Subpanel Size Calculator
Enter the floor area for lighting, and the heating, cooling, EV charger and other loads in watts, plus the number of circuits. The calculator applies a demand factor and gives the panel size and feeder.
- Subpanel size
- Feeder wire
- Spaces
Enter each load in watts from its nameplate.
Amps = (first 10 kW + 40% of the rest) ÷ 240 V × 1.25. A garage with 600 sq ft of lighting, a 5 kW heater, a 7.7 kW EV charger and 3 kW of tools totals 17.5 kW. That calculates to about 68 A, so a 100 A subpanel with 3 AWG copper.
Subpanel size chart
Copper feeder at 75 °C. Search for a size.
| Subpanel | Copper | Aluminium |
|---|---|---|
| 60 A | 6 AWG | 4 AWG |
| 100 A | 3 AWG | 1 AWG |
| 125 A | 1 AWG | 2/0 AWG |
| 150 A | 1/0 AWG | 3/0 AWG |
| 200 A | 3/0 AWG | 250 kcmil |
Amps.
Load and subpanel
Demand factor applied. Slide the load.
Breakers fill the panel.
How to size a subpanel
Loads to panel. Step through it.
Loads
List them.
Check the main panel has room for the feeder breaker.
60, 100 or 200 amp
Subpanel sizes. Compare them.
Small.
Common.
Large.
A 100 A subpanel covers most garages and workshops.
Before you wire
Subpanel checks. Tick each item.
Tick each item as you go.
Get a permit and inspection for subpanel work.
What you get
Subpanel sizing. Each tile explains one result.
Panel.
Wire.
Count.
Calculated.
In main panel.
Keep neutral and ground bars separate in a subpanel.
Frequently Asked Questions
No question matches that search.
What size subpanel do I need for a garage?
Usually sixty to one hundred amps, more with an EV charger.
What wire size for a one hundred amp subpanel?
About three AWG copper or one AWG aluminium.
Can I put a subpanel on a one hundred amp service?
Only if the load calculation shows the main service has room.
Does a subpanel need a ground rod?
A detached building usually needs one.
Why are neutral and ground separate in a subpanel?
So neutral current does not flow on the ground wires.
How many spaces should a subpanel have?
Your circuits plus about a quarter more for later.
Power where you need it
Size the subpanel for its loads and the garage or shop has power to spare.
More size calculators
Each one has its own calculator, worked examples and answers to common questions.