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Transformer question 480 corner ground delta to 240 high leg, only need 240 single

gbent

Diamond
Joined
Mar 14, 2005
Location
Kansas
I need to install a transformer to get 240V. Supply is 480V corner ground delta. My question is how to properly ground X4. With X4 ungrounded, all three legs are 247 phase to phase, and 117V to ground. X4 shows 60V to ground. Is this just a floating voltage that will have no current if grounded?
 
The supply type is mostly irrelevant if you are installing a transformer to get your desired output.

The secondary of a transformer is a "separately derived system" with no inherent relation to any grounding system. You need to re-establish the ground in order to comply with NEC if you are installing a "branch circuit" off that secondary.

Pretty much any wire "can" be grounded to re-establish the ground on the secondary (to protect against the high voltage breaking through from the primary). With 240V, the two most popular ways to get it are:

1) with a straight 240 L-L delta transformer, corner grounding. This takes a special distribution box, or replacing one fuse with a short. You cannot have a fuse in the grounded conductor (neutral).

2) With a "high leg", "farm", or "lighting tap" transformer (all the same thing), one winding of the delta is center-tapped for grounding. That gives 240 V 3 phase, with the option of also getting 120-0-120V single phase from the tapped winding. The grounded point is a regular neutral for the 120/240 portion, but NOT a standard neutral for the 3 phase.

There is not going to be much availability of a 240V L-L wye transformer, because the voltage to the neutral is not a useful one (around 139VAC).
 
JST, thank you for your reply.

You wrote:

2) With a "high leg", "farm", or "lighting tap" transformer (all the same thing), one winding of the delta is center-tapped for grounding. That gives 240 V 3 phase, with the option of also getting 120-0-120V single phase from the tapped winding. The grounded point is a regular neutral for the 120/240 portion, but NOT a standard neutral for the 3 phase.

Will X4, the center tap, become the neutral? And should this then be bonded with the ground in one, and only one, location? What did you mean by "NOT a standard neutral for the 3 phase"?
 
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Will X4, the center tap, become the neutral? And should this then be bonded with the ground in one, and only one, location? What did you mean by "NOT a standard neutral for the 3 phase"?

The center tap on one winding becomes the ground point for the secondary, yes. It gets grounded to whatever NEC ground you have, building steel, the green wire of the 480 supply, a etc. Yes, pick ONE to connect it to, and bond THAT to the distribution breaker box for the transformer (if any).

It is NOT the neutral for the 3 phase because it does not have the same voltage to all the wires. It will be the neutral for the two wires at the ends of the tapped winding, with 120V to each, but will be about 200V from the "high leg" or "stinger" leg, which is conventionally wired with orange wire or ID to show what it is.

The option of a lighting tap exists for 240V 3 phase, but 208V naturally has a normal neutral, with 120V from that neutral to all phase wires. If you have the option of using 208, that could be a good move, because it is simple with less to remember.

But 208 is not always acceptable. Most 240V motors don't really "like it", although they may be able to accept it. Higher current for the same power, and less power than marked if you stay within the marked current. It's 13% less voltage than nominal 240V.
 
Thanks again JST, very much. I'll get it finished this morning.

Dumb question - You indicated you only need 240 single, if that's just 240V single phase why are you looking at three phase transformers at all? All you need is one of the extremely common 240/480 x 120/240 single phase transformers in the appropriate size. As noted it's a "separately derived system" so you just ground the center tap on the 120/240 side and be done with it. The 240/480 side you just configure for 480 and leave that center tap unconnected.
 
Dumb question - You indicated you only need 240 single, if that's just 240V single phase why are you looking at three phase transformers at all? All you need is one of the extremely common 240/480 x 120/240 single phase transformers in the appropriate size. As noted it's a "separately derived system" so you just ground the center tap on the 120/240 side and be done with it. The 240/480 side you just configure for 480 and leave that center tap unconnected.

Easy answer, I already owned a sufficient 3 phase transformer.

It's wired and working. Thanks.
 








 
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