I am not talking about “average” or “normal” running. What I want to know is, what is the largest amount of current (i.e. amps) that a refrigerator will draw at once, presumably when the compressor is running? If you need a reference point, its Energy Guide says it uses 427 kWh/year, which I work out to be 0.41 A @ 120 V, and its specifications sheet says its “connected load” at 120 V is 360 W, which is 3.0 A. Should I assume that 3.0 A is the peak?
I ask because I intend to connect it to the same outlet as my microwave (I don’t have much choice), which is 1000W, so that would draw 8.3 A when running, and the plug is on a 20 A line.
The issue is that there is probably an inrush current when the compressor starts, could by up to 3x the running amperage. Newer, well maintained equipment will have less of an issue.
3 amps would be the load when the compressor is actually running, but it draws considerably more current for a second or two while the compressor is starting up, maybe ~8 amps. So if your fridge compressor happens to start up while you’re microwaving a burrito, that particular circuit breaker might briefly see 16 amps or so before settling back down to 11.3A. If you’re on a 20A breaker, then you should be fine.
Also note that breakers do not trip immediately when their rating is exceeded. Residential (and light duty commercial) breakers have two independent trip mechanisms - thermal and magnetic. The thermal mechanism handles overloads has a “time to trip” curve - the breaker will trip if the load current stays above the rated current for a specified time. For example, NEMA requires that a 20A breaker trip within one hour for a 135% overload (27A) and within two minutes for a 200% overload (40A). The time to trip accounts for the fact that the branch circuit wiring protected by the breaker takes some time to heat to a damaging temperature.
The magnetic trip mechanism handles short circuits and is also called “instantaneous trip”. When the current (actually the change in current) exceeds a certain level the breaker trips instantly (in a few tens of milliseconds, anyway).
TL;DR - Circuit breakers do not trip instantly when they rated current is exceeded. Small overloads will take up to an hour to trip the breaker.
Kill-A-Watt meters are great, they work really well for stuff like this, albeit maybe not for brief peak load when spooling up. I agree 8 amps sounds closer than 3, for peak load.
Also keep in mind, for refrigerators the compressor is not running constantly. Figure about a 33% duty cycle, something like that. Maybe closer to 50% in the summer? Circuit breakers can get “tired” too, eventually, and start to trip early.
I’ve been running a fridge and microwave on the same breaker since I had to move. The temporary apartment only had 15A breakers, and there was just one breaker for most of the plugs. (The other one only worked for the A/C and lights.) And now we decided to put the microwave on top of the fridge in the new house, and plug them up to the same outlet. It’s never been a problem. (It is a 20A breaker, I believe.)
I do believe our original fridge says it uses 3 Amps. It is one of those efficient models. However, the house came with a fridge, one that was apparently still here when the old owners moved out years ago before it was renovated. So it might be less efficient.
It also seems pretty rare for the things to go off right away with a single spike. While we didn’t have problems running the fridge and microwave on the same breaker, the plug for the A/C originally didn’t work, so we also had the A/C on the same breaker. And, if you turned on the microwave and the A/C came on, it would not trip right away.
And these were some very old circuit breakers, that had been there since the building was built, likely since the 1940s. (Though it was built by the military—it was originally for officers and their wives—so it may have been higher than normal quality.)
Holy cow! Do ground fault interrupters do the same? I’ve been told that convection heaters under windows have to be amperage over rated, I don’t recall by how much, 50%?
Maybe that’s a requirement for a built-in with a hood or something but most microwaves are counter or table top types and can’t have dedicated outlets. How does the outlet know if you’re using it for the microwave or a blender or a panini press or an immersion circulator, anyway?
Modern codes require a minimum number of 20A outlets based on on counter space. They can’t enforce how they’re used, but there also has to be a separate dedicated outlet for a refrigerator. Refrigerators and freezers are more critical than a microwave oven since they could trip a breaker unnoticed, resulting in food spoilage.
My new 20.5 cu ft refrigerator consumes 89 watts after the initial start up. Dollar wise, this equates to about $60+ per year using roughly 1+ KWH per day based on slightly over my cost of .15 cents per KWH.
I know that single-use fuses can be designed to blow slowly or quickly when the design current is exceeded by a modest amount, the former being intended to deal with expected short-term current spikes due to things like motors starting up. I’m not certain, but I think circuit breakers are also available with slow- and fast-tripping designs.
@Marvin_the_Martian pointed out that most modern breakers are combination, but there are motor-load specific rules to allow over-sizing the breaker to handle inrush current.
The electrician said the microwave oven needed to have its own outlet because of the codes. He called it a ‘dedicated’ outlet, but he may have been speaking colloquially. In fact, I do use both sockets. I have the food processor plugged into one of them. But I’ve never used both the microwave oven and the food processor at the same time. (I generally process food with a chef’s knife. The food processor is used for grating cheddar.)
The proper term is a dedicated circuit, not dedicated outlet. A circuit can have multiple outlets, a dedicated circuit will only have one outlet, for the appliance that it’s reserved for.