Transformer protection

[mgtack]

I have a client that wants to install a 3 phase 75 KVA transformer back fed for a few receptacles. They currently have a 208 service but need a 50 amp 3pole, 4 wire 480 volt receptacle and a 30 amp 3 pole, 4 wire 480 volt receptacle. They may want future receptacles but that remains to be seen. My questions is how do I size the breaker and wire on the primary and secondary side if it's a step up transformer? I heard there are problems with tripping. Do i need an adjustable trip breaker? Can someone show me the calculation for both?

 

[Jljohnson]

A couple of items to point out for you....
1) I suggest you order the correct transformer for this application and do not use a standard 480 primary-208/120 secondary transformer and backfeed it. XO can cause some major problems if bonded to building steel or water pipe in a "backfed" situation. We tend to be creatures of habit and when we see XO, we want to bond it in the worst way
2) The incoming feed to a transformer is always referred to as the "primary" the outgoing is the "secondary". With that in mind, your primary volyage would be 208, secondary either 480 or 480/277, depending on whether the 4-wire that you refer to is a neutral conductor or an equipment grounding conductor.
3) Overcurent protection and feeder size would be calculated per article 450, just like any other transformer, just using your lower voltage as the primary and your higher voltage as the secondary

 

[augie47]

As Jim noted ExI=W works no matter if the transformer is fed normal or back fed.
Reference Art 450.3(B) for transformer protection, Art 240.21(C) for conductor protection and Art 250.30 for grounding.
You need to keep inrush in mind. I would think perhaps a 250 amp or larger breaker for your primary (208) and a 100 amp for your secondary

 

[JDBrown]

1) I suggest you order the correct transformer for this application and do not use a standard 480 primary-208/120 secondary transformer and backfeed it.

I'll second that suggestion. We ran into some trouble a while back with an AHJ because of a transformer's UL listing or manufacturer's instructions (I forget which). At any rate, the Building Dept. wouldn't approve the design because we couldn't find a suitable transformer listed for 120-277V, rather than 277-120V. Luckily, this happened during the plan check phase, not after equipment had been purchased. It's been my experience that some AHJs are picky like that and some aren't, but it never hurts to check before you place your purchase order. And if the Inspector tells you back-feeding will be fine, you may want to get it in writing. If I had a nickel for every time a Plan Checker or an Inspector suddenly developed amnesia...

 

[templdl]

I have a client that wants to install a 3 phase 75 KVA transformer back fed for a few receptacles. They currently have a 208 service but need a 50 amp 3pole, 4 wire 480 volt receptacle and a 30 amp 3 pole, 4 wire 480 volt receptacle. They may want future receptacles but that remains to be seen. My questions is how do I size the breaker and wire on the primary and secondary side if it's a step up transformer? I heard there are problems with tripping. Do i need an adjustable trip breaker? Can someone show me the calculation for both?

If I am to understand you correctly you presently have a 75kva, 480delta-208y/120 and want to feed it with 208, 3ph, 3w which you can do by simply connecting the 208v 3ph to the X1, X2, and X3 and omit the neutral bringing the EGC along and bonding that to the enclosure. Make sure that the X0 connection of the transformer is not connected to anything and left floating.
That leaves the 480v side which is a delta with H1, H2, and H3 and that's where the problem is. The is no "wye" to connect to. All that you have is a 480v delta. If all that is needed is 480v 3ph and you don't need 277v you mat not require 480y/277v. If 480v 3ph is acceptable then you must consider how to ground it which is to use a corner grounded 'B' phase. As such you have to pat close attention to how a grounded 'B' phase panel is applied, that the 'B' conductor is a grounded conductor and your EGC is brought out as a EGC you normally would.

 

[mgtack]

Much Thanks

Much Thanks

This really helps. I'm not clear on the corner ground. On the secondary side they only want a panel to feed 3 phase 4 wire receptacles. Three ungrounded conductors and a ground. So to be clear, on the primary side, it would be fed with 3 ungrounded conductors H1,H2,H3 with no neutral to XO and a ground. On the secondary side, X1, X2, X3 and the ground would feed the new panel. The ground on the primary side would bond to the transformer casing and then continue on to the new panel. Does it also need to bond to the B phase on the secondary side? I recently saw a transformer that was back fed without grounding any phases for an a/c unit.

 

[mgtack]

Calculations

Calculations

So, for a 75 KVA transformer: 75000 / (208 x 1.732) = 208 amps
208 amps x 1.25 = 260 amps OCPD
Next step up = 300 amp breaker

So primary conductors must be 350 kcmil

Secondary side:
75000 / (480 x 1.732) = 90 amps
secondary conductors #3

Did I do this correctly?

 

[templdl]

This really helps. I'm not clear on the corner ground. On the secondary side they only want a panel to feed 3 phase 4 wire receptacles. Three ungrounded conductors and a ground. So to be clear, on the primary side, it would be fed with 3 ungrounded conductors H1,H2,H3 with no neutral to XO and a ground. On the secondary side, X1, X2, X3 and the ground would feed the new panel. The ground on the primary side would bond to the transformer casing and then continue on to the new panel. Does it also need to bond to the B phase on the secondary side? I recently saw a transformer that was back fed without grounding any phases for an a/c unit.

You avoided responding to if 277v was required or not as noted in your response. And, yes, with a delta other is no neutral. And more likely than not you would want a grounded 'B' phase. And, yes, the EGC would originate where the 'B" phase/is grounded. You must then be familiar with how to order the correct panel and connect the A, B, and C phases and their insulation colors. The EGC will be no different than a normal system.

You imply 3ph4w which would actually be 480y/277 which you won't get from a 480v 3ph3w transformer secondary.
And just because you imply that you have an EGC doesn't mean that you have 3ph4w.
From the question that you have I am concerned about your knowledge regarding such an installation. However I would strongly recommend that contact someone locally that would be able to provide you with assistance. Should you miss interpret something as we described the results could be disastrous.

 

[mgtack]

Tempdld

Tempdld

Sorry to be vague. It is not on purpose. 277 volts is not needed. The application is this; the customer has a couple of machines that use 3 pole, 4 wire outlets at 480 volt. (3) hots and a ground. There will be no other applications, they will not be using for other than the machines mentioned. Does there need to be a corner ground to the B phase? What is the advantage of doing this? As mentioned before, I have seen this on a previous system and it signed off on by the AHJ and an engineer. I know that does not mean it is correct.
Thanks for the help.

 

[augie47]

If the 480 side is indeed "Delta" with no neutral terminal, you can operate it as an "ungrounded" system but you would need to install a ground detector as noted in 250.21(B) or you can ground "B" phase and operate as a grounded system.
IN either event you need to establish a grounding electrode system per 250.30.

I would second templdl's comment concerning working with someone experienced with this arrangement since a misstep could be disastrous as he mentions.