480 Delta calculations

[Chrism308]

I have worked with all type voltages except 480 delta and would like some input. I have a electrical service with 480v 3 wire Delta which is feeding several busways and IFS panels, not that familiar with the 480 delta. The IFS panels will be fed with 480v 3 wire and have integrated transformers for 277v buss and 208/120v buss. My question is when calculating the loads on the 480v delta, will the VA for a phase be VA/480?
If a air rotation unit is 41.2A, would it be 19,776VA per phase?
In the IFS panels, the 480v is feeding transformers for 277v and 208/120v buss. Wouldn't those VA loads be divided by 480 also to get amps per phase?
I think my real question is when all loads are fed from the 480v delta, is the phase voltage 480v to ground.
Thanks for any input

 

[winnie]

Calculations for 480V delta are the same as calculations for 480/277V wye.

The relation between amps and VA for a load on a delta system is VA/V/sqrt(3), just as you would use for a load on a wye system.

In a 480V delta system the voltage to ground may be undefined (ungrounded), in which case it will generally 'float' to roughly 277V phase to ground, or it may be corner grounded (480V to ground on 2 phases, 0V to ground on the 3rd), or it may be 'high leg' (240V to ground on 2 phases, 416V to ground on the 3rd). But in all cases the VA to A calculation is the same.

For purposes of approximating the VA assigned to each phase, just use VA/3 for 3 phase loads and VA/2 for single phase loads. This is only an approximation, for an accurate calculation you need to know the phase angle of each load and do a bunch of vector math...not usually worth the effort.

-Jon

 

[kwired]

I have worked with all type voltages except 480 delta and would like some input. I have a electrical service with 480v 3 wire Delta which is feeding several busways and IFS panels, not that familiar with the 480 delta. The IFS panels will be fed with 480v 3 wire and have integrated transformers for 277v buss and 208/120v buss. My question is when calculating the loads on the 480v delta, will the VA for a phase be VA/480?
If a air rotation unit is 41.2A, would it be 19,776VA per phase?
In the IFS panels, the 480v is feeding transformers for 277v and 208/120v buss. Wouldn't those VA loads be divided by 480 also to get amps per phase?
I think my real question is when all loads are fed from the 480v delta, is the phase voltage 480v to ground.
Thanks for any input

Air rotation unit? if it is a three phase motor drawing 41.2 Amps @ 480 volts the total VA is 480 x 41.2 x 1.732 = 34.252 kVA. If it is single phase 480 volts @ 41.2 amps then total VA is 19.776 kVA, but is across two legs instead of all three.

If you have 208/120 bus - then you must have a three phase transformer deriving it, loading of that is no different then any other 480 x 208/120 transformer. The 277 bus? If it is only a two wire 277 volt system then you have a single phase transformer with 277 secondary. Again total connected VA is only on two of the primary legs. If it is a separately derived three phase system then you will have 480/277 on the secondary of that system and loading is same as loading of the 208/120 system just at a different volt level.

Add: is physically possible to derive a 277 volt neutral with an auto transformer but you create other code issues because you can't ground that neutral if one of the phase conductors is already grounded - that would be a short circuit between those two points.

 

[Chrism308]

Thanks for the input. I did calculate the 480v delta phases the same as 480/277v wye, just second guessing myself. It is 480v delta (no high leg) and all equipment is 3 phase on the 480v delta buss, also has breakers on this buss feeding the transformers for the 277 section and 208/120 section.
On panel loads I used the air rotation unit load at 41.2 amps - 480x41.2/1.73 = 11,434 per phase
With 5 air rotation units each phase total VA would be 57,170 and I divided by 277 for phase connected load at 206 amps
I was second guessing myself, wondering if the 57,170 VA should be divided by 480v on each phase.
By the responses it appears that I am correct by calculating the phase loads the same as I would a 480/277v wye