The load calculation that decides your quote

It is a sheet of arithmetic that estimates the largest demand your house can plausibly place on the service at one time, and compares that to what the service is rated for. It is not a measurement and it is not a guess. It is a standardised method with published demand factors, and the reason those factors exist is that appliances do not all run at once — the range and the dryer and the water heater and the air conditioning coincide far less often than the sum of their nameplates suggests.An electrician who runs one can tell you whether a charger fits. An electrician who looks at the panel cover cannot. That is the whole difference, and it is worth a few thousand dollars.

This is the optional calculation for adding a load to an existing dwelling unit. The inputs are things you can read off the house: square footage, the small-appliance and laundry circuits, and the nameplate ratings of appliances fastened in place — range, oven, dryer, water heater, dishwasher, disposal.The demand factors are the interesting part. The first 8 kVA of the total counts at 100 percent and everything above that counts at 40 percent, which is the code's way of saying that a house with a lot of appliances does not run them all at once.Its virtue is that it needs no data. An electrician can do it standing in your kitchen. Its weakness is structural: it counts nameplates, so it counts a 12 kW range that has never drawn 12 kW and an electric furnace sized for the coldest hour of the decade. On a gas-heat house that is fine. On an all-electric house it can fail a service that has never once been stressed.

This one uses what your house actually did. The electrician takes the maximum demand recorded over a one-year period, multiplies it by 125 percent, adds the new load, and compares the result to the service rating. Where twelve months of data is not available, the code permits a minimum 30-day recording of maximum demand at 15-minute intervals instead.The 125 percent is the code's cushion for a year that was not representative. The new charger load is itself a continuous load and is counted at 125 percent of its continuous current.The virtue is obvious: it replaces a worst-case theoretical total with observed behaviour, and observed behaviour is nearly always far lower. The weakness is that it is only as good as the year it measured — a year without an unusual cold snap, or a year when the house was empty for two months, is not a fair sample. That is a judgement call for the electrician and, if they want it, for the AHJ.

Same house, two methods, two answers. Neither is a trick; both are in the code.

Three things matter here, and the third is where honest electricians disagree with each other.First, EV charging is a continuous load, so the branch circuit and its overcurrent device are sized at 125 percent of the charger's maximum current — a 48-amp charger on a 60-amp circuit, a 32-amp charger on a 40-amp circuit.Second, the 2023 NEC added section 220.57, which requires the EVSE load in a service calculation to be counted at 7,200 volt-amperes or the nameplate rating, whichever is larger. That 7,200 figure is a 30-amp charger at 240 volts, and it exists so that a new service is sized for a real charger even if a small one is installed today. If your jurisdiction is still on an earlier code cycle, 220.57 does not apply to you — and adoption is genuinely uneven, with plenty of jurisdictions two cycles behind.Third, and this is where honest electricians differ: exactly how the EVSE load interacts with the 220.83 demand factors is read more than one way in the trade, and inspectors are not uniform about it either. We are not going to pretend there is a single settled answer. What we will say is that the reading your AHJ applies is the one that governs, and that a good electrician will tell you which reading they used and why.

NEC 625.42 gives two ways to reduce the number the calculation has to accommodate rather than the number the house produces.Under 625.42(A), where an energy management system in accordance with Article 750 manages the charging equipment, the load counted on the service and feeder is the maximum the EMS permits — not the charger's nameplate. Under 625.42(B), charging equipment with restricted access to an ampere-adjusting means complying with 750.30(C) may be counted at its adjusted setting. Restricted access means a sealed cover, a tool, a locked door or a password limited to qualified personnel, and where the adjustment changes the rating, the adjusted rating goes on the label.These are not loopholes; they are the code's own mechanism for fitting modern loads onto existing services. They are also the reason a good electrician asks what charger you bought before running any arithmetic.

Ask two questions and request one document. Which method did you use, 220.83 or 220.87? And can I have the calculation sheet?A load calculation sheet is a normal deliverable, not a favour. It is one page. Any contractor doing this work regularly produces them for permits already.

A service upgrade is $1,500 to $3,000 for the electrical work alone, before the panel at $500 to $5,000 and before whatever the utility charges for the service drop and the meter. It is the largest single line item available on a residential charger project, and whether it appears is decided entirely by this sheet.So when two quotes are thousands apart, this is nearly always why. One bidder ran the arithmetic and one estimated from the panel label — or one used 220.83 and the other used 220.87. Neither is dishonest. But only one of them can show you the page.Get the data from your utility before either of them visits. It costs you a phone call and it removes the most expensive assumption on the project.