Step Up Transformer (design check please)

busman

busman

Senior Member
Location
Northern Virginia
Occupation
Master Electrician / Electrical Engineer
All,

I've never had to design a step-up transformer before. If you would, please give me a quick sanity check on what is below. The service here is 208/120V Wye with plenty of capacity for this chiller. Chiller is only available at 480V. The 350A CB is already there (left over from a former 208V chiller). This existing CB is not adjustable trip. I have a small concern that it might trip on inrush of the transformer, but I have an adjustable trip unit I could swap for it. Any comments would be greatly appreciated.

Thanks much,

Mark

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The #4/0 primary conductors would need to be the same size as the OCPD or sized by applying the next size up rule. The GEC could be the same size as the #6 SSBJ feeding the secondary circuit breaker. Although not depicted the SBJ could also be #6.
 
infinity said:
The #4/0 primary conductors would need to be the same size as the OCPD or sized by applying the next size up rule. The GEC could be the same size as the #6 SSBJ feeding the secondary circuit breaker. Although not depicted the SBJ could also be #6.
Click to expand...
Infinity,

Thanks. The primary and secondary conductors will be a combined length of 25' or less (sorry, that was in the notes, but missed the snapshot). Using 240.21(B)(3) (2014 NEC) am I good? I made the GEC larger for resistance to physical damage.

Thanks so much,

Mark
 
Whenever possible purchase a transformer designed as 208 Delta to 480Y/277.

The taps will work correctly.
The inrush will have been optimized.
You will have a true neutral point for grounding the output.
 
jim dungar said:
Whenever possible purchase a transformer designed as 208 Delta to 480Y/277.

The taps will work correctly.
The inrush will have been optimized.
You will have a true neutral point for grounding the output.
Click to expand...
Yes, I specified explicitly in the notes that this be designed as a step-up transformer. With that said, if you had specific language you would recommend for the Note, I would love to have it. As I said, this is my first one-line diagram with a step-up transformer. What I had was, "Transformer C1 shall be designed specifically for use in voltage step-up applications."

I have an adjustable trip breaker in case I need to fix inrush.

Yes. This is a steel tower with ground bars at each level connected to the GES.

Thanks so much,

Mark
 
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I think your language is sufficent. You could add a specific prohibition against 'reverse fed' units, but why get wordy.
 
IMHO you already specify that the transformer is 208V delta - 480/277V wye. That makes it pretty clear that the 208V side is the primary.

You might want to specify the primary taps available, but unless the site has known supply weirdness (voltage is always > 5% high for example) you would only want to specify a very common tap arrangement so that you don't get caught paying for a custom transformer.

See https://americas.hammondpowersoluti...hnical-articles/transformer-tap-specification for a discussion of taps and the possible arrangements for your transformer.
 
winnie said:
IMHO you already specify that the transformer is 208V delta - 480/277V wye. That makes it pretty clear that the 208V side is the primary.

You might want to specify the primary taps available, but unless the site has known supply weirdness (voltage is always > 5% high for example) you would only want to specify a very common tap arrangement so that you don't get caught paying for a custom transformer.

See https://americas.hammondpowersoluti...hnical-articles/transformer-tap-specification for a discussion of taps and the possible arrangements for your transformer.
Click to expand...
Thanks. This site is supplied by a dedication 12,470V feeder and 500 kVA transformer, so voltage fluctuations has never been an issue, but it's a good thought and I will specify taps.

Mark
 
The breaker that you are specifying does have an adjustable magnetic trip, or is the existing too old? so you should be good there.
 

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