Methodology
The same math your electrician runs, in plain English
ChargeRight calculates your house under NEC 220.83(A), the section the code wrote for an existing home adding a load, and runs NEC 220.82 alongside it - the optional method for new construction and full service changes. Same math a licensed electrician puts on a permit drawing. No black box: every number behind your verdict is on the report, including which section bound.
What the calculation actually sums
The optional method gives the homeowner credit for what real homes really use, not the worst case the standard method assumes. It is in the code on purpose.
NEC Article 220 Part IV holds two optional load calculations. 220.82 sizes a dwelling unit's whole load, which is the new-construction and full-service-change case. 220.83 is titled Existing Dwelling Unit and is written to determine whether an existing service has capacity to serve additional loads. Adding an EV charger to a house that already has a panel is that second question, so 220.83(A) is what binds for most homeowners. Either way the calculation adds up your home's general lighting load, your fixed appliances at their nameplate ratings, the larger of heating or A/C per NEC 220.60, and the new EV circuit at the continuous-load sizing the code requires, then compares the demand-factored total to your service amperage. I built ChargeRight to run it for $49.99 instead of a service call.
Which section applies to your house
Both sections are code-legal. Which one applies is decided by your situation, not by your state - the NEC edition a state adopts does not pick the method.
- 220.83(A)Existing home, adding a charger. The usual case, and the one most ChargeRight customers are in. The code section is titled Existing Dwelling Unit and exists to answer whether a service already in the wall can carry one more load.
- 220.83(B)Existing home, adding a charger and new heating or A/C. Same section, different subsection, because new space-conditioning equipment changes how the existing and new loads get credited.
- 220.82New construction, or a full service change. The Optional Method for sizing the whole load of a dwelling unit from scratch. If you are already replacing the panel and the service, this is the calculation that sizes the new one.
Your report prints the section that bound your verdict at the top of the math, and shows the other methods beside it so you can see whether they agree.
General lighting and receptacle load
based on the size of your home, exactly the way the code prescribes it
Small appliance and laundry circuits
the code-required allowance for every small appliance and laundry circuit
Appliance nameplates
every fixed appliance you actually have, at its real nameplate rating
Heating or A/C - the larger of the two
NEC 220.60 lets the smaller load drop out, because they do not run at once
Electric heat strips
credited at the reduced factor the code allows when the conditions apply
The EV charger
sized for continuous load the way the code requires for a charging circuit
The demand factor
One demand factor is why most homes pass
After every load is summed, both optional methods let a large share of the total drop to a reduced value, because a real home never runs everything at once. 220.83(A) and 220.82(B) each do this at their own threshold. That single step is why most homes do not actually need a service upgrade when a charger gets added: a 200 amp service handles far more diverse, non-coincident load than a worst-case standard calc admits.
The code is public - you can read 220.82 and 220.83 at any library. What the report buys you is the calculation run correctly on YOUR panel, under the section that fits your situation: every threshold, every reduced factor, every intermediate number, applied where the code applies it and printed line by line. Bathroom branch circuits live in 210.11(C)(3), heat strips get their own reduced factor when the conditions apply, and the largest-motor surcharge belongs to the Standard method, not here. ChargeRight keeps each rule where the code keeps it.
Not an electrician? Your main panel is the gray metal box your meter feeds. ChargeRight checks if it has room for the charger before anyone upsizes it.
Three steps to a verdict
Add it all up
Sum the general load, every small appliance and laundry circuit, every fixed appliance nameplate, the larger of heat or A/C, and the new EV charger at its code-required continuous sizing.
Apply the optional demand factor
The code lets a big share of your total drop to a reduced percentage, because real homes never run everything at once. The exact threshold and percentage come from whichever section applies to your situation - 220.83(A) for an existing home adding a charger, 220.82 for new construction or a full service change - and both are printed on your report.
Compare to your service
If the demand-factored total fits under your panel rating, you can charge without upsizing the service. If it does not, you get an honest no with the exact number.
What the report tells you
- Whether your panel already has the capacity for the charger your car needs
- The exact circuit size to install (the 30, 40, 48, or 50 amp answer)
- A one-page report you can hand a licensed electrician for the install
- The math behind every number - not a black-box verdict
Run the math on your home
ChargeRight runs this exact calculation against your panel and your car. Get an honest verdict in a few minutes.
Check my panel$49.99 - results in minutes