Will 100 amps still carry the house afterwards?

The question comes up on almost every site walk, usually phrased as a hope. We are not changing much electrically, so we probably do not need to touch the panel, do we? Sometimes the answer is no. More often the house has been living at the edge of its capacity for a decade, and the renovation is what pushes it over.

The National Electrical Code gives an optional calculation for dwellings in Article 220 Part IV. It is simple enough to do at a kitchen table, and running it against the appliances you are planning to buy, rather than the ones you own, is the single most useful thing you can do before choosing a range.

How the calculation works

Add the general lighting and receptacle load at 3 volt-amps per square foot of conditioned area. Add two small-appliance branch circuits at 1,500 volt-amps each and a laundry circuit at another 1,500. Add the nameplate rating of every fixed appliance.

Then apply the demand factor, which acknowledges that nobody runs everything at once: 100 percent of the first 10,000 volt-amps and 40 percent of the remainder. Finally add the larger of the heating or the cooling load at 100 percent, because those two never run at the same time. Divide by 240 volts and you have the demand in amps.

The pattern this produces

Scroll the table sideways to see every column.

Sample calculations using the optional method. Your figures depend on actual nameplate ratings.

Signals you are already at the limit

• A panel with no empty spaces and tandem breakers filling the slots that exist
• A 60, 100 or 125 amp main on a house that has been extended or gained electric appliances
• Lights dimming noticeably when the well pump or the air conditioner starts
• A fuse box rather than a breaker panel, or a panel brand insurers now flag
• Aluminium branch circuit wiring from the mid-1960s to mid-1970s, which is a safety issue separate from capacity

What an upgrade actually involves

Replacing the service entrance conductors, the meter base, the main panel and the grounding electrode system. On an overhead service the utility drops and reconnects the service; on an underground service there is usually trenching, which is where the days go.

Inside, every circuit reconnected to the new panel has to meet current protection requirements. Bedrooms, living areas, kitchens and laundry need AFCI protection; kitchens, bathrooms, laundry, garages, basements and exterior receptacles need GFCI. You cannot simply move forty old circuits across unchanged, which is why this work belongs in the rough-in phase while the walls are open.