480-240 3 ph. Delta dry type transformer grounding.

[JP23]

A 15KVA dry type 480-240V 3 phase 3 wire dry type delta-delta transformer with secondary OCP is
feeding a single piece of 3 phase 3 wire 240V equipment in a steel building.
There is a 5% convenience x6 tap on the transformer not being used. I listed the code sections that seem
to apply but still leave me with questions as to the interpretations.

1) Is X6 required to be bonded? 250.20 (B) seems to say no.
2) Is a grounding electrode conductor to building steel required to supplement the EGC?

 

[don_resqcapt19]

You need a grounding electrode and grounding electrode conductor per 250.30(B)(1) & (2), if you keep the system ungrounded. If you ground it, you need to apply the rules in 250.30(A), but in both cases a grounding electrode and GEC is required.

If you do not ground the secondary you need to install a ground detector as required by 250.21(B)

If you ground it, you would typically ground one of the phases and make it a corner grounded system. The grounding conductor must comply with the identification requirements in Article 200.

 

[petersonra]

1. No. Delta connections are not connected to ground USUALLY, although there are some exceptions, such as corner grounded.

2. The GEC is probably required, but it does not actually supplement the EGC. They are separate conductors with seperate functions. keep in mind the building steel is not the grounding electrode but just the bonding point to the GE. Take a close look at the requirements.

 

[JP23]

Thank you for your quick replies and direction. I'm more than willing to do the homework and even
find it enjoyable. Knowing I'm on the right path helps a lot. Your tips are much appreciated!

 

[jim dungar]

Does your X6 convenience tap provide 120V to ground?

 

[winnie]

You are _required_ to ground if doing so reduces phase-ground to less than 150V. You have a 240V secondary with an X6, which is 120V to two of your phases (< 150V) but 208V to ground on the 3rd phase.

You are also _required_ to ground if you are using the X6 to supply a 'neutral' circuit conductor.

So per NEC you may leave this secondary ungrounded. However depending upon the equipment connected this is probably a poor design choice.

Ungrounded delta supplies are often intentionally used for 'continuity of service'; equipment can keep running in the event of a single ground fault. But it can subject equipment to strange voltages during such faults, which can damage the equipment. I personally would not use an ungrounded system if I could avoid it.

-Jon

 

[JP23]

Does your X6 convenience tap provide 120V to ground?

Yes, it is a convenience tap only rated at 5% of the transformer capacity as I understand it.

 

[JP23]

You are _required_ to ground if doing so reduces phase-ground to less than 150V. You have a 240V secondary with an X6, which is 120V to two of your phases (< 150V) but 208V to ground on the 3rd phase.

You are also _required_ to ground if you are using the X6 to supply a 'neutral' circuit conductor.

So per NEC you may leave this secondary ungrounded. However depending upon the equipment connected this is probably a poor design choice.

Ungrounded delta supplies are often intentionally used for 'continuity of service'; equipment can keep running in the event of a single ground fault. But it can subject equipment to strange voltages during such faults, which can damage the equipment. I personally would not use an ungrounded system if I could avoid it.

-Jon

Thanks Jon, so corner grounding seems to be the best option. Do you agree or is this type of transformer not recommended for this purpose?
The equipment is a portable cord connected air conditioner if that makes any difference.

 

[winnie]

If you choose to ground, then ground X6. This minimizes neutral to ground voltage.

Off hand I am not sure if you are even allowed to corner ground if X6 is available.

J ou b

 

[JP23]

If you choose to ground, then ground X6. This minimizes neutral to ground voltage.

Off hand I am not sure if you are even allowed to corner ground if X6 is available.

J ou b

Thank you Jon. I will check on that also.
The manufacturer tech said that this type of transformer generates a lot
of calls from contractors trying to understand how to connect it properly.

 

[JP23]

For anyone visiting this thread that is interested in learning about corner grounding, there's a
video from Mike Holt on Youtube that is short and to the point. Thanks Mike!