# How to predict when the electrical infrastructure on my 1950s rental is going throw a breaker?

I live in an old house; it has old electrical infrastructure. Last night it was really hot, and we were running an external dishwasher on the same circuit as the TV and a fan and a ceiling fan. Only the TV is connected with an extension cord because there just isn't a three prong outlet in that room, but sadly it's still on the same circuit. The TV is an older flat screen from maybe 2003 or so, the dish washer is brand new.

When it was hot last night the breaker kept switching off, and I had to unplug everything from that circuit and reset the breaker.

Is there any way to calculate how much a breaker is likely to be able to handle, so I don't have to keep running up and down the stairs to fix this?

I do have things running in other parts of the house as well, but they aren't on the same circuit. They only blew once or twice; the one in question blew like 4 or 5 times last night.

• What was the ambient temperature near the breaker? – Tester101 Jun 21 '16 at 12:08
• The breaker is in the basement, so I wanna say probably about 75 degrees. Upstairs it's like 85 and probably about 80 on the first floor. – leeand00 Jun 21 '16 at 12:12
• Circuit breakers may fail, e.g. simply through aging over the years. If you're confident you didn't overload the circuit beyond what the breaker is rated for, get a new breaker of the same rating and see if that improves the situation. – JimmyB Jun 22 '16 at 11:38
• @JimmyB it's not my breaker, but it'd be great to prove it to the land lord. – leeand00 Jun 22 '16 at 11:39
• Swap the breaker with one from another circuit and check :) – JimmyB Jun 22 '16 at 11:41

Get yourself a Kill-a-Watt power meter and measure each of your loads on each circuit. They also make upscale units with backlights and storage so you can unplug it and look at the stored data in a bit more comfort.

Take Watts or VA (whichever is larger) and divide by 120 - this gives Amps. Or take "amps" right off the Kill-a-Watt. Tally up all the amps for each load on each circuit.

Then compare it to the number on the circuit breaker that keeps tripping. The number will be 15 or 20 depending on the size of the wire.

I'd like to break you out of the mentality that your "infrastructure is old, and that's what happens, and resetting the breaker all the time is normal, safe and a burden of living in an older home". That is all false.

The above test will tell: If it's tripping well below rating, it's a bad breaker. Otherwise it's tripping because you are overloading it. Stop doing that.

Breakers protect wires. They are designed to warm up at about the same rate as the wires in your walls, and trip when they get too hot. Trouble is the breaker may cool off much faster than your wall wiring, so by repeatedly resetting it, you're letting the wall wiring "keep getting warmer", defeating the breaker's protection.

My hunch is this newer dishwasher has an integral heater, so it's meant for a dedicated 20A circuit, and using 14-15A all itself. That, however, does not explain why the other circuit trips.

One coping strategy may be to tell the dishwasher not to use the heater.

Like I say, the Kill-a-Watt tells all. Once you have the numbers, if the answer isn't obvious, bring your data back here and we'll help.

A breaker will trip according to its trip curve, at 40°C (104°F). In this condition, it should be able to carry 100% of the rated load. So if you have a 20 ampere breaker, at 40°C it should be able to carry 20 amperes. It's often recommended (and sometimes required), to only load a circuit to 80% its capacity.

In most cases, devices are listed by the amount of watts they use, rather than the amount of current they draw. Luckily it's fairly easy to convert back and forth, as long as you know the system voltage.

`Watts = Volts * Amperes`, `Amperes = Watts / Volts`.

So a 20 ampere breaker on a 120 volt system, should be able to supply 2400 watts. Applying the 80% safety margin, that's 1920 watts. A 15 ampere breaker can supply 1800 watts at 100%, or 1440 watts at 80%.

To estimate how much power you're using, add the wattage rating of all the devices on the circuit.