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

Electrical Technology

How to Choose the Correct Wire Size for a 55A Breaker and Load Circuits Based on NEC?

Although, the 55-Amp breaker is not a standard size breaker mentioned in the NEC Table 240.6(A), but still they are commonly used in heavy-duty circuits to power high-demand electrical equipment such as UPS systems, large air conditioners, industrial machinery, electric furnaces, and other high-power devices.

These breakers are typically available in 2-pole configurations for 240V and 1-pole for special purpose applications in 120V systems. In this technical article, we will explain how to determine the correct wire size for a 55A breaker and load circuit in AWG for personal and circuit protection, following the guidelines outlined in the National Electrical Code (NEC) NFPA 70.

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

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What is the Right Wire Size for a 55A Breaker?

According to NEC Table 310.15(B)(16) (formerly Table 310.16), the standard wire size for a 55-ampere breaker is #6 AWG copper wire. If using aluminum conductors, #4 AWG aluminum wire is required due to aluminum’s lower conductivity and higher resistance compared to copper.

The ampacity ratings for #6 AWG copper and #4 AWG aluminum wires are:

60°C (140°F): 55A
75°C (167°F): 65A
90°C (194°F): 75A

For a 55A circuit, you should ideally use #6 AWG solid copper wire, as it ensures safety, better conductivity, and efficiency. Solid copper wire is preferred over stranded wire in residential and industrial applications due to its durability and stable electrical properties.

Good to Know:

For a 55A circuit breaker, use #6 AWG for copper conductors and #4 AWG for aluminum conductors.
A 55A breaker is typically used as a dedicated circuit for a single high-power device, such as water heater, furnace etc.
Use 14-2 with ground for 120V, 55-Amp breaker and load circuits.
Use 14-3 with ground for 240V, 55-Amp breaker and load point.
For a 240V, 55A breaker, you should use 6-3 with ground for proper installation.
Longer runs exceeding 50 feet (15.25 meters) may require a larger wire gauge to compensate for voltage drop.
According to NEC 310.15(B)(2)(a), add 20% of additional ampacity for every 100 feet (30.50 meters) of wire length to account for voltage drop.

How to Select the Right Wire Size for a 55A Breaker?

Let’s explore how to determine the correct wire size for a 55A breaker and load for both continuous and non-continuous circuits according to NEC guidelines.

Continuous Load Circuit

The NEC’s 125% rule dictates that the maximum overcurrent protection device (OCPD) should handle 125% of the continuous load. To ensure safety, no more than 80% of the continuous load should be connected to the OCPD.

55A × 80% = 44A

Based on this rule, the load circuit’s amperage should not exceed 44A. This means you may only connect a maximum continuous load of 44A to a 55A breaker and load circuits.

For instance, if an appliance has a Minimum Circuit Ampacity (MCA) of 44A, the correct breaker size would be 55A based on the 125% rule:

44A × 125% = 55A

According to NEC Table 310.15(B)(16), the #6 AWG copper wire can carry 55A at 60°C (140°F) and 65A at 75°C (167°F), which is suitable for a 55A breaker and branch circuits.

Non-continuous Load Circuit

For non-continuous loads, the conductor size should be no less than 100% of the load, which also applies to OCPDs.

A 55A breaker can safely handle up to 55A of non-continuous load circuits. However, you must consider factors like distance and ambient temperature ratings as per NEC 110.14(C) and 310.15(B)(2)(a).

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

How Many Amps Can a 55A Breaker Handle Safely?

A 55A breaker handles a maximum of 55 amps of current. According to the NEC, you should not load breakers to more than 80% of their rated capacity for continuous loads (defined as a load lasting three hours or more). Therefore, a 55A breaker is suitable to use for a maximum of 44A continuous load circuits.

55A × 80% = 44A

For non-continuous loads, the breaker can handle up to 55 amps safely.

Use a 55A breaker for a 44A continuous load circuit.
Use a 55A breaker for a 55A non-continuous load circuit.

These ratings comply with NEC Sections 210.19(A), 215.2, and 230.42(A) for continuous and non-continuous loads, and 110.14(C) for ambient temperature considerations.

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How Many Watts Can a 55A Breaker Hold?

To calculate the power capacity of a 55A breaker and circuit, use the following formula:

Watts = Volts × Amps

120V Circuit

In a 1-Pole, 120V circuit, the max power a 55A breaker holds:

55 A × 120 V = 6,600 W

Applying the safety factor (80% load):

55 A × 80% = 44 A

This way, you may connect up to 5.2kW of resistive load circuit to a 55A breaker and load circuit.

55A × 120V = 5,280 watts

It allows to use 55A breaker with a 120V, 4.5kW to 4.8kW electric water heater elements or other resistive load circuits.

Good to know: 55A breakers are not for general use, but used for special purposes in an 120V circuit. The maximum rating of outlet in an 120V circuit is 50A i.e. NEAM-14-50R.

240V Circuit

In a 2-Pole, 240V circuit, the maximum power a 55A breaker can hold is:

55 A × 240 V = 13,200 W

Applying the 80% rule for continuous load:

55 A × 80% = 44 A

44A × 240V = 10,560 watts

Therefore, a 55A breaker can safely supply up to 10,560 watts of continuous load in a 240V circuit, such as for an electric furnace or industrial machine.

Related Post: What is the Right Wire Size for a 30A Breaker and Outlet?

How Many Outlets Can be Put on a 55A Breaker?

55A breakers are typically used for dedicated circuits and outlets for high-wattage appliances. It is not recommended or code-compliant to use a 55A breaker to power multiple smaller outlets. Instead, it should be dedicated to a single device.

Theoretically, you can connect a maximum of 44A continuous load to a 55A breaker. If each outlet draws 1.5A:

44 A ÷ 1.5 A ≈ 29 outlets

However, keep in mind that a 55A breaker should be used for a single high-power appliance or a dedicated circuit rather than multiple outlets.

What are the Suitable Types of Cables for a 55A Breaker?

When choosing cables for a 55A breaker and circuit, select a conductor that can handle the current without overheating. The correct cable size depends on the conductor material (copper or aluminum) and the insulation type.

Copper Conductors:

#6 AWG Copper Wire: This is the standard for 55A circuits, offering an excellent balance between flexibility and current-carrying capacity.
Insulation Types: THHN (Thermoplastic High Heat-resistant Nylon-coated) or THWN (Thermoplastic Heat and Water-resistant Nylon-coated) are recommended due to their durability and high-temperature rating.

Aluminum Conductors:

#4 AWG Aluminum Wire: Since aluminum has lower conductivity, a larger wire size is necessary for the same current.
Insulation Types: Use THHN or XHHW (Cross-linked High Heat Water-resistant) insulation for aluminum conductors.

Applications of 55-Amp Breakers

55-amp breakers are commonly used in applications that demand higher power. Some typical uses include:

Large Air Conditioners: These require significant power, making a 55A breaker suitable.
Electric Furnaces: High power electric furnaces often run on 55A circuits.
Industrial Machinery: Heavy-duty equipment in industrial settings may need 55A circuits for proper operation.

Good to Know:

The ampere rating of a single receptacle on an individual branch circuit should not exceed that of the branch circuit.
Calculations and wire sizes mentioned are for purely resistive loads like heating elements.
For inductive loads (e.g., motors), refer to NEC Article 440, particularly sections 440.22 and 440.32.
A 55A branch circuit requires #6 AWG copper or #4 AWG aluminum wire, as per NEC Table 310.15(B)(16) and 210.24.(1).
You may use a 55A breaker for a 44A continuous load and a maximum 55A non-continuous load, according to NEC sections 210.19(A), 215.2, and 230.42(A).
It is against code to use smaller gauge wire sizes, such as 10 or 8 AWG, with a 55A breaker and circuit.
According to NEC 210.21(B)(3), it is permissible to use a 44A or two 55A outlets on a 55A circuit if there are multiple receptacles on the circuit and only one operates at a time.

Important Note: When installing a 55A outlet on a 55A circuit, dedicate it to a single appliance or outlet with a maximum 44A load. Drawing more than 44A continuously from a 55A breaker or outlet can lead to overheating, serious injury, or fire hazards.