Wire sizing for a 3 phase step up transformer

M

mchmela

Member
Location
Florida
Occupation
Electrician
Im on a project right now where carrier sent us the wrong air handlers (should've been 208v and the sent 460 volt) and our company's solution is to install a step up transformer. The A/H is only 15A and my foreman had us wire them in 12 AWG in and out. When I looked up the calculations and sized it myself it looks like we should be running minimum 6 AWG.

Since this was an unexpected scenario we're just submitting an as built for engineering. Can anybody shed some light on this and let me know if we're going to haVe to change the wire size?

For additional information they purchased 15kva step up transformers and the wire lengths are less than 10ft.
 
Assuming 3 phase power, the full load current of a 15kVA transformer at 208 volts is 41.6 amps. I would be looking at a 50 amp OCPD with 8AWG for the primary, based on the assumption that everything is rated 75°C.

Is the 480 volt secondary wye or delta.? If delta you will have to run it as a corner grounded system or install ground fault detection.
 
We actually have a fused disconnect before and after the transformers both fused at 15A and for grounding we're grounding it to building steel. These air handlers don't have a neutral so we don't have one in the transformers.
 
mchmela said:
These air handlers don't have a neutral so we don't have one in the transformers.
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These transformers will be separately derived systems. As others have said, their secondaries will need to be grounded or a ground detection scheme needs to be employed.
Are there "electronics" associated with these air handlers?
 
mchmela said:
They're solely for the AHUs
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But do the AHUs contain drives? Are there controlling electronics which might contain filters connected L-G? These tend not to like corner grounded and ungrounded systems.

None of this is an issue, if the transformers are connected 480Y/277.
 
mchmela said:
We actually have a fused disconnect before and after the transformers both fused at 15A and for grounding we're grounding it to building steel. These air handlers don't have a neutral so we don't have one in the transformers.
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First off; 15A on the 208V side isn't going to cut it, you are likely going to clear those fuses the instant the 15kVA transformer is powered up... You need a 60A disconnect with AT LEAST 50A fuses.

"Grounding it to the building steel" doesn't address the Code requirement for EITHER a corner rounded Delta or a Ground Fault monitoring system on the 480V side as mentioned by Don, and the VFD issue mentioned by Zbang is important too. The AHU might have built-in VFDs, you need to check because if so you might want to just get a transformer with a 480/277V "primary" even though you don't need to use the neutral.

And DO NOT ground the X0 terminal on the 208V side, regardless of what you do on the other side..
 
They're delta configured on the secondary windings. The AHU may have a VFD but I'm not that familiar with their equipment. Since this was the solution my managers came up with im assuming it's the correct set up but you know what they say about assumptions. We did not ground the X0 terminal, it's just empty. #12 seemed too small but since I didn't have the information at the time I couldn't dispute it.

As I said, my foreman was the one who made the wire size decisions but to be honest I'm not sure he knows the ins and outs of transformer wire sizing. I appreciate all the feedback from y'all. Mike Holts forum is a treasure trove when it comes to information.
 
mchmela said:
I guess it was faster to get these transformers than the new motors. The decisions this school makes baffles me.
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It is baffling to me too. There is no way I would accept the wrong equipment and have to buy transformers and deal with that cost not to mention the energy losses forever. Also sounds like they got the wrong transformers which is just bad.
 
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