What is the Right Wire Size for 90A Breaker and Load?

Electrical Technology

How to Determine the Correct Wire Size for 90A Breaker and Load Circuits Based on NEC?

A 90-amp breaker is commonly used for large electrical loads like heavy-duty appliances, industrial machines, and electric vehicle chargers. To ensure safety, efficiency, and NEC compliance, choosing the right wire size is crucial. This article will guide you through the key factors to consider when selecting wire for a 90A breaker and outlet, based on NEC standards.

According to the NEC, the recommended wire size for a 90-amp circuit is #2 AWG copper or 1/0 AWG aluminum. However, the final choice may vary due to factors like voltage drop, distance, temperature, and wire insulation type. For specific cases, consult a licensed electrician to ensure your installation is compliant with NEC and local regulations.

When selecting the right wire for a 90A breaker, consider the following factors which affect the wire size selection:

Voltage Drop: Over longer distances, voltage drop becomes a concern. For wire runs exceeding 50 feet, increase the wire size to reduce voltage drop and maintain efficient power delivery.
Temperature Rating: The wire’s ampacity (current-carrying capacity) depends on the temperature rating of the insulation. For instance:
- At 60°C (140°F), #2 AWG copper wire can carry 95 amps.
- At 75°C (167°F), it can handle 115 amps.
Wire Insulation Type: Common insulation types like THHN and THWN are heat- and water-resistant, offering better ampacity and performance under different conditions.

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Correct Wire Size for a 90A Circuit

Based on NEC Table 310.15(B)(16), the suitable wire sizes for a 90A breaker are:

#2 AWG copper
1/0 AWG aluminum

These sizes ensure safe operation of a 90-amp circuit, but ambient temperature and distance can influence the exact wire needed. Copper wire is often preferred due to its higher conductivity and durability compared to aluminum.

At different ambient temperatures, #2 gauge copper and #1/0 gauge aluminum wire types have the following ampacities:

#2 AWG Copper:

- 95 Amps at 60°C (140°F)
- 115 Amps at 75°C (167°F)
- 130 Amps at 90°C (194°F)

1/0 AWG Aluminum:

- 100 Amps at 60°C (140°F)
- 120 Amps at 75°C (167°F)
- 135 Amps at 90°C (194°F)

These values reflect the importance of considering temperature and material when choosing the correct wire for a 90A breaker.

Good to Know:

The recommended wire size for a 90A breaker is #2 AWG copper or 1/0 AWG aluminum.
A 90A 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.
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).
Ensure the wire’s temperature rating matches the ambient conditions to avoid overheating.

Selecting the Correct Wire Size for a 90A Breaker

Continuous Load Circuit

For continuous loads, NEC requires that the load not exceed 80% of the breaker’s capacity. For a 90A breaker:

90A × 80% = 72A

This means a 90-amp breaker can handle up to 72 amps continuously.

If your equipment has a Minimum Circuit Ampacity (MCA) of 72A, and you need to determine the right size of breaker for this 72-Amp continuous load circuit, the breaker should handle 125% of the load current amperes.

72A × 125% = 90A

Thus, for continuous loads, use #2 AWG copper or 1/0 AWG aluminum wire to carry up to 72 amps safely.

Non-Continuous Load Circuit

For non-continuous loads, the breaker can handle the full 90 amps without derating. Both #2 AWG copper and 1/0 AWG aluminum wires are sufficient for such loads.

Good to Know: Always factor in voltage drop and ambient temperature when choosing wire size for any load.

How Many Amps Can a 90A Breaker Safely Handle?

According to NEC guidelines, a 90A breaker can safely handle:

72A for continuous loads (90A × 80% = 72A)
90A for non-continuous loads

These values align with NEC sections 210.19(A) and 215.2.

Good to know:

Use a 90A breaker for 72A continuous load circuits or maximum 90A 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 90A. (NEC 625.23 – 2011).

How Many Watts Can a 90A Breaker Hold?

To determine the wattage for a 90-amp breaker, use the formula:

Watts = Volts × Amps

120V Circuit:

A 1-P, 90A breaker on a 120V circuit can hold:

- Maximum Watts = 90A × 120V = 10,800W
- Continuous load = 72A × 80% = 72A
- Safe Wattage = 72A × 120V = 8,640W

240V Circuit:

A 2-P, 80A breaker in a 240V circuit holds:

Maximum Watts = 90A × 240V = 21,600W
Continuous load = 72A × 80% = 72A
Safe Wattage = 72A × 240V = 17,280W

Based on the above calculations, you can safely connect up to 17,280 watts of load on a two pole, 80A breaker in a 240V circuit. This way, you may use a single load up to 90A or multiple load circuits in the subpanel but the total load amperes should not exceed 90A.

Good to Know:

A 90-amp breaker can handle a safe load of 8,640 watts in a 120V circuit and 17,280 watts in a 240V circuit.
Similarly, a 90-amp breaker can be used for maximum 10,800 watts in a 120V circuit and 21,600 watts in a 240V circuit.

Suitable Cable Types for a 90A Breaker:

For a 90A circuit, several types of cables can be used depending on the application and the environmental conditions. The cables must meet the ampacity requirements and comply with the National Electrical Code (NEC) standards. Here’s a breakdown of the different types of cables commonly used:

THHN (Thermoplastic High Heat-Resistant Nylon-Coated) Cable

THHN is a versatile and widely used wire type that is rated for high temperatures, up to 90°C in dry locations. They are Lightweight, heat-resistant, and easy to pull through conduits. THHN cables are commonly used in residential, commercial, and industrial wiring, primarily for indoor installations.

THWN (Thermoplastic Heat and Water-Resistant Nylon-Coated) Cable

THWN is similar to THHN but designed for use in wet locations. They are Water-resistant, ideal for damp or wet conditions and commonly used Often used in outdoor or underground conduits where moisture may be present.

XHHW (Cross-Linked Polyethylene High Heat Water-Resistant) Cable

XHHW is another cable type rated for high temperatures and wet conditions, but with a more durable insulation material. They have excellent resistance to abrasion, heat, and moisture, making it ideal for harsh environments and Suitable for use in conduits, both indoor and outdoor, including underground applications.

USE-2 (Underground Service Entrance) Cable

USE-2 cable is designed for direct burial and underground service entrance applications. They are specifically designed for underground use, offering good water resistance and mechanical durability. USE-2 cables are used mainly for underground feeders and other buried installations.

SER (Service Entrance Rated) Cable

SER cable is commonly used for subpanels and is suitable for residential service entrances. They are flexible, easy to install, and available in multi-conductor configurations. SER cables are Ideal for indoor installations where the cable runs through walls and floors.

MC (Metal Clad) Cable

MC cable features a metallic armor for protection against physical damage. They offers superior mechanical protection, making it suitable for exposed installations. MC cables are used in areas where extra protection is needed, such as in industrial environments or where the cable might be exposed.

RHH/RHW-2 (Rubber High Heat Resistant / Rubber High Water Resistant) Cable

RHH/RHW-2 cable is designed for high-heat and wet locations, making it ideal for more extreme environments. These types of cables have high durability and resistance to both extreme temperatures and moisture. They can be used in both dry and wet locations, including industrial and outdoor applications.

Applications of a 90A Breaker:

A 90A breaker is used in circuits that require higher current capacities, often for heavy-duty residential, commercial, or industrial applications. Below are the typical applications of a 90A breaker:

Main and subpanels for large homes or buildings
Electric furnaces and heat pumps
Large HVAC Systems (Heating, Ventilation, and Air Conditioning)
Electric Vehicle (EV) charging stations
Hot tubs and spas
Industrial equipment and machinery
Heavy duty machinery, compressors, pumps and large motors
Commercial kitchen equipment
Welding machines
Solar inverter systems
Agricultural equipment
Backup power systems (generators)
Commercial freezers or refrigeration units

Good to Know:

A 90A circuit needs #2 AWG copper wire or #1/0 AWG aluminum wire, according to NEC Table 310.15(B)(16) (formerly 310.16) and section 210.24.
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 90A breaker can support a 72A continuous load and 90A 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 90A breaker.
When installing a 90A circuit, dedicate it to a single appliance with a maximum continuous load of 72A. Drawing more than 72A 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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