Maximum amps in circuit box

aliciazimSeptember 26, 2008

How do I tell if I am at the maximum amps that my circuit box can handle? Do I count up the individual switches? Or can I assume that if there are slots left in the box that I can add something? I don't want it overloaded, for obvious reasons, but I need two more 20 amps circuits added, and if I count all the amps of the circuits in there now, we are already over the limit!

It is a 220 amp Cutler-Hammer box if that helps.

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The short answer is to add the circuits. Adding the amperages of the existing breakers reveals nothing meaningful. Every circuit will not be fully loaded simultaneously. A professional can do a load analysis for you and calculate demand factors for your premises. But adding the two breakers will also be their advice.

    Bookmark   September 26, 2008 at 12:06PM
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You need to do a demand load calculation. Two 20 amp circuit won't use any power. It all depends on what you connect to those circuits. You could have breakers in your panel that are not supplying any current.

A demand load calculation takes into account things like size of house, do you have electric dryer- furnace, hot water heater, stove. Are there any other large loads like compressor, AC, etc?

    Bookmark   September 26, 2008 at 2:36PM
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Agreeing with Bus Driver...

Adding up the nominal size of the circuit breakers in a main panel is all but meaningless. Furthermore, if your main breaker has no history of tripping, you are not likely to be on the verge of overloading your service with two new 20 amp circuits. (Although that's always possible!)

BD's right in noting that it is unlikely for all of circuits to be fully loaded simultaneously. Some of the reasons are obvious at face value. Air conditioning and electric heat, for example, do not usually draw power at the same time.

To supplement BD's explanation a bit, the foremost purpose of each circuit breaker is to protect the wiring fed by the breaker from overheating. Let's take the case of a 5,000 watt hot water heater. When it is on and "cooking water" for all it's worth, it draws roughly 21 amps (5000W / 240 volts = 20.8A). Because that's more than a 20A circuit can safely handle, it's wired for the next size up--a 30A circuit. (In addition, because a hot water heater is considered a "continuous load", conservative usage assumptions are required.) Even when the heater is on and drawing full power, it only uses 70% of the circuit's capacity.

And that's more of an exceptional case than a typical case. A 15A lighting circuit with eight 75W bulbs turned on is only drawing a third of the circuit's capacity, and so forth. That's why the lighting load of a house is usually estimated at 3 watts per square foot rather than adding up the lighting circuit breaker sizes. Remember, those 15-amp circuit breakers for lighting are to protect the wires, and do not reflect anyone's estimate of your actual power demand for lighting.

Another very significant factor is that your electrical service consists of two "poles". If you have a 200A service, each of the two poles can draw 200A at 120V.

Now, a 240V circuit uses both poles, so its full load--let's say 20 amps for the purpose of illustration--might gobble up 20 amps of your total service rating. However, two fully loaded 20-amp circuits on opposite poles only account for 20 amps. Thus, if you add the two 20A circuits together, you're overstating the maximum load-against-service-capacity by a factor of two.

A demand load analysis is the usual way of assessing whether you're nearing (or over) the point of overloading your service. (Actual measurement under controlled conditions is another way.)

    Bookmark   September 26, 2008 at 2:43PM
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