750KVA transformer manufactured as step-down, submitted as step-up: accept or revise/resubmit

The OP says this.


. If the contractor has a transformer with a 208 V secondary and a 480/277 V primary, I don't see why that would be inappropriate to use back wired, assuming the manufacturer approves back wiring it.

if the 480 side is delta, than obviously it is not usable.
Right, but what are the chances of having a "step down" transformer with a wye primary and Delta secondary?
 

kwired

Electron manager
Location
NE Nebraska
seems unlikely but how else would the contractor ever get it to work right for this application?
OP needed step up, don't know if he actually needed to supply 277 volt loads. Can cause further complications though if supplying say a panel with "slash rated" 277/480 breakers in it, because he would need breakers rated at least straight 480, or if there is VFD or something that wasn't designed for ungrounded or corner grounded delta for input.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
I am not entirely sure what he means by that. If the contractor has a transformer with a 208 V secondary and a 480/277 V primary, I don't see why that would be inappropriate to use back wired, assuming the manufacturer approves back wiring it.

if the 480 side is delta, than obviously it is not usable.
I agree. We are ASSUMING that the use of the phrase " ...the same transformer configured as a step-down." is referring to a common 480 Delta primary to 208Y120 secondary, in which case all of the naysayers would be correct; that cannot provide 480Y277 when stepped up. But is that what he really submitted on? Because if it WAS a Y-Y transformer or a 480Y277 Primary to 208 Delta secondary, it would be fine (so long as the manufacturer provided documentation on it being suitable for step-up as per 450.11 (B) as Tom Baker said).

And that last point is crucial, not just because it's now part of the NEC, but because the REASON why it was added is because some transformer designs CANNOT be reverse fed without dangerous (to the transformer) consequences. The NEC is making in incumbent on the installer to assure that. The resubmittal should include that documentation (IMHO) and woudl further prove that he had selected the correct configuration.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
As the specifier you should have stipulated which was the winding arrangement on the HV and LV as well as if the neutral point is to be earthed.
I disagree.

The OP specified the kVA rating (750kVA) the primary voltage (208V) and the secondary voltage (480/277).

The most common way to achieve this is a delta:wye transformer. But perhaps the supplier has some weird beastie on the shelf that provides the required characteristics which is not a delta:wye transformer. As long as the transformer has the required characteristics then the OP should accept it.

However it is most likely the case that a 480 to 208V transformer 'in reverse' will not have the required characteristics. But something like a 208V wye:480V wye with a tertiary delta winding might be just fine 'in reverse'.

The OP specified something, the supplier came back with something that will likely not meet the specified requirements. At this point it is up to the supplier to demonstrate that what they want to use does in fact meet the reqirements.

-Jon
 
Thanks for all the responses, I think they were as helpful as any I've sought here in the past.

Conclusion:

What I hadn't mentioned was this was all for a temporary setup - a temporary 300T chiller. Since it was temporary, I was willing to cut the contractor some slack in the submittal. The mechanical contractor had informed us the 300T chiller was only available in 480 (460)V, and the facility only had 208V, hence the requirement for the step-up xfmr. The chiller itself only required delta power, but it had available single phase and single pole (277V) aux connections, which I wanted to use for the temporary heat tracing. Ultimately though, I lost that battle and they're running temporary 120V circuits about 200' from inside the building, down multiple corridors, through a temporary construction opening, to the temporary chiller, all for two heat trace circuits which could have been powered from right there. But, that's means and methods, so whatta ya do? I just said fine.

But, as a result of other conversations related to this matter, I lost confidence in the electrical contractor's prudence, capability and reliability, and took the hard line and required a submittal that met the design specifications. At that point (losing the heat trace "battle"), I wasn't about to go back and say, okay, never mind, delta:delta will work after all. Given all we've been through with this lowest bid contractor, I wouldn't have put it past them to come back at a later point and say they needed the 277 after all and try to scratch out a change order. Of course they'd have ultimately failed to win the change order, but with all the effort associated with contesting it, I certainly wouldn't have considered it a "win" for us.
 
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