Air Conditioners and Power
June 5, 2010 4:35 PM   Subscribe

ElectricityFilter: How many A\C units can I plug in?

My rental house came with a window-mounted air conditioner in storage in the garage, and my landlord said we could use it if we wanted. I just hauled it out and installed it in the master bedroom. It is an LG that is "designed for use on a 15amp, 115volt circuit".

I went and looked at my breaker box, and the one labeled "bedroom outlets, living room outlets, bathroom outlets" is a 20amp circuit.

I would like to put a second (exactly the same) window unit in the spare bedroom so when we have guests they can sleep in comfort too. Given the above, is it a bad idea two have two of these bad boys on a 20amp circuit that may, potentially, also be powering a bunch of other things like a computer + LCD monitor, printer, laptop charger, box fans, etc?
posted by jeffamaphone to Home & Garden (26 answers total)
 
It won't work. If they're both on, they'll try to draw 30 amps (eventually, when they're both running at the same time) and the breaker will blow.

Yes, this is a bad idea. Not dangerous, mind; the breaker will do the job it's supposed to do and prevent your wiring from burning your house down. But really annoying.
posted by Chocolate Pickle at 4:41 PM on June 5, 2010


By the way, if that really is a 20 amp circuit and it has all that other stuff on it, the breaker may blow quite often even with only one 15-amp aircon. 5 amps isn't all that much power for computers and printers and TVs and the like. (It's only 600 watts.)
posted by Chocolate Pickle at 4:52 PM on June 5, 2010


Hairdryer on in the bathroom + AC on in the bedroom blew a 20 amp circuit pretty regularly in one of my former houses, so I'd say you're already in iffy territory even without another AC unit.
posted by donnagirl at 5:17 PM on June 5, 2010


"designed for use on a 15amp, 115volt circuit

Check for another sticker somewhere that says how many watts it uses. I doubt it draws all 15 amps.
posted by gjc at 5:30 PM on June 5, 2010


check you breaker box again to see how the circuits are distributed.

I seem to recall some place I lived had at least two circuits for the outlets in every room (I may have been a code requirement).

But then again, maybe the reason the AC is in the garage is that you can't run it and a TV at the same time.
posted by MonkeySaltedNuts at 5:31 PM on June 5, 2010


Okay, thanks. My suspicions are confirmed.
posted by jeffamaphone at 5:44 PM on June 5, 2010


Seconding gjc. It is almost certain that the air conditioner you are looking at uses fewer than 15 Amps. There should be another sticker that will list the actual Wattage and Amperage. Alternatively, if you find a BTU/hr rating, you can multiply that by 10 to approximate the Wattage if the air conditioner isn't terribly ancient. (Then divide the Wattage by 115 to find the Amperage.)

As for donnagirl's hair dryer: Most hair dryers take a LOT of electricity when in use; more than all but the very largest 115 Volt air conditioners.
posted by Juffo-Wup at 6:03 PM on June 5, 2010


It doesn't matter one lick what the wattage is. Two window AC units on the same circuit is still going to blow a 20 amp fuse.

Besides, you can't just write down a single number to characterize how much power is required. When the unit starts up it is going to draw a huge current spike, much higher than the average usage. So even if it says X watts, it's going to temporarily draw X*5 watts for that first second or so and that is going to cause the breaker to trip. When they wrote on the plate "designed for a 15 amp circuit" they are telling you this exactly: don't try to put two of these things on a 20 amp circuit.
posted by Rhomboid at 6:37 PM on June 5, 2010


My electrician told me that a 15 amp circuit is rated to blow the fuse after 20 minutes of a full 15 amp load. Same with 20 amps.
posted by StickyCarpet at 6:43 PM on June 5, 2010


What you could do is put both ACs on timer switches.
AC1 is powered for the first 29 minutes of each hour and after a minute AC2 is powered for the second 29 minutes.
posted by MonkeySaltedNuts at 7:01 PM on June 5, 2010


AC1 is powered for the first 29 minutes of each hour and after a minute AC2 is powered for the second 29 minutes.

Might as well just run the stronger AC for the entire hour.
posted by knave at 7:25 PM on June 5, 2010


Turn off that breaker and take a nightlight around to see if you have any remaining live outlet in either bedroom. You never know. Those labels have a way of being wrong (out of date).
posted by fritley at 7:35 PM on June 5, 2010


My electrician told me that a 15 amp circuit is rated to blow the fuse after 20 minutes of a full 15 amp load. Same with 20 amps.

Fuses are only found in very old buildings. Breakers have been code for more than 50 years.

Breakers blow nearly instantly if the current exceeds their rating. ("Slow blow" is dangerous.) And in practice, current usage is hardly ever exactly flat. If you're right on the edge like that, you're going to wander over and the breaker will blow.

It is almost certain that the air conditioner you are looking at uses fewer than 15 Amps.

The vast majority of the time it will use much less than 15 amps. But when motors spin up they draw far more current than when they are at speed. If it's a 1000 watt aircon it could very easily draw 15 amps for a second or two when it turns on.
posted by Chocolate Pickle at 7:50 PM on June 5, 2010 [1 favorite]


It doesn't matter one lick what the wattage is. Two window AC units on the same circuit is still going to blow a 20 amp fuse.

Not if they're 1000 watt AC units. In other words, yes it does fucking matter.

jeffamaphone: The guys telling you to look for how many watts your unit will use are steering you in the right direction. The total amount of power in watts available for a given circuit is found by multiplying the amperes by the voltage. In this case, a 20 amp 120 volt (if this is nominal in your area) circuit will be able to power devices totalling 2400 watts. If you have the units linked above and there are no other devices with serious demands on the circuit, you are good to go.
posted by ferdinand.bardamu at 8:05 PM on June 5, 2010


The vast majority of the time it will use much less than 15 amps. But when motors spin up they draw far more current than when they are at speed. If it's a 1000 watt aircon it could very easily draw 15 amps for a second or two when it turns on.

Lol. Yeah, those air conditioner motors have some killer inrush current.


(Hint: The motors in window air conditioners are tiny and just run the fan. The compressor is what draws the most current.)

Please, no one listen to anything Chocolate Pickle has to say about electricity and electronics ever again. He obviously does not know what he is talking about.
posted by ferdinand.bardamu at 8:09 PM on June 5, 2010


What makes the compressor go?
posted by Chocolate Pickle at 8:15 PM on June 5, 2010


And that compressor is driven by what? An electric motor.

Your insistence on making everything about power is asinine. Household electric capacity is measured by current: the wiring is rated for a certain number of amps, the breakers are rated to trip at a certain number of amps. Furthermore watts doesn't always equal volts times amps when dealing with alternating current, because you have real power (W), reactive power (VAR) and apparent power (VA), the ratios of which depend on the kind of load and whether it's linear or non-linear.
posted by Rhomboid at 8:23 PM on June 5, 2010


Compressors can draw a lot of current when they start up. Ever seen the lights dimming when the fridge's compressor starts up?

When they say 15 A, they mean it.
posted by Monday, stony Monday at 8:28 PM on June 5, 2010


(Hint: The motors in window air conditioners are tiny and just run the fan. The compressor is what draws the most current.)

Please, no one listen to anything Chocolate Pickle has to say about electricity and electronics ever again. He obviously does not know what he is talking about.


Tough talk from someone who doesn't know that a compressor pump contains a motor.

The power rating on an air conditioner is there to let you know how much power the device consumes on average, so you can compare device efficiency. The current rating is there to let you know the maximum current when the fan, compressor, microprocessor, etc., are all running at once. This is useful information because (1) current heats up wiring and can set your home on fire; and (2) breakers and fuses are designed to break the circuit to prevent this from happening. Jeez, Chocolate Pickle pointed this out in the first comment.

Running two 15 A devices on a 20 A circuit is foolhardy.
posted by Mapes at 8:28 PM on June 5, 2010


AC1 is powered for the first 29 minutes of each hour and after a minute AC2 is powered for the second 29 minutes.

knave Might as well just run the stronger AC for the entire hour.

I thought the situation was window units in two bedrooms. Such are often run with the bedroom doors closed.
posted by MonkeySaltedNuts at 8:36 PM on June 5, 2010


TIAS. There are several possible outcomes:

1) It works.

2) It doesn't work.

3) The apartment burns down.

I ordered those in descending order of likelyhood.
posted by jrockway at 11:18 PM on June 5, 2010


Chocolate Pickle: Breakers blow nearly instantly if the current exceeds their rating. ("Slow blow" is dangerous.) And in practice, current usage is hardly ever exactly flat. If you're right on the edge like that, you're going to wander over and the breaker will blow.

This is wrong. Nearly all residential breakers have a time delay. Otherwise you would be popping breakers every time some appliance turns on because most motors pull several times their rated load briefly when starting.

Here are the characteristics of a typical residential breaker. You can see from the chart that the breaker will handle 6 times its rated current for up to 1 second. That's 120 amps for a nominal 20 amp breaker. It will handle two times rated current, or 40 amps for up to 15 seconds.

These current and time characteristics are conservatively designed. It takes a long time for 12 gauge house wiring to heat up even with high currents. To trip a breaker instantly (that is, about 10 milliseconds) requires perhaps 200 amps for that instant.

You can run a load at the breaker's rated current indefinitely. For these two air conditioners, you are concerned about their running power, not their start up power, since the breaker's time delay will handle start up. The electrical plate on the back of the air conditioner will either give you watts or amps. The combined power for the two air conditioners (plus whatever else is on the same circuit) should not exceed 2400 watts or 20 amps. Starting current should not be an issue unless they both happen to start at the same instant. This occurrence may pop the breaker but is not necessarily dangerous.
posted by JackFlash at 11:25 PM on June 5, 2010


Well after searching the unit, the manual and the manufacturers website, I cannot find any information about watts or its electricity consumption.

The unit is an LG LWHD1006R, but their site doesn't have any manuals / tech sheets for it.
posted by jeffamaphone at 9:43 AM on June 6, 2010


A typical 10,000 BTU air conditioner will use about 1000 to 1100 watts and about 9.5 amps. Two air conditioners will be very close to the maximum for a single 20 amp circuit not even considering other devices on the same circuit. You won't be able to run both at the same time.
posted by JackFlash at 10:29 AM on June 6, 2010


When I say fuse I mean breaker. I'm from Appalachia and that's the way we talk.

If we still had fuses, this would be a no-brainer, just put a penny in there.
posted by StickyCarpet at 2:31 PM on June 6, 2010


Please, no one listen to anything Chocolate Pickle has to say about electricity and electronics ever again.

I'm sorry, I can't promise to do that.
posted by StickyCarpet at 2:33 PM on June 6, 2010


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