Electrical – How to calculate if a 15 amp circuit will be adequate for your needs

electrical

I have a 15 amp circuit and I would like to have the following things plugged into it:

definitely will have:

  • tv
  • 4 lights
  • microwave
  • mini-fridge

could possibly have:

Is this too much for a 15 amp circuit? How do I go about calculating what a 15 amp (or any size) circuit can handle?

Best Answer

Are you in North America with 120V power? Your 15A circuit has 1800 watts and 1800 VA (a different way to measure watts). The microwave, oil heater, and electric cooktop are each large heating loads. Any one of them draw nearly the entire circuit capacity (1440 out of 1800 watts). No two of them may ever run at the same time, but of course ther's no way to enforce this, so you will get breaker trips everytime you mess up.

Them mini-fridge will be small, probably no more than 300 watts when running.

In a "power is precious" situation you really, really want to be using LED light bulbs, and those will be tiny - 5-15 watts each.

TVs vary, but presuming it is not a tube TV, and is not gigantic, it will draw under 100 watts. If it has a "wall wart" or lumpy DC power supply, it's all but certain it draws less than 100 and maybe even 50 watts. You didn't mention a cable TV box (which can draw more than the TV).

All of these small loads can coexist with one of the large loads.

You didn't mention any decked out PC gaming rigs or laser printers; eqch of those can draw power like a large load.

All of the large loads will also add a lot of heat to the room, which may matter if it's summer.

How to calculate

A 15A circuit (assuming it's all yours) can supply nominal 1800 watts or VA of electricity. That's 15A x 120V.

All your appliances will have a rating of either amps, watts, or VA. If it's amps, multiply by 120V to make it comparable to watts and VA. The difference between watts and VA comes up for certain electronic or motor loads. VA will be higher, and you must use that in place of watts.

On some loads, their rating is excessive compared to what they actually draw, e.g. A 900 watt PC power supply rated to draw 10A (1200W). (the difference is due to internal conversion losses). if the PC has just a few components inside(i.e. only one modest video card and hard drive), it will draw considerably less than 1200W, and you can only measure that with a device such as a "Kill-a-Watt". Make sure to measure at worst-case, with the hard drive spinning and the video card rendering hard.

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