Idle-Time Transformer Power Consumption

A shop owner tries to decide whether there are significant power savings to be achieved by turning a transformer off at night and on weekends. December 29, 2008

Question
Anyone know how much power, if any, a 75kva transformer (480v -> 208v) may consume when there is no load on it? Is it worth it to turn the power into it off at night and on the weekends?

Forum Responses
(Dust Collection and Safety Equipment Forum)
From contributor L:
You should shut the transformer down any time you won't need it for a couple hours. It is very costly on the light bill, with a 75 KVA transformer left idling. A 3 phase 480 volt 75 KVA transformer would have 90 amps on the primary side (480) and would have 210 amps on the secondary side (208) if the volts are 208. The amps would be less on the secondary side if you are using 240 volts, then the amps on the secondary side would be about 181 amps. If you don't have the 75 KVA transformer yet, it would be better to get the size transformer that you will need. If you plan to use several large machines at the same time, you would need the 75 KVA transformer. If it's not being used, shut the monster off.

I just finished setting up a 240 volt 3 phase converter and a 15 KVA 480 volt Delta/Delta transformer in my shop.

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From contributor B:
Actually, the above analysis is not correct. The currents listed above are when the transformer is "loaded," actually transferring power to a load. At this point the transformer is about 90-95% efficient at transferring power to the load (i.e. if you actually had 75 KVA of load connected to the transformer, it would give off 3.75 to 7.5 kilowatts of power in the form of heat). However, when the transformer is connected to the primary voltage and there is no load on the secondary, the transformer will consume almost no power. I looked up some technical specification on dry-type distribution transformers in this size and found a no-load power consumption of about 150-200 watts. So at 24 hours per day and 10 cents/kilowatt-hr, cost to leave the transformer idling will be (0.2 kw x 0.1$/kw-hr x 24 hrs =) $0.48 per day. (Further, if you are using the transformer for half the day, the cost to leave it run overnight is about 24 cents).

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From contributor L:
Those figures were for a 75 KVA transformer under load. That is a lot of transformer. I have talked to several electricians that install transformers and they all state to turn them off when not in use. My guess is a 75 KVA transformer will cost you more than a few pennies to leave on all night. We haven't even talked about safety. Would you feel safe with that 75 KVA transformer being on and no one being around, if something should go wrong with it? I sure wouldn't.

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From contributor C:
Contributor L, I have been running a 75kva transformer in my shop for 5 years. The only time it has ever been off is during a power outage, or if we need to hook up a new disconnect to the line. As far as cost, I agree it costs a bit more to leave on 24/7, maybe a dollar a day is what my electrical company rep told me.

However, the in-rush spike from turning on the transformer is far more expensive than letting it sit and idle. A 75kva transformer is about the same size as an 80HP motor. I know my bill will show turning on an 80HP motor a couple times a day without a soft start on it, which you can't use on a transformer.

I'm not sure what your safety concerns are. As long as the transformer is in good shape, wired correctly with proper guage wire, and you aren't emptying the dust collector into it, there aren't really any safety issues. Keep it clear so it can breathe and walk away.

To the original questioner: find your nearest electrical recycling center and talk to them. You can buy a used transformer that may not be shiny anymore for a lot less money than new.

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From the original questioner:
Thanks. Below is the e-mail I received from Jeffersonelectric when I asked them about it:

"A transformer consumes power any time it is energized, even when there is no load on the secondary. The "No Load" losses on a 75 KVA transformer would be around 550 - 600 watts. You can turn the unit off when you are not using it, but you have to make sure the fuses or CB can handle the in-rush when you power the unit up."

Using that info, it looks like I could save 25-30 dollars each per month to shut them off nights and weekends.