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Old 05-14-2012, 09:41 PM   #1
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Convert 277vac to 220vac / Old transformer died


I have a building that previous owner took two leads off 480 3 ph and ran it thru underground conduit to an out building to avoid a permit hassle.
I re-tested voltage again today and it was 275.8 vac. In junctions box I have a white, black and ground wire....the white and black test to 275.8 volts. I did not test white or black to ground.

The 277 was hooked into a GE transformer #9T21B9105 which is a 480 primary to 240, etc single phase transformer with multiple taps. The transformer has died (the wires all melted together and out voltage dropped to 40 vac) and I am looking for a lower cost (replacement GE unit is $2,200) to power a 220vac, single phase 60 amp panel. Panel is used to run lights, plugs...nothing heavy.

This setup was done by a licensed electrician who has since moved away and he brought in this used 15kv transformer that he acquired for $300.

Is there a lower cost transformer that I can buy used to get this building back up. I am looking to get the transformer and then bring in an electrician for final installation.

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Old 05-14-2012, 09:43 PM   #2
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Convert 277vac to 220vac / Old transformer died


Call an electrician. Let him find the parts. This is not something to mess around with.

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Old 05-14-2012, 10:15 PM   #3
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Convert 277vac to 220vac / Old transformer died


Are you sure that the feed is two phases at 480 volts?

Go measure the phase to ground voltages (black to ground and white to ground)
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Old 05-14-2012, 10:36 PM   #4
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Convert 277vac to 220vac / Old transformer died


Wow, I agree with KBuz, get a pro. This installation is not kosher whatsoever and needs re-vamped to code, talk about phase imbalance. I wonder how the stepped down voltage from the transformer in the existing service is going to get a neutral to a single phase 220 vac 60 amp panel ?
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Old 05-14-2012, 10:45 PM   #5
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Convert 277vac to 220vac / Old transformer died


not kosher, but hey, but at least the installation avoided the dreaded "permit hassle"
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Old 05-14-2012, 11:49 PM   #6
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Convert 277vac to 220vac / Old transformer died


This is a bit confusing; two leads from a 480 3 source will give you 480, not 277.

277 is one phase of a 480 3 wye system to neutral/ground.

If the original set-up was indeed two phases of a 480 system (seems like it with a 480 transformer), there would be 480 between black and white and 277 between black and ground and 277 between white and ground.

Before investing in a transformer, you need to verify voltage black to white, black to green and white to green.

It sounds a lot like either the black or the white wire is broken somewhere.

It's common to use two legs of a 480 system to power a transformer that feeds a 120/240 single phase panel.

If the 480 wires are #12s, a 10 KVA transformer would be my choice. The 480 side is 20.8 amps and the 120/240 side is 41.6 amps. If they're #10s, a 15 KVA unit would be 31.2 amps on the 480 side and 62.5 amps on the 120/240 side.

Rob
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Old 05-15-2012, 06:32 AM   #7
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Convert 277vac to 220vac / Old transformer died


Quote:
Originally Posted by Jeeps View Post
Wow, I agree with KBuz, get a pro. This installation is not kosher whatsoever and needs re-vamped to code, talk about phase imbalance. I wonder how the stepped down voltage from the transformer in the existing service is going to get a neutral to a single phase 220 vac 60 amp panel ?
You get 120/240 volts from a transformer (of appropriate voltage rating or turn ratio) with a center tapped secondary (output), regardless of the primary (input) voltage. The panel neutral is connected to the transformer center tap. Either the panel neutral or the transformer center tap (not both) is bonded to the ground(ing electrode) system.

While you could configure the underground line to go with either a 277 volt to 120/240 volt transformer or a 480 volt to 120/240 volt transformer, the 480 volt system would be more efficient for a longer distance because a smaller percentage of energy would be lost due to voltage drop for a given set of wires running between the buildings.
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Last edited by AllanJ; 05-15-2012 at 01:17 PM.
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Old 05-15-2012, 11:08 AM   #8
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Convert 277vac to 220vac / Old transformer died


Quote:
Originally Posted by k_buz
Call an electrician. Let him find the parts. This is not something to mess around with.
I agree with k_buz. Hire a pro.
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Old 05-29-2012, 10:46 PM   #9
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Convert 277vac to 220vac / Old transformer died


UPDATE.....I called an electrician out to assess the situation and offer guidance. He ran a couple of tests and told me right away that I had a broken wire to outbuilding.
I found out the previous electrician buried a 10-2 SEU wire about 3" under as he didn't want to dig. I abandoned the old wire and trenched a 14" deep cut from power to outbuilding. I then ran 1" schedule 80 PVC conduit with three #8 wires
and electrician will be back out tomorrow to make final connections on each end.

Thanks for the recommendations as I was over my head and would have screwed up big time. We do have 480vac at the source and will be powering up a GE transformer (that had not died as originally thought) that will step power down to 240vac to power up a 60amp panel. Should be up and running tomorrow if electrician shows as promised.
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Old 05-29-2012, 11:51 PM   #10
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Convert 277vac to 220vac / Old transformer died


Quote:
Originally Posted by micromind View Post
This is a bit confusing; two leads from a 480 3 source will give you 480, not 277.

277 is one phase of a 480 3 wye system to neutral/ground.

If the original set-up was indeed two phases of a 480 system (seems like it with a 480 transformer), there would be 480 between black and white and 277 between black and ground and 277 between white and ground.

Before investing in a transformer, you need to verify voltage black to white, black to green and white to green.

It sounds a lot like either the black or the white wire is broken somewhere.

It's common to use two legs of a 480 system to power a transformer that feeds a 120/240 single phase panel.

If the 480 wires are #12s, a 10 KVA transformer would be my choice. The 480 side is 20.8 amps and the 120/240 side is 41.6 amps. If they're #10s, a 15 KVA unit would be 31.2 amps on the 480 side and 62.5 amps on the 120/240 side.

Rob
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