Sub Panel Sizes: How to Size a Subpanel and Its Feeder

Sizing a subpanel is three decisions in a fixed order, and doing them out of order is how people end up with a feeder that doesn't match the breaker or a subpanel their service can't support.

1. What's the calculated load?
2. What feeder breaker does that load require?
3. What conductors does that breaker require?

Plus one question that comes before all three: can the main panel spare it?

First: can your service support it?

This is the question a subpanel doesn't answer.

A subpanel adds circuit positions, not capacity. Every amp it delivers still flows through the feeder breaker in the main panel, and through the main breaker, and through the service. If your calculated demand is already close to your service rating, a subpanel changes nothing.

Run a whole-house load calculation including whatever new loads the subpanel will serve. Use the home electrical load calculator, and see how to do a home electrical load calculation for the method.

Two possible outcomes:

• Service has headroom → a subpanel solves your slot problem
• Service is near capacity → you need a service upgrade, or load management, before adding load. See Electrical panel upgrade and service upgrade vs panel replacement

You also need two adjacent free positions in the main panel for a double-pole feeder breaker, and the panel's busbar must have capacity for it. See can I add a breaker to my panel.

Step 1: Calculate the load

Add up what the subpanel will actually supply, using the NEC's calculation method — not the sum of nameplate ratings, which overstates real demand because the code applies demand factors reflecting that not everything runs at once.

Typical inputs:

• General lighting and receptacle load, based on the area served
• Small-appliance and laundry circuits, where applicable
• Fixed appliances with their demand factors
• Motor loads, with the largest one factored per the rules
• Continuous loads at 125% — this is the one people forget

Continuous loads are those expected to run for three hours or more, and they must be sized at 125% of the running current. In a residential subpanel that typically means EV charging — see the Level 2 charger breaker size calculator — and sometimes electric heat. See dedicated circuit requirements.

Step 2: Choose the feeder breaker

The feeder overcurrent device in the main panel goes at or above the calculated demand, at a standard breaker size.

Common residential subpanels: 60 A and 100 A. Smaller (30–50 A) suits a modest workshop or a few circuits; larger suits an ADU, a heavily loaded garage, or a subpanel destined to carry EV charging plus shop equipment.

Build in headroom. A subpanel is disruptive to resize later, and loads only ever grow — EV charging, a heat pump, a shop tool, a hot tub. If the incremental cost between 60 A and 100 A is small and the service can support it, take the larger one. That said, don't oversize past what the service can genuinely deliver; that's just moving the constraint.

Step 3: Size the conductors

The rule: the feeder conductors must have an ampacity that supports the feeder overcurrent device protecting them. That sounds simple and isn't, because ampacity is not a single number.

What determines it:

This is why there's no single correct "wire size for a 100 amp subpanel." It comes from the NEC ampacity tables applied to your specific installation, and it's exactly the kind of question where a rule of thumb from the internet produces an unsafe answer.

Use the wire and breaker size reference for the general relationship, and have the actual feeder sized by the electrician doing the work against your adopted code edition.

Note also: feeders to a subpanel use four conductors — two ungrounded, a neutral, and an equipment grounding conductor, kept separate throughout. See subpanel grounding and neutral separation.

The equipment grounding conductor is sized from its own table based on the feeder overcurrent device, not the same size as the ungrounded conductors.

Voltage drop on long runs

Ampacity keeps conductors from overheating. Voltage drop is a separate concern about performance, and it becomes significant on long feeders — a detached garage, a shop at the end of the property, a barn.

Excessive voltage drop causes motors to run hot and start poorly, lighting to dim, and heating elements to underperform. The NEC treats voltage drop as a recommendation rather than a hard requirement for most feeders, but it's a real design consideration and the fix is upsizing the conductors.

Rule of thumb: if the run is long — think tens of metres rather than a few — voltage drop probably drives the conductor size rather than ampacity. Tell your electrician the run length; it changes the answer.

See feeding a detached building for the other requirements that come with a separate structure.

Choosing the panel itself

Beyond the feeder:

• Busbar rating must be at least the feeder overcurrent device. It can be higher — a 100 A-rated load center fed by a 60 A breaker is normal and fine. It must never be lower.
• Circuit positions. Count the branch circuits you're installing, then add spares. Panels are cheap; adding positions later isn't. See how many circuits can a panel hold.
• Main breaker or main lug. A subpanel in the same building often uses main lugs, since the feeder breaker upstream provides the disconnect. A subpanel in a separate structure requires a disconnecting means at that building. See main breaker vs main lug.
• Breaker compatibility. Only breakers listed for that panel may be used — see breaker brand compatibility.
• Working clearance. The subpanel needs the same clear working space as any panel. Plan the location around it, not the other way round. See electrical panel working clearance requirements and where electrical panels cannot be installed.

Permits and who does the work

Installing a subpanel means landing a feeder breaker in an energized main panel, where the main lugs remain live behind the deadfront even with the main breaker off. It's permitted and inspected work in essentially every jurisdiction.

This is licensed-electrician territory in nearly all cases. See electrical work homeowners can legally do.

The bottom line

Check the service first — a subpanel adds slots, not amps, so a whole-house load calculation decides whether it solves your problem at all. Then size in order: calculated load, feeder breaker at or above it, conductors from the NEC ampacity tables for your specific installation. Don't take a wire size from a rule of thumb; too many variables feed it. Build in headroom on the feeder, size the panel's busbar at or above the feeder breaker, and factor voltage drop on any long run.

Run the numbers with the home electrical load calculator.