What is a Service Disconnect?

Do I need a service upgrade?

Service capacity isn’t necessarily related to the number or size of breakers in your existing breaker panel, despite what a home inspector or carpenter or general contractor may tell you about needing a service upgrade.

A Service Disconnect is the first, required, point where the building owner may disconnect from the service entrance (utility-connected) conductors, using an accessible, manually-operable switch. There may be up to six (6) separate disconnects for any building’s electrical service. The disconnect must provide protection  (fuse or breaker) against excessive current flowing to the building, based on the capacity of the building wiring system AND the calculated load of all equipment connected to the building wiring system.

200A electric service upgrade permits connection of a larger solar PV grid-tie via ‘backfeed’ circuit breakers

How do I tell the size of my electrical service?

Do I need to upgrade my service for more breakers or to add central AC?

Several factors affect the capacity of your electrical service, but the size of the overcurrent device (breaker or fuze) protecting everything on the building (load) side of the service disconnect should tell you your service’s capacity, since all the other components of the system must legally be capable of carrying more power than the fuse or breaker will let through. Most single-family homes will be either 100A or 200A. Apartments in a multi-family building may have 60A services each, and businesses may have whatever is needed to supply the business’s loads: typically from 60A up to 800A or more, on single- or multi-phase systems where multiple phases increase the comparative power capacity of the system over single-phase systems of the same Ampere rating.

Many homes in the Philadelphia area — especially those inside City limits — feature mostly or all gas-fired major appliances, minimizing load on the electrical service. A 100A electric service in a home with all gas or oil-fired appliances (water heater, central space heating, range, dryer) may be capable of a central cooling system. A good HVAC installer will do a heating load analysis based on your square-footage, type of construction, number of windows, and a fudge-factor for draftiness of doors/windows — and provide this analysis to you with their estimate upon request, along with the electrical requirements of the equipment they propose to use to meat the estimated cooling load. Can you run this HVAC system on your existing 100A electrical service? If you have no 220V appliances, multiply your occupied square feet by 3, multiply the proposed HVAC circuit breaker rating in Amperes by 240, and if the sum of these two products is 20,000 or less, then you may be able to add central AC without upgrading your 100A service. If you have any 220V electric appliances or are close to the 20,000 threshold, contact me for an electrical load calculation to determine existing capacity and/or a quote for upgrading to a 200A service.

Service Capacity is different from Breaker Capacity (number of circuits)

Service capacity relates to total power; breaker capacity relates to number of circuits. There is no direct relationship between total demand power and number of circuits, although more circuits often means more demand power. Note, however, that most households will find a way to connect and use about the same amount of power per square foot, regardless of the electrical service size, or how many circuits or receptacles there may be. Occupants of under-equipped houses tend to compensate by the use of window air conditioners, multi-outlet expanders (‘plug strips’), and/or extension cords. Doing so in older buildings can put occupants and the building at risk.

Service capacity is ultimately a power rating, comparable to engine horse-power in a car. A larger engine size can carry more people up a given hill, but if there are only 4 seatbelts in the car, you can only safely carry 4 people up the hill, no matter whether the engine is 95 hp or 2,000 hp. Similarly, a 100A service might serve a distribution/breaker panel (aka ‘fuse panel’ ‘breaker box’) with only 2 breaker positions (seatbelts), or it might serve another panel featuring 20 positions (seatbelts), each of which can accomodate 1/2-size breakers for a total of 60 possible circuits. The same capacity service can supply dramatically different numbers of circuits in these two examples.

Many installers — especially, but not only, general contractors or other non-electricians who simply don’t know any better — will sell customers an electric service power capacity upgrade when upgrading circuit capacity by replacing only the distribution/breaker panel is a more cost-effective alternative. I recommend that you do not accept a recommendation for a service upgrade unless and until the contractor has provided you with a load calculation in accordance with National Electric Code (NEC), that demonstrates the need for increased capacity. Also, check out my post on the option of twin breakers for squeezing new life out of your breaker panel.

See also:

• Residential Costs Guide
• Frayed service entrance cable
• Can I put twins in my breaker panel?

200A electric service replacement and renovation provided proper grounding and bonding and a logical layout with upgraded breaker(circuit) capacity for each tenant unit.