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What is the Correct Wire Size for 25A Breaker and Load?

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How to Find the Correct Wire Size for a 25A Breaker and Circuit Based on NEC?

25A circuits are less common compared to 15A, 20A, or 30A circuits. However, if you encounter a situation requiring a 25A circuit, this guide will help clarify how to select the correct wire size for a 25A breaker and outlet. Choosing the right wire size is crucial for the safety and protection of both the device and operator, as well as ensuring compliance with NEC standards.

Typically, a 10 AWG wire is the appropriate choice for a 25A circuit. However, factors such as distance, ambient temperature, and the installation environment must also be considered. Understanding the load capacity and the proper applications of a 25A breaker is essential for safe and efficient electrical installations, helping to prevent potential hazards. Always adhere to National Electrical Code (NEC) – NFPA 70 guidelines, and consult a professional electrician for complex installations.

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Suitable Wire Size for a 25A Breaker

According to the National Electrical Code (NEC) 2020-2023, the recommended wire size for a 25A breaker is #10 AWG (American Wire Gauge) both for copper and aluminum.

The #10 AWG copper and aluminum wires has the following ampere capacity (ampacity) at different ambient temperature based on NEC Table 310.15(B)(16) (formerly Table 310.16).

#10 AWG Copper

30 amps at 60°C (140°F)
35 amps at 75°C (167°F)
40 amps at 90°C (194°F)

#10 AWG Aluminum

25 amps at 60°C (140°F)
30 amps at 75°C (167°F)
35 amps at 90°C (194°F)

This is based on the NEC’s ampacity charts, which consider the maximum current-carrying capacity of a wire under normal conditions. Using a 10 AWG wire ensures that the conductor can safely handle the required amount of load current without overheating. Using the wrong or smaller gauge sizes than recommended gauge could lead to potential fire hazards, electric shock and damage the overall circuit.

Good to know:

For a 25A circuit breaker, the correct wire size is #10 AWG for both copper and aluminum.
Use 10-2 with ground for 120V, 25-Amp breaker and load circuits.
Use 10-3 with ground for 240V, 25-Amp breaker and load circuits.
Longer runs (when the distance is more than 50 ft (15.25 meters) require an upgrade and larger wire gauge size to compensate for voltage drop.
According to the NEC – 310-16, add 20% of additional ampacity for every 100 feet (30.50 meters) of distance (for example between main panel and subpanel) to counter the voltage drop.

Right Wire Size for a 25A Breaker and Load

The following example will elaborate how to find the correct wire size to use with a 25A breaker and a general purpose outlet for both continuous and non-continuous load circuits.

Example:

Continuous Load Circuit

The rule of thumb or 125% rule based on NEC shows that the rating of the maximum overcurrent protection (MOCP) of Overcurrent Protection Device (OCPD) i.e. fuse or breaker should handle 125% of the continuous load. As a safety factor, only 80% of the continuous load should be connected to the

(Overcurrent Protection Device i.e. fuse or breaker).

25A × 80% = 20A

Based on the calculation, Do not connect more than a 20A continuous load circuit (e.g., a 20A outlet or water heater) to a 25A breaker, based on the calculation.

When the minimum circuit ampacity (MCA) i.e. load circuit’s amperes are 204A, to find the right breaker size for this circuit:

20A × 125% = 25A

According to NEC Table 310.15(B)(16), the 10 AWG wire size can carry 30A at 60°C (140°F) and 35A at 75°C (167°F).

Non-continuous Load Circuit

For non-continuous loads (e.g., lighting circuits), the conductor size should be no less than 100%. The same applies to overcurrent protection devices (OCPDs).

Hence, a 25A breaker can be used for a maximum of 30A circuits (e.g., outlets and lighting points) while considering the ambient temperature rating [Refer to 110.14(C) and 310.15(B)(2)].

Amps Capacity of 25A Breaker

A 25A breaker is designed to handle up to 25 amperes safely. Breakers are engineered to trip and disconnect the circuit when the current exceeds the rated capacity, preventing potential overheating and fire hazards.

However, it’s important to note that the continuous load on a breaker should not exceed 80% of its rating.

25A × 80% = 20A

This means that for continuous loads, the safe operating current should not exceed 20A for a 25A breaker.

Use a 25A breaker for a 20A continuous load circuit.
Use a 25A breaker for a 25A non-continuous load circuit.

The above ratings of the breaker, and the ampacity of the wire size, comply with the National Electrical Code (NEC) – Sections 210.19(A), 215.2, and 230.42(A) for continuous and non-continuous loads, and 110.14(C) for ambient temperature.

The above calculations are based on NEC Table 310.15(B)(16) and 240.4(A) through (G), illustrating that the 10 AWG copper wire size can carry 30A at 60°C (140°F) and 35A at 75°C (167°F).

Watts Handling Capacity of 25-Amp Breaker

The wattage capacity of a breaker is determined by multiplying the current rating by the voltage of the circuit.

120V Circuit

For a standard 120V circuit, a 25A breaker can handle:

With an 80% of safety factor

The safe limit of wattage

It shows that no more than 2,400 watts of load should be connected to a 25A breaker. For example, you may use 25A breaker with 2kW water heater element in 120V circuits using.

240V Circuit

For a 240V circuit, the calculation is:

Multiplying the safety factor

The safe limit of wattage

For example, you may use 25A breaker with 4kW – 4.5kW water heater element in 240V circuits using #14 gauge wire.

Based on the above calculation, no more than 4,800 watts of load having 20A of current should be connected to a 25A breaker. The safe limit of maximum wattage to a 25A breaker is:

120V Circuit (Continuous Load): W
240V Circuit (Continuous Load):

These calculations assume a resistive load, such as heaters or incandescent lighting. For inductive loads like motors, the inrush current and power factor must be considered.

How Many Outlets Can be Installed on a 25A Breaker?

The NEC does not specify an exact number of outlets for a given breaker size, but it does provide guidelines based on load calculations.

As a general rule, it’s advisable to install up to 10 outlets on a 25A circuit, assuming each outlet is intended for general-purpose use and not high-draw appliances. Each outlet is assumed to draw 1.5A (180W at 120V), which allows for safe operation within the breaker’s capacity.

For more details, refer to the NEC table – 210.21(B)(3) for receptacles rating for various size circuits which clarify the circuit rating and receptacle rating in amperes.

What Happens if a 14 AWG Wire is Connected to a 25A Breaker?

Connecting a 14 AWG wire to a 25A breaker is a violation of the NEC and poses significant safety risks. A 14 AWG wire is only rated for 15A, and using it with a 25A breaker could result in the wire overheating, as it cannot safely carry the higher current. This could lead to insulation breakdown, electrical fires, and damage to connected equipment. Always ensure that the wire gauge matches the breaker rating to avoid such hazards.

Suitable Types of Cables for a 25A Breaker

For a 25A breaker and circuit, the most suitable types of cables include:

NM-B (Non-Metallic Sheathed Cable): Commonly used in residential wiring, NM-B cables are available in 10 AWG and are suitable for dry, indoor locations.
THHN/THWN (Thermoplastic High Heat-resistant Nylon-coated/Water-resistant): These cables are commonly used in conduit and can handle higher temperatures, making them suitable for a 25A circuit.
MC (Metal-Clad) Cable: This type of cable is often used in commercial and industrial settings and provides added protection with its metal sheath.

Applications of a 25A Breaker

A 25A breakers are typically used for mid-range appliances that require more power than standard outlets provide. Common applications include:

Electric Water Heaters: Most residential electric water heaters fall within the 20A to 30A range.
Window Air Conditioners: Larger units that demand more current may require a dedicated 25A circuit.
Commercial Lighting: In some commercial settings, 25A circuits may be used for lighting systems that draw more power than typical residential lighting.

Good to Know:

The ampere rating of a single receptacle installed on an individual branch circuit should not exceed that of the branch circuit.
The above calculations for breaker and wire sizes are only applicable to purely resistive load circuits, such as lighting.
For inductive loads, such as air conditioner compressors and fan motors in HVAC systems, refer to NEC Article 440, particularly sections 440.22 and 440.32 (2017). If not sure, consult an HVACR contractor, instructor, and electrician.
A 30A branch circuit needs #10 AWG wire size for copper – NEC Table 210.24.(1).
A 30A outlet can be used for a 24A (continuous) load and a maximum 20A (non-continuous) load (210.19(A)).
It is against the code to use a 30A outlet to draw 30A on a 25A breaker.
It is against the code to use smaller gauge wire sizes (e.g., using 12 AWG) instead of the recommended wire size, which is #10 AWG with a 30A breaker and outlet.
According to NEC 210.21(B)(3), it is permissible to use a 20A outlet on a 30A circuit if there are multiple receptacles on the circuit.
Important Note: If you install a 30A outlet on a 30A circuit, it should only be used for a maximum of a 30A load. Drawing more than 30A from a 30A outlet will overheat the circuit, potentially causing serious injury and fire hazards.

What is the Correct Wire Size for 25A Breaker and Load?