Two constraints, and only one is visible

When to use this: adding a circuit to a panel that already exists — an EV charger, a hot tub, a heat pump — and wanting to know before you buy the breaker.

“Is there room in the panel?” is two separate questions that people merge into one:

• Spaces. Physical breaker positions. Visible through the door, easy to count, and almost never the real problem.
• Amps. The calculated load against the panel rating. Invisible, requires arithmetic, and usually the binding constraint.

A panel can be full of empty spaces and out of capacity, or out of spaces with capacity to spare. Those need completely different fixes, and one of them costs twenty times the other.

Which problem you have decides the cost

A subpanel adds spaces, not capacity — it is fed from the existing service and shares it. If the constraint is amps, a subpanel changes nothing whatsoever. That distinction is worth being clear about before anyone quotes for one.

Do not add up the breakers

The most common way people estimate panel load is to total the breaker ratings, and it overstates the answer enormously. A typical house has 200 A or more of breakers on a 100 A service and is entirely compliant.

Circuits are sized for their worst case, and NEC Article 220 applies demand factors because they never all run at once — 3,000 VA of lighting at 100% and the rest at 35%, ranges from Table 220.55, the larger of heating or cooling but never both. Our load calculator does that arithmetic; this page starts from its answer.

Typical loads

VA for common additions. The continuous ones count at 125% under 210.19(A)(1), which is the same rule as “a breaker may carry only 80% continuously” stated from the other end.

The EV charger rows are where 100 A services break. A 48 A charger is 11,520 VA on paper and 14,400 VA in the calculation — around 60 A of a 100 A service before anything else is switched on.

Tandem breakers

A tandem fits two circuits into one full-size position. They are permitted only where the panel is listed for them, and only in the positions the label identifies — often just the bottom few.

NEC 408.36 limits a panelboard to the number of overcurrent devices it is marked for. A panel rated for 20 devices does not become a 40-device panel because tandems physically fit in the slots, and inspectors check this.

Note also that a tandem gives you two 120 V circuits, not a 240 V one. A hot tub or EV charger needs two adjacent full-size positions on opposite busbars, which a tandem cannot provide.

Load management, the third option

Where the constraint is amps and a service upgrade is not practical, NEC 625.42 recognises load management for EV charging: a device that monitors the service and pauses or reduces charging when the house draws heavily. The circuit is then sized on the managed maximum rather than the charger's rating.

It is increasingly the answer for older housing stock, and it is considerably cheaper than moving a service. The same principle appears in 220.70 for energy management systems more generally.

How full is too full?

Nothing prohibits running a panel to 100% of its calculated capacity. It is not good practice. Above roughly 80% it is worth asking what is coming in the next few years — because heat pumps, induction ranges and EV chargers are precisely the loads people add to older panels, and each one arrives as a surprise.

If you are considering an upgrade anyway, sizing for one circuit and then upgrading again in two years is the expensive path. Our load calculator will size the service properly, and wire size handles the feeder to any new subpanel.

How this calculator is checked

Technical reviewer

Salamot Hok

Electrician · 10+ years of installation work in Bangladesh and the wider South Asian region

He reads the result the way an installer would: are the defaults values people actually meet, does the warning fire where you would stop and think, and is the answer something you could buy and fit? The code figures themselves come from the published standards cited below, not from him — that boundary is set out on his profile.

• The maths lives in a pure function with its own test suite, asserted against worked examples from published references and standards. A calculator does not ship until those tests pass.
• 2 sources cited by name and linked, so any figure on the page can be traced back to the document it came from.
• Last reviewed 17 August 2026. Review dates are advanced only when the page is actually re-read, never to look fresh.
• Unusable input returns no answer. Where the inputs do not describe a real design, the calculator says so and withholds the number rather than printing a plausible-looking wrong one.

The full process is written up in the methodology and editorial policy. Results are engineering guidance, not a code sign-off — see the disclaimer. If a result looks wrong, tell us; corrections are answered before anything else.