Delta/Wye to make a neutral

[scnkapc]

I have ran into a situation where the engineers fed some 480v Heat Trace Panels with an MDP that does not have a neutral. The heat trace cabling requires 277v with a neutral. The Heat Trace Panels are 225Amps. Can I use a delta/wye 100kva transformer to create a neutral. We would intercept the feed to the Heat Trace Panel and feed it into the transformer and then feed the panel with 480v 225a with a neutral. Is this feasible?

 

[ptonsparky]

Yes.

 

[don_resqcapt19]

What is the actual load? 225 amps at 480 is 187kVA.

 

[tortuga]

Is the 480 delta supply from a utility ?

 

[scnkapc]

No. The 480volt MDP is fed from local 480v switchgear inside the plant.
The actual demand load will vary depending on the heat trace cycling on and off.
In doing a quick calculation by only having OCPD on the primary side, it would be 208.33 amps with a OCPD of 300 amps and a 100kva delta/wye transformer should be sufficient.

 

[D. Castor]

Yes, it's completely feasible. I haven't evaluate your transformer sizing. It's just like sizing any other transformer.

 

[jim dungar]

100kva delta/wye transformer should be sufficient.

I think 112.5kVA is the standard power transformer size for 3-phase.
But shop around, there are some Drive Isolation Transformers in non-standard sizes.

 

[winnie]

Careful there.

Code allows you to have primary OCPD significantly greater than the transformer primary current rating. This doesn't mean that code magically increases the actual current rating of the transformer, just that the OCPD has to be capable of starting the transformer, including transformer inrush current.

A 100 kVA 480:480/277V transformer has a current rating of 120A. You can have a primary OCPD of 300A (2.5 * 120) along with secondary OCPD of 150A (1.25 * 120) but you really shouldn't be loading that transformer more than 120A. (Yes, transformers can tolerate overload, and AFAIK there isn't a code requirement to not overload a transformer.)

The actual load of your 208A of heat trace may be far less than 208A, given that it cycles on and off. But I think you need to evaluate the actually load more carefully.

 

[scnkapc]

Would we be better off using a 112.5kva or a 125kva transformer?

 

[winnie]

You will be better off using a standard transformer than something custom: 75, 112.5, 150 or 225 kVA.

The size you need will depend on a more careful load calculation. If you don't want to think about it you need 225 kVA. (You said that you have 208A at 480V = 173 kVA but that is all of your heat trace 100% on, which we both agree is likely not the case.)

I don't know how much actual load you have and can't help you with that calculation.

 

[jim dungar]

You will be better off using a standard transformer than something custom: 75, 112.5, 150 or 225 kVA.

But some DIT transformers may come in non-traditional sizes as off the shelf products.

This is a common issue, where designers may not be aware that Single Phase and Three Phase transformers have few sizes in common.

 

[augie47]

You have likely done so but if not I would suggest you check the actual utility voltage available. A feed from another existing 277 source might be more economic

 

[kwired]

Is this a delta source or is it a wye source but you just don't have a neutral at the panel where these will be fed from?

Seems it possibly may be much less cost to pull a neutral if there is one that can be extended (not knowing what factors may be present that maybe makes this rather difficult to do). Or depending on what POCO may or may not be willing to do for you might even be less cost to change to wye source if it is currently delta?

 

[tortuga]

Is this a delta source or is it a wye source but you just don't have a neutral at the panel where these will be fed from?

Seems it possibly may be much less cost to pull a neutral if there is one that can be extended (not knowing what factors may be present that maybe makes this rather difficult to do). Or depending on what POCO may or may not be willing to do for you might even be less cost to change to wye source if it is currently delta?

Yeah thats where I was going also. Utilities here prefer the wye setup, the last 480 delta service I worked on got changed for free on the utility side to a grounded wye. Its not super simple or cheap, but the pencil pushers ran the numbers and it penciled out, especially when you figure the 24/7 vampire load losses on the larger transformers.

 

[scnkapc]

Thank you all for your feedback, I appreciate it.

 

[Deltaforce]

I don't know how much actual load you have and can't help you with that calculation.

But for transformer size, 100% heat trace panel connected load( non-continuous operation), 125%(continuous operation)

 

[Deltaforce]

As no industry accepted standard diversity factors above

 

[jim dungar]

But for transformer size, 100% heat trace panel connected load( non-continuous operation), 125%(continuous operation)

But just because someone chose a 225A panel does not tell us anything about the loading on the transformer.
The NEC does not provide guidance for the actual sizing of transformers and motors. You can make assumptions based on OCPD sizing, but those are based on the output of the transformer not its nameplate KVA.

 

[Deltaforce]

But just because someone chose a 225A panel does not tell us anything about the loading on the transformer.

Panel connected load know

You can make assumptions based on OCPD sizing, but those are based on the output of the transformer not its nameplate KVA.p

But ocpd interupt rating not leas transformer fault current rating

 

[jim dungar]

But ocpd interupt rating not leas transformer fault current rating

Which has nothing to do with the sizing of OCPDs, it only affects their AIC.

Panel connected load know

Which was not given by the OP and cannot be inferred by anything except the OP comment about using a 100kVA transformer.