Electrical Sub Panel Sizes: What Does Your Business Need?

A sub panel is a small electrical panel that feeds off the main panel. It holds circuit breakers that are different from the main panel’s and it usually has 4 to 12 slots. Just like with the main panel, there are several electrical sub panel sizes, so if your business or commercial property needs this addition, then you have to know which size fits your needs best. 

There are many similarities between a main panel and a sub panel, they both have the same two hot bars, same neutral bus bars and ground ones. However, the sub panel does not have the main shut off breaker, which can discontinue electricity throughout your entire system. 

Another difference is that, since the sub panel is usually installed to service a certain area of your business, when talking about electrical sub panel sizes, it’s actually the load you need to pay attention to. Determining the safe electrical load for a sub panel requires several calculations. One can find more information on load calculations in Article 220 of the National Electrical Code.

Why Use Sub Panels?

There are two main reasons your electrician could recommend adding a sub panel to the electrical system of your business or commercial property. The first one is there’s no more room to install circuits on the main breaker panel. The second one is to have a control point near the area that the circuit serves. 

In the first case, if the sub panel is not meant to serve a certain area, such as when you expand your business or add extra office space, then it’s generally installed next to the main panel. Since its sole purpose is to have space for more circuit breakers, it simply sits next to the main and connects through a feeder cable from the main panel. 

If the sub panel’s purpose is to serve a dedicated area, then it will be installed as close to the area it serves as possible, and the feeder cable is run from the main panel to the sub panel. This is usually a 3-wire cable, with 3 insulated conductors, and a bare copper ground wire. 

Once the sub panel is installed and connected, it’s ready to take individual circuit breakers for additional circuits. This is great if you’re expanding your office space, adding new equipment or manufacturing lines, anything really that would require a dedicated circuit. Beware though, that the sub panel doesn’t bring additional power, it just helps manage the power from the main panel more efficiently. 

Electrical Sub Panel Sizes & Capacity

As mentioned above, when adding a sub panel to the electrical system of your business or commercial property, you need to take into account your power needs and whether or not the current system can support the load. 

To that extent, there are 2 capacity issues to consider:

• the sub panel’s amperage capacity;
• the extra demand created by the sub panel.

Whether it’s serving a different area or it just houses extra breakers for additional circuits, the sub panel must have the right amount of amperage capacity. The electrical sub panel sizes to consider will depend on the application. For instance, smaller equipment, light power tools or lounge area entertainment systems will be adequately served by a small sub panel, ranging from 30 to 60 Amps.

On the other hand, heavy duty equipment, lifts, or entire space expansions will require a bigger sub panel, in the 60 to 100 Amps range. So, the installation can be configured from 30 amps – 240 volts to 100 amps – 240 volts. 

Electrical Sub Panel Load Calculation

As said, there are several calculations required in order to determine the safe gross electrical load for a sub panel. If the sub panel is used to serve a certain area that’s away from the main panel, you’ll need to know the square footage of that area. You’ll also need to know the type of equipment or electrical devices you’ll be powering through the sub panel.

The advantage of load calculations is that not only will they help with the sizing for a sub panel, but also help with finding the correct amperage and the feeder cable size. Here are the three main calculations to take into account.

Load Calculations According to the NEC

When calculating the electrical sub panel sizes and loads, as well as breaker and feeder calculations, electricians must follow the local electrical code. Oftentimes, these local codes are in accordance with the National Electrical Code, or the NEC. This means that they follow the “long form” load calculations. 

The form takes into account the electrical usage, or the wattage, of the equipment, devices or appliances served by your sub panel and sums it up. The total is then divided by 240 and the result is the minimum required amperage capacity. 

Coverage Area Calculations 

As mentioned before, the area that the sub panel is supposed to serve bears great importance in finding the right size. You can start by adding up the total area of the space to which the sub panel will supply power. This is particularly helpful if the additional area is office space, as it is a good way to determine the load of general lighting and circuits for receptacles. The square footage is calculated by multiplying the length of the space by the width. If there are multiple rooms, add up the square footage of all of them to find the serviceable area. Then, multiply that toale by 3 (watts).  

Wattage Calculations 

The equipment, devices or appliances powered by the sub panel have a great deal of impact on load calculations. The type of room also comes into play here. For instance, if you’re adding an office kitchen, then you need to account for small appliances, such as microwave, coffee maker, etc, as well as fastened in place appliances, such as a dishwasher or food disposer. 

All of these require different wattage, and large equipment or a large number of devices will require specified wattage. Once you have the wattage figured out, multiply by 1 if you have less than  4 fixed devices, and by 0.75 if you have 4 or more. In the end, you might need to add 25% of the largest motor load to get the wattage total, in order to meet the extra load that these require when starting up.