How to Determine the Right Size Capacity of a Subpanel?

How To Determine The Right Size Capacity Of A Subpanel? . Determining the right size capacity of a subpanel is crucial for ensuring the safety and efficiency of your electrical system. Whether you are upgrading your home electrical system, adding a workshop, or setting up a new appliance-heavy area, understanding subpanel capacity prevents overloads, fire hazards, and compliance issues. This guide simplifies the process, from understanding subpanel basics to performing load calculations, and provides actionable insights for homeowners and professionals.

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What is a Subpanel?

A subpanel, also known as a secondary panel, is an extension of the main electrical panel. It distributes power to specific areas or circuits, offering better management and flexibility for large electrical systems.

Key Features of a Subpanel

• Distribution Control: Allows separate circuit management.
• Safety Enhancement: Reduces overload risks.
• Convenience: Offers localized control for specific zones.

Applications of Subpanels

• Garages or workshops.
• Home additions.
• High-demand areas like kitchens or media rooms.

Read More : Which is More Dangerous 50Hz or 60Hz in 120V/230V and Why?

Sizing the Suitable Capacity of a Subpanel or Main Lug

A subpanel or main lug (also known as a remote panel in the NEC) is a smaller service panel installed downstream from the main breaker box or load center in a home. Typically, a subpanel is installed in a separate section of a property, such as a barn, garage, workshop, or another part of a building for specific applications.

When wiring a subpanel or main lug, it is important to calculate the suitable size of the subpanel based on the specific load points. The rated capacity of the subpanel should not exceed the peak amount of power used by the electrical appliances and other connected load points, such as outlets and receptacles.

The subpanel is wired to the main panel through a feeder using two hot wires (Hot 1 and Hot 2), connected to a 240V two-pole breaker, and a ground wire. The neutral in the subpanel is only required when there are 120V circuits, GFCI/AFCI outlets, or breakers, in addition to 240V circuits. Note that bonding in the subpanel is not required because the ground and neutral are already bonded and grounded in the main service panel.

The purpose of both the main panel and the subpanel is the same: they distribute the power, control and protect the wired circuits against electrical faults, electric shocks, and surges. A subpanel is mainly used to provide electricity via additional circuit spaces and separate power control management and protection for the circuits.

Read More : How to Wire a Double 3-Way Combination Switch Device?

Understanding Subpanel Capacity

The capacity of a subpanel is determined by its amperage rating, which indicates how much electrical current it can safely handle. Common sizes include 50-amp, 100-amp, and 200-amp panels.

Main Panel vs. Subpanel

• Main Panel: Primary source of power for the entire property.
• Subpanel: Supplementary, serves specific areas or functions.

Factors Influencing Capacity

• Total electrical load requirements.
• Distance between the main panel and the subpanel.
• Future expansion needs.

Read More : How to Wire a 240V Simultaneous Water Heater Thermostat?

How to Size a Subpanel or Main Lug?

For load calculation, multiply continuous loads (lasting 3 or more hours, e.g., water heaters) by 125% per NEC 310-14 and add 100% of non-continuous loads (like light bulbs, TVs).

Total Load = 125% * Continuous Loads + 100% * Non-Continuous Loads

To account for the fact that all loads are unlikely to operate simultaneously, NEC 220.110 and Table 220.42 recommend applying a demand factor. The first 3 kVA is rated at 100%, while the remaining non-continuous load is rated at 35%.

For example, with a total load of 24,000 watts, the demand factor reduces it to 10.35 kW.

3 kW (100% * 24 kW) + 7.35 kW (35% * (24 kW – 3 kW)) = 10.35 kW

For more details, refer to NEC sections 220.110, 220.14, 220.42, 220.45, 220.53, 220.55, and 517.22.

Let’s follow the example below to determine the appropriate subpanel capacity and number of circuit spaces:

Example:

Suppose we need to install a subpanel for three circuits having total load of 8,540 watts for both 120V and 240V circuits with the following ratings:

• General Lighting = 120V, 1500 W
• 1 x Outlets : 120V  x 12A = 1440 W
• 1 x Electric heater: 240V x 23.3A = 5600 W

Read More : How to Wire a 120V Simultaneous Water Heater Thermostat?

Solution:

Typically, general lighting and smaller appliances are connected to 15A or 20A, 120V single-pole breakers. Larger appliances, such as air conditioners, water heaters, and electric ranges, are connected to 20A to 50A, 240V double-pole breakers.

Calculating the Wattage

To find the appropriate subpanel rating, we must first calculate the total wattage of the appliances that will be connected to the subpanel or main lug:

1500 W + 1440 W + 5600 W = 8540 W

Calculating the Amperes

Next, we calculate the amperage rating of the appliances. Since the voltage levels are 120V and 240V (in the US), we use the higher voltage level, 240V, for calculation:

I = P ÷ V

Where:

• I = Current (in amperes)
• P = Power (in watts)
• V = Voltage (in volts)

Substitute the values:

I = 8540 W ÷ 240V

I = 35.6A

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Applying a Safety Factor

To ensure proper and smooth operation of circuit breakers in high-temperature conditions, apply a safety factor of 125% (1.25) to the calculated amperage. The NEC suggests that the OCPD devices should be able to handle up to 125% of the load current. This complies with NEC 310.14, 705.12(B)(3), 210.19(A) 220.12-16 and 310.15.

125% × 35.5A = 44.48A

Future Expansion

To allow for future expansion and more breakers and circuits in the empty slots, it is recommended to add 20% to the calculated amperage:

20% × 44.48A = 53.38A

Based on this estimate, the next available and suitable size main switch breaker for the subpanel is 60A.

NEC 310.15(B)(16) suggests that a 6 AWG wire is the appropriate feeder size for a 60A subpanel. If the distance exceeds 140 feet (42 meters), use a 4 AWG wire instead of 6 AWG. For the ground wire, use #8 AWG.

Read More : What Are the Types of Switches – Construction and Working

Subpanel Space and Circuits

The example above for given load requires three breakers: two 1-pole, 120V breakers and one, 2-pole, 240V breaker, occupying four spaces.

Therefore, a minimum 4-space, 8-circuit main lug is necessary. For future expansion, a 6-space, 12-circuit subpanel is recommended for a 60A setup designed for an 8.5kW load.

Why Determining the Correct Subpanel Size is Important

Choosing the right subpanel size ensures the system’s longevity, safety, and compliance with local electrical codes. Improper sizing can lead to frequent tripping, overheating, or worse, electrical fires.

Consequences of Incorrect Sizing

• Overloaded circuits.
• Increased energy costs.
• Damage to appliances and wiring.

Read More : Types of Electrical Wires and Cables

Types of Subpanels and Their Capacities

50-Amp Subpanel

Ideal for light loads such as lighting circuits or small workshops.

• Applications: Outdoor lighting, tool sheds.
• Limitations: Insufficient for high-power appliances.

100-Amp Subpanel

Suitable for medium loads, including garages and small apartments.

• Applications: Home offices, kitchens.
• Benefits: Balances capacity and cost.

200-Amp Subpanel

Designed for heavy loads in large properties.

• Applications: Large workshops, multi-unit systems.
• Advantages: Supports high-demand appliances.

Read More : How to Connect a Portable Generator to the Home Supply?

Common Mistakes to Avoid When Sizing a Subpanel

1. Ignoring Future Needs

Underestimating load requirements can lead to costly upgrades.

2. Incorrect Wire Gauge

Using incompatible wiring can cause overheating and fire hazards.

3. Overloading Circuits

Exceeding the subpanel’s capacity leads to frequent breaker trips.

4. Skipping Professional Consultation

Always consult a licensed electrician to ensure compliance with local codes and safety standards.

Read More : How to Wire an AFCI Breaker?

FAQs

1. How do I calculate the load for a subpanel?

Use the formula: Amps = Total Wattage ÷ Voltage, and factor in future needs.

2. Can I install a subpanel myself?

While possible for experienced DIYers, hiring a professional is recommended for safety and compliance.

3. What is the largest capacity subpanel I can install?

The largest capacity depends on your main panel’s capability and local code regulations. Consult an electrician for guidance.

4. Is a 100-amp subpanel sufficient for a garage?

Yes, for most garages, a 100-amp subpanel is adequate for lighting and standard power tools.

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