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Old 09-06-2008, 05:54 PM   #1
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connecting a 240v A/C to my 480v box?


I need to connect an A/C unit that requires 240v single phase in my shop. I have a 480v 3phase breaker box and a 240 3phase breaker box. If I run it on the 240 box, it will only see 208v, so can I connect it to the 480v box between one of the hot leads and neutral?

thanks

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Old 09-06-2008, 05:56 PM   #2
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connecting a 240v A/C to my 480v box?


Honestly, what do you think?

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Old 09-06-2008, 06:11 PM   #3
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connecting a 240v A/C to my 480v box?


208V/240V is the manufacturers markings ...208V could read 215V easy on 2 legs to your unit....one leg of your 480V is 277V with a neutral...thats 37Vs above your 240Vstay with the 240V and run 2 legs over and a green ground 3rd wire
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Old 09-06-2008, 06:22 PM   #4
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connecting a 240v A/C to my 480v box?


3 phase systems come in basically 2 types, wye and delta. The wye is almost always 4 wire, the delta can be 4 wire, or 3 wire with one phase grounded.

Most likely you have a 4 wire wye system, in which case one leg to neutral is 277 volts. This would be too high for a 240 volt unit. If your system is a 4 wire delta, then 2 of the legs are 240 to neutral, the other is 416. In this case, using one of the 240 volt legs and the neutral is OK.

It is very rare that a unit like this will not run on 208. Most of these have a voltage rating of 208/240. Some of them also have a current rating at both voltages, such as 13/11 amps.

In case the unit absolutely must have 240 volts, a buck-boost transformer can easily be installed. You can either buck some of the 277 off, or boost the 208 up. To properly size such a transformer, we'll need to know the amperage of the load, and whether or not it needs a neutral. I don't think I've ever seen an A/C unit that needed a neutral.

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Old 09-06-2008, 06:32 PM   #5
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connecting a 240v A/C to my 480v box?


There are two delta 4 wires that are 3phase 240. Phase to phase is 240 volts. Phase A and C to neutral is 120 volts and phase B to neutral is 208 volts. Phase B being the high leg.

To my knowledge there are no 240 volt three phase configurations in the USA that are 208 phase to phase.

Quote:
so can I connect it to the 480v box between one of the hot leads and neutral?
Nope... 240 volt single phase is a phase to phase circuit in the USA no neutral. If you have 4 wires then a phase to neutral is going to get you 277 volts.
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Old 09-06-2008, 06:52 PM   #6
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connecting a 240v A/C to my 480v box?


Quote:
Honestly, what do you think?



Quote:
on the 240 box, it will only see 208v
What happened to the other 32V?


In MOST cases, 208/240...whatever it takes

Your 480 panel is 277 to ground and 480 phase to phase.

Your other panel is 120 to ground and 208 between phases
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Old 09-06-2008, 06:55 PM   #7
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connecting a 240v A/C to my 480v box?


Thanks for all the info and quick replies!

My 240v breaker box has black, red, blue, and neutral white. If I measure across the colors, I get 208, not 240.

My 480v box has black, red, blue, and neutral white. For a 3phase system, I thought each leg would be 240v in relation to neutral, and I could use one leg for the 240v air conditioner.
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Old 09-06-2008, 07:11 PM   #8
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connecting a 240v A/C to my 480v box?


Your '240 box' is actually a 120/208 3 phase 4 wire wye system. As you know, there isn't 240 volts available here.

If the 480 volt box is a 4 wire wye, the phase to neutral voltage will be 277. If it is a 4 wire delta, the phase to neutral will be 240 on 2 of the legs, 416 on the other.

If there are single pole breakers on all 3 legs, it is a wye. If there are single pole breakers on 2 legs, but one leg has only 2 pole or 3 pole breakers on it, it's likely a delta. If the POCO meter says '277V' somewhere on it, it's a wye. If it says '480v', it's a delta.

You can measure voltage in the 480 panel, but be careful. 120 out of control goes 'pop'. 480 goes 'BLAM!!'

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Old 09-06-2008, 07:39 PM   #9
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connecting a 240v A/C to my 480v box?


Quote:
I thought each leg would be 240v in relation to neutral

That is a logical assumption.

120 plus 120 = 240....sometimes.

Other times 120 plus 120 = 208.

277 plus 277 = 480.

Don't blame me, I didn't invent the stuff.
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Old 09-06-2008, 07:59 PM   #10
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connecting a 240v A/C to my 480v box?


Ah...got it now. With three phase, it's a division factor of sqrt of 3, so a 480 panel wye has 277v phase to neutral. Thanks for clearing that up.
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Old 09-06-2008, 08:31 PM   #11
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connecting a 240v A/C to my 480v box?


Is there an easy way to step down the 277v to 240v? The ac unit is rated at 240v and 20A.

thanks
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Old 09-06-2008, 08:39 PM   #12
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connecting a 240v A/C to my 480v box?


Someone please tell me how your going to operate a 240 volt single phase a/c on 277 volts phase to neutral?

EDIT: Just saw post #4

Last edited by Stubbie; 09-06-2008 at 08:53 PM.
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Old 09-06-2008, 08:46 PM   #13
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connecting a 240v A/C to my 480v box?


Quote:
Originally Posted by micromind View Post
It is very rare that a unit like this will not run on 208. Most of these have a voltage rating of 208/240. Some of them also have a current rating at both voltages, such as 13/11 amps.
I agree.

Auto123, have you checked the motor nameplate to verify it can't be run on 208v?
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Old 09-06-2008, 09:06 PM   #14
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connecting a 240v A/C to my 480v box?


The a/c unit does run on 208v. It's connected now. However, when it kicks on, it causes quite a voltage drop. Enough to effect some other equipment, which turn off because of the a/c starting. Those pieces of equipment can be run at either 120 or 208/240, and are currently set up on 120. Maybe I should set them up to run on 208/240?

thanks
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Old 09-06-2008, 10:36 PM   #15
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connecting a 240v A/C to my 480v box?


What kind of place is this?

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