480 VAC Transformer Question
480 VAC Transformer Question
(OP)
I'm installing controls for a treatment facility. The power company has a bank of transformers that they said they are going to connect in a 480 VAC 3-wire delta to me.
I have a 45KVA 480 by 120/208Y transformer that I wish to connect to their supply. How would I create the neutral if the power company is feeding 3-wire? Would it meet code if I were to drive some ground rods and use them as VAC ground reference? I've grounded buildings before but without a transformer.
Thanks everyone.
striped.bass@erols.com
I have a 45KVA 480 by 120/208Y transformer that I wish to connect to their supply. How would I create the neutral if the power company is feeding 3-wire? Would it meet code if I were to drive some ground rods and use them as VAC ground reference? I've grounded buildings before but without a transformer.
Thanks everyone.
striped.bass@erols.com





RE: 480 VAC Transformer Question
RE: 480 VAC Transformer Question
1. The neutral of 208V/120V 3-phase 4-wire system shall have a solidly grounded neutral to some suitable grounding rods, water pipes in front of water meter (or provide jumper across the watermeter) according to NFPA 70-1999 National Electric Code.
2. Consult the transformer manufacturer for suitable protective devices. Else, stay on a very conservative side with your own selected protective devices, e.g.
Iprimaryfuse=45000VA/(3**0.5 x 480V)=54.1Amps ~ 50A fuses (or 3phase 600VAC circuit breaker) on the primary and
Isecondaryfuse=45000VA/(3**0.5 x 208V)=124.9Amps ~ 125A fuses (or 3phase ~250VAC main circuit breaker) on the secondary side. A 125A 208V/120V 3phase 4wire distribution panel with 30 to 42 circuit breaker spaces could be used, e.g. by Square D, General Electric, Cutler-Hammer, Siemens, etc.
RE: 480 VAC Transformer Question
I would think that good practice would be to use ground conductors connected between distribution transformers, distribution panels, and utilization equipment. This system of ground conductors may be connected to one grounding point (at the transformer wye) or to many depending on the size of the system.
For overcurrent protection you should use fuses or circuit breakers on the primary and secondary of the transformer and you should also consider ground fault protection on the secondary side. Finally, be sure that the primary overcurrent protective device is rated to interrupt the short circuit capacity of the utilities transformer bank.
RE: 480 VAC Transformer Question
RE: 480 VAC Transformer Question
"The system neutral will connect to the transformer wye and should be grounded."
I apologize for any confusion caused by not specifically pointing out that the above mentioned "system neutral" would have to be on the secondary side of the referenced delta-wye stepdown transformer. Thank you for the opportunity to clarify since anyone acting on my post based on your interpretation of what was presented would be in for a nasty surprise.
RE: 480 VAC Transformer Question
RE: 480 VAC Transformer Question
RE: 480 VAC Transformer Question
RE: 480 VAC Transformer Question
RE: 480 VAC Transformer Question
NEC Art. 450-2(b)(2) allows the following for a transformer which has primary & secondary protective devices -
- Secondary - Fuse or CB rated at not more than 125% (or nearest higher standard rating) of transformer full load current
- Primary - with secondary protection as above, primary overcurrent protection may be rated up to 250% of the transformer full load current; individual circuit protection is not required in this case
Using the above criteria -
- Secondary protection rating not greater than 124.9*1.25 = 156A; Select 150A Fuse or CB
- Primary protection rating not greater than 54.1*2.5 = 135A; Select 90A fuse or CB
I checked the coordination of the above combination (using Buss FRS-R Fusetron fuse curves) and the primary & secondary fuses will operate selectively. A 50A primary/125A secondary combination definitely doesn't coordinate - the primary fuse will blow ahead of the secondary fuse for secondary faults. Depending on the fuse type selected, there may also be issues with transformer inrush current for the 50A fuse.
The bottom line of the above is to remember that the overcurrent protection is fault protection for the transformer circuit, NOT overload protection. If you want close overload protection, you probably need to look at winding temeperature monitoring for alarm and trip.
RE: 480 VAC Transformer Question
RE: 480 VAC Transformer Question
The code allows primary (not secondary) protection sized up to 150A (135.3 A, or next larger standard fuse or breaker), where the secondary is protected with a device rated at 125% or less of rated current.
I chose 90A in order to get closer protection, although any of the sizes that you mention, plus 150A, could also be selected.
RE: 480 VAC Transformer Question