The first photograph we take on any site visit is not of the kitchen. It is of the open electrical panel, with the door label visible. That picture, plus the square footage of conditioned space, is enough to answer a question that decides the shape of a lot of renovations: is the service big enough for what you are about to ask it to do.
The calculation is a demand calculation, not a sum
People assume a load calculation adds up every appliance and compares the total to the breaker. If that were true, almost every house in Jacksonville would be failing constantly. A 2,400 square foot house with an electric range, a dryer, a water heater and a four ton heat pump has a connected load far larger than its service rating.
What the code does instead is recognise that you do not run everything at once. Under the optional method for an existing dwelling, the totals are assembled and then discounted: the first 8,000 volt-amperes counts in full, and everything above that counts at 40 percent. The mechanical load, being the one thing that genuinely does run for hours at a stretch in a Florida summer, is added afterwards at its full value with no discount.
What goes into the pile
- Three volt-amperes for every square foot of conditioned floor area, which covers lighting and general receptacles as a blanket allowance.
- 1,500 volt-amperes for each small appliance branch circuit in the kitchen and dining areas, with a minimum of two, plus another 1,500 for the laundry circuit.
- The nameplate rating of every fastened-in-place appliance: range, ovens, dryer, water heater, dishwasher, disposal, built-in microwave, pool equipment, well pump.
Then the discount is applied, the larger of the cooling or heating load is added at 100 percent, and the whole thing is divided by 240 volts to get amperes.
Why it matters before design, not after
A typical Mandarin ranch from the 1970s has a 150 amp service. Run the numbers on the house as it stands and it often comes out in the low hundreds, comfortably inside the rating. Now add an induction range, a second wall oven, a heat pump one size larger because the addition changed the load, and a car charger. The same calculation can land past 180 amps, and that is before anyone talks about a tankless electric water heater, which on its own can add the equivalent of a hundred amps of connected load.
Finding that out during design means the service upgrade is a line in the contract. Finding it out at rough-in means a change order, a utility appointment you did not schedule and, on a bad week, a delay at inspection while everyone waits for a meter can.
Capacity is not the only limit
There is a second constraint that catches people out. A panel has a listed number of circuits and a physical amount of space. Renovated circuits usually have to pick up arc-fault or ground-fault protection, and those breakers are sometimes physically larger than what they replace. We have seen 200 amp panels with plenty of headroom on paper and no room left to put a breaker in. That is a panel replacement, not a service upgrade, and it is a different conversation and a different price.
Two things that change the answer
Older branch wiring
In Riverside and Springfield, cloth-insulated wiring and the occasional surviving knob-and-tube circuit are common. That wiring cannot be extended and insurers increasingly will not cover it. On a whole-home scope in those neighbourhoods we assume a full rewire until the walls prove otherwise. Houses wired between roughly 1965 and 1973 sometimes have aluminium branch circuits, which are workable with approved connectors at every termination but need to be found early.
Load management
Where a service upgrade is genuinely difficult, listed load-management devices can let a large load such as an EV charger share capacity with the rest of the house, dropping the calculated demand. It is a real answer and sometimes a cheaper one, but it has to be designed rather than assumed, and not every jurisdiction reviews it the same way.
What to do with this
If you are planning a renovation, photograph the inside of your panel door and count the square footage of conditioned space. That is almost everything a contractor needs to give you a straight answer about capacity on the first visit. Anyone who cannot give you one, or who says the panel looks fine without doing the arithmetic, is guessing with your budget.
The panel photograph is the cheapest twenty dollars of risk reduction in the whole project.
Written for this demo site. Code references, tolerances and figures are illustrative and vary by jurisdiction, by product and by house. Always confirm with the authority having jurisdiction and a licensed design professional.
