Decision Guide
100-Amp vs 200-Amp Service: Which Does Your Home Need?
Published September 9, 2026 · Updated September 9, 2026
Short answer
100-amp service covers most homes running gas heat, gas water heating and one AC system. You need 200 amps when the NEC Article 220 load calculation exceeds 100 amps — typically at three or more large electric loads.
The honest test
Count your large electric loads: heat pump or electric furnace, central air, electric range, electric dryer, electric water heater, EV charger, hot tub, shop, ADU, well pump.
Two of those on 100 amps is usually fine. Four or more almost never is. Three is where the calculation has to be done rather than guessed.
Gas appliances change the answer completely. A house with gas heat, a gas range and a gas water heater has far more headroom than an all-electric house of the same size — which is why the electrification decision and the panel decision are really one decision.
What the calculation does
Under NEC Article 220 a dwelling load calculation totals general lighting load by square footage, small-appliance and laundry circuits, fixed appliances, the larger of heating or cooling, then applies demand factors that account for the fact you never run everything at once. The result is a number in amps, compared with your service rating.
This is why square footage alone is a bad proxy. A 1,600 square foot Bench ranch that has gone all-electric does not need the same service as a 1,600 square foot house still on gas. Any contractor quoting 200 amps without asking what is in the house is quoting a part number rather than a design.
The Treasure Valley version of this question
Locally, the decision is usually forced by one of three additions.
A heat pump conversion. The load that most often turns an adequate 100-amp service into an inadequate one, because cold-climate units with electric resistance backup draw heavily in a real winter.
An EV charger. A 48-amp unit is a continuous load sized at 125 percent under NEC 625.
A shop or ADU. Common on the valley's larger lots. A shop with a welder or compressor is a genuine load needing its own subpanel and feeder, not a circuit borrowed from the house.
If two or three of those are in your plans, do the calculation once against the finished picture. Upgrading twice is the expensive path.
When 100 amps is genuinely fine
- Gas heat, gas water heater, gas range
- One AC system
- No EV, no hot tub, no shop
- Panel in sound condition with open slots
If that is you and the panel is not a Federal Pacific or Zinsco, leave it alone. A service upgrade you do not need is $1,500 to $4,000 of nothing.
The middle options people skip
A subpanel. Solves "no open slots" when the service has capacity. Frequently the right answer.
A single dedicated circuit. If one appliance is tripping one circuit.
Load management. For EV charging, devices that shed or throttle the charger during household peaks can avoid an upgrade entirely.
Panel replacement without a service change. If the issue is an obsolete panel rather than capacity.
What each costs in Boise
| Job | Range |
|---|---|
| 100A to 200A service upgrade | $1,500 - $4,000 |
| Licensed electrician, hourly | $80 - $140 |
| Trip charge | $50 - $100 |
| DOPL permit | Scaled to declared job value |
What about 400 amps?
Rare in residential and usually the wrong answer — but it comes up in Eagle and on larger custom builds with multiple HVAC zones, a shop, a pool and two EVs. It is a materially bigger job involving different service equipment. If someone proposes it for an ordinary house, ask to see the load calculation.
Bottom line
Do not buy amperage by intuition. Count your large electric loads, and if the answer is three or more — or if a heat pump or EV is in your plans — have the NEC 220 calculation done before you take bids. Call 208-856-4826.
Related questions
Will 200-amp service lower my bill?
No. A panel distributes power, it does not consume or save it. What an upgrade buys is capacity, modern overcurrent protection, current GFCI and AFCI coverage, and a system your insurer will write.
How do I find out what service I have?
Open the panel door and read the rating on the main breaker: it will say 100, 125, 150 or 200. If there is no main breaker and you see screw-in fuses, you likely have 60 amps, and that is a replacement conversation regardless of load.
Can I upgrade later when I actually need it?
Yes, and often you should. The exception is when you are already opening the service for another reason — doing the meter base, grounding and panel in one visit is meaningfully cheaper than doing them twice.