How much of an A/C can my circuit handle?

Old 04-28-04, 09:45 AM
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How much of an A/C can my circuit handle?

I am having trouble figuring out how much my circuit can handle.

It's 15 amps.

I currently have a 27" TV, CD, DVD, TiVo, Cable Box, Computer, Monitor, Printer, Two Lights, a Fan, and a Cell Phone Charger running on this circuit.

Is there a way for me to figure out how much of an Air Conditioner would be able to run safely? Is there some sort of electrical meter you can buy, plug in, and it will tell me how much of the 15 amps are being taken up at any given time?
Old 04-28-04, 09:53 AM
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There are devices that can measure the current being used. However, consumer orinted devices tend to be less accurate than you would want.

Without regard to that, I wouldn't try to put an AC on this circuit, or any circuit with an existing load.
Old 04-28-04, 11:15 AM
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I agree. I can't imagine you could run any air conditioner on this circuit.
Old 04-28-04, 12:33 PM
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I've got to concur with the racraft and John; you really don't want to run an air conditioner on this circuit. But you have made a wise decision to be concerned and to evaluate the situation. While you have lots of electronics on this circuit, you don't have any outstanding large loads, so you may have the capacity for a small air conditioner. You have to take a closer look at the numbers.

Additionally, even if you have the capacity for a small air conditioner, you probably don't want to do this. The problem is that motor loads (like air conditioners) draw lots of current each time they start up (eg each time the thermostat turns the compressor on), and any other device on the circuit will see a significant voltage drop. The voltage drop and inductive surges every time the air conditioner cycles could cause damage to the electronics.

That said, on each appliance you should have a 'name plate' which describes how much power the device requires. This will specify the number of amps or watts. To convert watts to amps, just divide by voltage (this ignores power factor, but is close enough for now). Add up the amp ratings for all of the devices connected to the circuit. This will give you a conservative estimate of your maximum load. Now calculate the total for the devices that you will have running at the same time; this will give you a conservative estimate of your normal load in use.

Now evaluate how much capacity you have remaining. Your maximum _continuous_ load is only supposed to be 80% of the circuit rating, or 12A. You can go up to 15A for short periods, however, and can even exceed 15A for very short time periods (eg. a few seconds for starting a motor). If your _normal load_ on this circuit is say 7A, then you could reasonably add a 5A air conditioner to this circuit. You will probably not like the results, but it would be safe.

If, on the other hand, you estimate that the load on this circuit is 11A, then you put yourself right in the grey zone where the air-conditioner would probably work, but would be overloading the circuit, yet might not trip the breaker. This you do not want to do. Breakers are not really designed to be used right up to the edge of tripping for extended periods of time.

A modern, efficient 5000 BTU air conditioner should have about a 5A rating. Do your sums, and you might be able to safely use this. Please report back here with your totals.


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