Will a 100 amp service run a workshop subpanel?
| Added demand14,400 VA at 240 V | 60A |
|---|---|
| Existing load must stay under | 40A |
| Share of the service this takes | 60% |
| Existing service | 100 A |
| What is being addeda 60 A feeder — the demand depends entirely on what goes in the shop | a workshop subpanel |
Notes
- Do not let anyone guess your existing load, including this page. NEC 220.87 gives the real method: take the highest demand recorded in the last 12 months from the utility, multiply by 1.25, and that is your existing load. Most utilities will supply the figure on request; where there is not a year of history, a recording ammeter left on the service for 30 days does the same job. That measured number, plus the 60 A above, is the answer.
- 60% of the service goes to this one thing. Whether it fits depends entirely on what the rest of the house already draws. Under 40 A of existing calculated load and it fits; over, and the service has to grow.
- This is not a continuous load, so it counts at its demand figure rather than 125%. Ranges and dryers get further relief in NEC 220.55 and 220.54 because they cycle: a 12 kW range is counted at 8 kW, since the elements are never all on at full for long.
- Load management can save the upgrade, and it is worth pricing first. An EV charger with a load-management device, or an energy-management system that sheds the charger when the range comes on, is a listed and code-recognised way to add a large load to a service that could not otherwise take it. It is frequently a fraction of the cost of a new service.
- A panel swap and a service upgrade are different jobs. If the panel is full but the service is adequate, a subpanel or a panel with more spaces solves it for far less. If the calculated load exceeds the service, new breakers change nothing — the conductors from the utility, the meter base and the mast all have to grow.
60 amps added, and the honest half of the answer
Adding a workshop subpanel puts 60 A onto the service — 60% of a 100 amp panel. Whether it fits depends on what the house already draws, and that is the number every other page on this subject invents.
Measure the existing load instead of guessing it
NEC 220.87 gives the method, and it is easier than the full calculation. Take the highest demand recorded on the service in the last twelve months, multiply by 1.25, and that is your existing load. Most utilities will provide the figure on request. Where there is not a year of history, a recording ammeter on the service for thirty days does the same job.
If that number is under 40 A, this addition fits.
When load management is the cheaper answer
A charger with a listed load-management device, or an energy-management system that sheds it when the range comes on, is a code-recognised way to put a large load on a service that could not otherwise take it. It is often a fraction of the cost of a new service, and it is worth pricing before anyone quotes a mast and a meter base.
A full panel is a different problem
If the breakers have run out but the calculated load is fine, a subpanel or a panel with more spaces solves it cheaply. Only when the load exceeds the service does the utility conductor, meter base and mast all have to grow.
Nearby sizes
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