG aluminum wire as per NEC Table 310.15(B)(16). When selecting wire, consider factors like voltage drop and ambient temperature. For continuous loads, limit the load to 80A to ensure safe and efficient operation.

Suitable Cable Types for a 100A Breaker

For a 100-amp breaker and circuit, various types of cables are used depending on the application, installation environment, and material (copper or aluminum). Below are some of the commonly used cable types:

THHN/THWN-2 (Thermoplastic High Heat-resistant Nylon-coated): THHN/THWN-2 are available in both material i.e. copper and aluminum. They are heat-resistant thermoplastic, suitable for wet and dry locations and typically used in conduit systems for residential and commercial wiring.

SE (Service Entrance) Cable: SE aluminum, SE-U (Ungrounded) and SE-R (Grounded) cables are generally sunlight-resistant and rated for wet locations. SE cables are used for connecting power from a utility transformer to the main breaker in homes or from main panel to the subpanel.

SER (Service Entrance Cable, Round): SER are Often aluminum, with three or four conductors (including ground). Commonly used for sub-panel feeds or large appliances. They are sunlight-restraint and suitable for wet or dry conditions. Service Entrance Cable are commonly used for sub-panel feeds or large appliances.

XHHW-2 (Cross-linked High Heat Water-resistant): XHHW-2 cables are available in copper or aluminum. Cross-linked polyethylene are suitable for wet, dry, or damp environments. They are commonly used in conduit systems or buried directly.

UF-B (Underground Feeder Cable): Under Ground Feeder Cable are mostly copper. They are designed for direct burial without needing conduit. UF-F are used in moisture-resistant and suitable for outdoor applications.

MC (Metal Clad) Cable: Metal cade cables are usually copper, but aluminum conductors are also available. They are in PVC jacket are suitable for indoor applications. Metal Clade cables, with metal armor for protection, are used for wiring in industrial and commercial buildings.

RHH/RHW-2 (Rubber High Heat/Water-resistant): RHH/RHW-2 are mostly copper. The rubber-based insulation make them highly heat and moisture-resistant. They are suitable for conduit systems and underground installations for both dry and wet conditions.

Direct-Burial Cable: These types of cables are available in both of copper or aluminum. They are specifically designed to withstand underground conditions, moisture-resistant. These cables are installed directly in the ground without conduit.

Applications of a 100A Breaker and Circuit:

A 100-amp breaker has a variety of applications, particularly in residential, commercial, and some industrial settings. Below are the common uses for a 100-amp breaker:

Main electrical service panes in home applications for power distribution
Sub-panels in residential homes.
Power distribution in small commercial buildings
Workshop or Garage Sub-Panels
Commercial applications like high power tools and equipment electric furnaces, and HVAC applications e.g. electric heat pumps and central Air-conditioning units
Solar Power Systems e.g. Photovoltaic (PV) and battery backup Systems
Temporary construction in construction sites as power distribution units (PDUs)
Heating Systems e.g. baseboard and electric water heaters
In backup generators – used in transfer switch for whole-house generators
Agricultural Settings i.e. barns, outbuildings and irrigation systems
RVs and mobile homes for RV hookups and temporary power stations for lighting and kitchen appliances.
Electric Vehicle charging i.e. High-power EV chargers and charging stations e.g. Level 2 EV chargers.

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Good to Know:

A 100A circuit needs #1 AWG copper wire or #1/0 AWG aluminum wire, according to NEC Table 310.15(B)(16) (formerly 310.16) and section 210.24.
A 100A service entrance in main or subpanel needs minimum #4 AWG copper wire or #2 AWG aluminum wire, according to NEC Table 310.12(A).
Ensure the ampere rating of the receptacle matches the branch circuit’s rating.
The breaker and wire size calculations are valid for resistive loads. For inductive loads (e.g., motors and HVAC systems), refer to NEC Article 440, especially sections 440.22 and 440.32.
A 100A breaker can support a 80A continuous load and 100A non-continuous load, following NEC sections 210.19(A), 215.2 230.42(A).
It’s against NEC code to use smaller wires (e.g., #6 AWG) with a 100A breaker.
When installing a 100A circuit, dedicate it to a single appliance with a maximum continuous load of 80A. Drawing more than 80A simultaneously will overheat the circuit and could cause serious injury or fire hazards.
Always consult a licensed electrician to ensure your installation complies with NEC standards.

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