Calculating wattage using an ammeter

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Old 06-12-20, 07:15 PM
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Calculating wattage using an ammeter

I am testing my air conditioner compressor to see if it is possible to run off my generator. I put a clamp ammeter on each hot wire leading into the A/C. At max power each hot line was drawing 12.5A. It's a 4 ton 18 SEER variable speed unit. So, that means 25A total. What I'm not sure is if I multiply 25A X 120V or 240V to come up with either 3,000 watts or 6,000 watts total. If anybody can advise, that would be much appreciated, thanks!
 
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Old 06-12-20, 07:47 PM
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Welcome to the forums.

The current the unit draws is circular. In other words...... the current measurement on the hot side will be equal to the neutral side or the opposite leg if a 240v circuit.

That means you measure current draw on one side only.
So that is 12.5amps total.
If the unit is 240v then it's 240 x 12.5 = 3000 watts
If the unit is 120v then it's 120 x 12.5 = 1500 watts

I'm guessing this is a 240v setup. Keep in mind there is also an indoor blower.
Also..... depending on the type of compressor.... you could need double the amperage during starting.
 
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Old 06-12-20, 08:00 PM
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A 4 ton A/C is going to be 240 volt. The largest 120 volt A/C unit I have ever seen is 12000 BTU's or 1 ton.

Your generator will need to output 240 volts to power the A/C
 
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Old 06-12-20, 08:00 PM
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Yes, definitely a 240 volt setup. Thanks much for the answer! I was surprised to hear how little power it took. I was expecting much more.
 
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Old 06-12-20, 08:04 PM
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The generator does put out 240V through an L14-30R plug.

my only problem now is that the A/C runs on a completely separate circuit breaker in the meter box. The transfer switch is only hooked up to the main circuit breaker leading into the house.

No idea how to solve that issue.
 
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Old 06-12-20, 08:16 PM
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You are measuring the current while the unit is running. It's starting that is the issue.
For a four ton unit you may need in excess of 7500 watts
 
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Old 06-13-20, 05:44 AM
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You will need a second transfer switch, this one in the air conditioner circuit, installed anywhere between the meter box and the A/C unit. This would have its own inlet to connect the generator cord.

Having a second transfer switch and inlet might come in handy in the future if you find you need more emergency power and choose to get a second generator instead of replace the first with a larger one.
,
 
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Old 06-13-20, 04:46 PM
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Thanks all! Just curious, why is the A/C on its own breaker in the meter box versus being run through the main breaker panel in the house? At first I thought it might be because of how much power it draws, but then I saw the A/C breakers are 50 A but so is the breaker for the oven. Is it just convenience because the A/C is only 10 feet away from the main panel?

I just wish it did run from the main breaker panel so I wouldn't need a 2nd transfer switch. Of course the obvious problem is that the generator can only be plugged into one at a time.
 
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Old 06-13-20, 04:57 PM
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By putting the A/C condenser on a breaker in the service disconnect panel keeps the excessive power load off the main panel as well it saves two spaces in the main panel.
 
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Old 06-13-20, 06:31 PM
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It may have been a shorter cable run too.
But mostly the condenser draws a lot of current at start-up. Unless the subpanel cabling is oversized, you'll likely see a possible dimming of lights every time it kicks on.

To solve your transfer switch problem, you could install a main panel interlock at the main panel (if you haven't already installed the transfer switch). This will allow you to switch your main panel and the whole house over to generator power. You could also look at moving the AC breaker to your subpanel, but you'll have to ensure the sub panel capacity has enough leeway.

Or consider if you really need the AC running when running on generator power. It will certainly use a lot more fuel if the gen can support it.
 
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