Explainer
100 amp vs 200 amp service which do you need?
What the numbers actually mean, how to tell what you have, and the honest answer to whether a modern home needs the upgrade.
What the number means
The amperage of your service is the maximum the utility connection and the main breaker can carry at once. It is a ceiling, not a consumption figure. A 200 amp service does not use more electricity than a 100 amp one; it can simply support more running at the same time.
Think of it as the width of a doorway rather than the number of people going through it.
How to tell what you have
Open the panel door and read the main breaker at the top. The number stamped on the handle is your service size: 100, 125, 150 or 200. If there is no single main breaker, the panel may be a split-bus design, which is older and worth having looked at for other reasons.
Two things that do not tell you the answer: the number of breakers in the panel, and the physical size of the enclosure. A large panel packed with circuits can still sit on a 100 amp service.
What each one supports
| Load | 100 amp | 200 amp |
|---|---|---|
| Gas heat, gas water, gas range | Comfortable | Comfortable |
| Central air conditioning | Usually fine | Fine |
| Electric range and dryer | Tight | Fine |
| Level 2 EV charger | Often not | Fine |
| Heat pump replacing gas heat | Rarely | Fine |
| Two of the above together | No | Usually fine |
| Workshop, pool, ADU | No | Depends on the rest |
The honest answer
A 100 amp service is not obsolete. Plenty of homes in this area run on one indefinitely with no problem, particularly where heating, water heating and cooking are gas. If nothing is tripping and you are not adding load, there is no reason to spend money on the strength of the number alone.
What changes the answer is electrification. Each of an EV charger, a heat pump and an induction range is a substantial continuous load. One of them may fit. Two rarely do.
The load calculation
Guessing from the age of the house is how people end up paying for an upgrade they did not need, or discovering mid-install that they did. The National Electrical Code sets out a method: general lighting by square footage, fixed appliances at nameplate, the larger of heating or cooling, plus specific allowances for ranges and chargers, with demand factors applied because not everything runs at once.
It takes about twenty minutes and it produces a number rather than an opinion. We do it before quoting an upgrade, and we will tell you when the answer is that you do not need one.
If you are between the two
Some homes come out just over 100 amps on paper. Three options before committing to a service change:
- Load management. A device that prevents two large loads from running simultaneously. Common with EV chargers and effective.
- Lower the draw. A 32 amp charger instead of 48. A soft starter on the air conditioner. Both reduce peak demand for a fraction of an upgrade.
- Stage it. If a remodel is coming, a service change is cheaper done alongside other work than as its own job.
Common questions
Is 100 amp service unsafe?
Not in itself. A 100 amp service that is properly installed, correctly loaded and in a sound panel is perfectly safe. What is unsafe is an overloaded service, a failing panel or bad terminations, and those happen at any size.
Will 200 amp service lower my bill?
No. Service size sets what you can draw, not what you pay for. Your bill reflects what you actually use.
Can I go to 400 amps?
Yes, and it is occasionally right for a large property with an ADU, a shop and multiple chargers. It usually means two 200 amp panels fed from a single service. For most homes it is more than the situation needs.
How do I know if my panel is full?
Count the empty slots. But full is not only about slots: a panel can be at capacity on load while looking half empty, and some have tandem breakers where the manufacturer does not allow them. The load calculation settles it.
Does an older panel need replacing even at 100 amps?
Sometimes, and for reasons unrelated to size. Federal Pacific, Zinsco and Challenger panels have a history of breakers failing to trip. If you have one, replacing it is worth doing whether or not you change the service size.
Not sure which you have?
We will run a load calculation and give you a straight answer, upgrade or not.