Grounding an SDS Local to a Machine

[barnesja2]

So I've seen some threads that may touch on this but I'm still confused.

I have a machine with a 480/120 3KVA xformer. Though the machine will be fixed during use we'd like to be able to pull it out for maintenance so we want to avoid additional connections. The machine builder says the low side of the xformer needs to be grounded to building steel or a grounding electrode per 250.30. My reading of 250 makes me think they are correct but I think we have quite a few machines that aren't grounded this way so one of us is wrong. I thought we may qualify for Exception 2, which speaks to the supply of the SDS being located inside of equipment rated as Suitable for Use as Service Equipment but don't see a lot of information on what qualifies equipment as SUSE.

Thanks for reading.

 

[petersonra]

So I've seen some threads that may touch on this but I'm still confused.

I have a machine with a 480/120 3KVA xformer. Though the machine will be fixed during use we'd like to be able to pull it out for maintenance so we want to avoid additional connections. The machine builder says the low side of the xformer needs to be grounded to building steel or a grounding electrode per 250.30. My reading of 250 makes me think they are correct but I think we have quite a few machines that aren't grounded this way so one of us is wrong. I thought we may qualify for Exception 2, which speaks to the supply of the SDS being located inside of equipment rated as Suitable for Use as Service Equipment but don't see a lot of information on what qualifies equipment as SUSE.

Thanks for reading.

All that exception allows is for you to use the GE tied to the service equipment as your GE for the SDS. I think there is another rule that allows you to use the EGC as a combined EGC/GEC as long as the wire meets the requirements of both. If so, then you would only need the one wire instead of two.

You can get rid of the need for the GEC by making the xfmr 1000VA or smaller. In this case maybe you replace the 3KVA xfmr with (3) 1KVA xfmrs.

In general, if you have a SDS it requires a GEC. Just because your company never bothered to install one does not make it OK.

And yes, it is a dumb rule for these small xfmrs. But it is the rule.

UL defines what equipment is suitable for use as service equipment. You can get a UL508a panel made that is listed as being suitable for use as service equipment. It is a little convoluted but it can be done.

ETA: added comment about combined egc/gec

 

[barnesja2]

All that exception allows is for you to use the GE tied to the service equipment as your GE for the SDS.

You can get rid of the need for the GEC by making the xfmr 1000VA or smaller. In this case maybe you replace the 3KVA xfmr with (3) 1KVA xfmrs.

In general, if you have a SDS it requires a GEC. Just because your company never bothered to install one does not make it OK.

And yes, it is a dumb rule for these small xfmrs. But it is the rule.

UL defines what equipment is suitable for use as service equipment. You can get a UL508a panel made that is listed as being suitable for use as service equipment. It is a little convoluted but it can be done.

Thanks! I suspected this was the correct answer though I had not considered 3x 1 KVA xfrmrs. That's a creative solution.

 

[petersonra]

let me expound on the idea of a combined egc/gec.

I am not entirely sure about this one. It seems to me that it was permitted at one time (maybe unintentionally) but a later code revision may have changed this.

If I recall the rule as it is now, you can use the GEC as an EGC but not vice versa.

I tried to find it in the code, but couldn't so maybe I am imagining it.

 

[petersonra]

Thanks! I suspected this was the correct answer though I had not considered 3x 1 KVA xfrmrs. That's a creative solution.

A lot of times these cabinets have much larger control xfmrs than actually needed. As long as you can legitimately call the xfmr a Class 1, 2, or 3 source you are good to go. If it feeds a small HP motor it is probably not going to be able to be called a class 1 source.

 

[Smart $]

let me expound on the idea of a combined egc/gec.

I am not entirely sure about this one. It seems to me that it was permitted at one time (maybe unintentionally) but a later code revision may have changed this.

If I recall the rule as it is now, you can use the GEC as an EGC but not vice versa.

I tried to find it in the code, but couldn't so maybe I am imagining it.

Not your imagination. 2011 NEC added 250.121, then 2014 NEC added the Exception.

250.121 Use of Equipment Grounding Conductors. An
equipment grounding conductor shall not be used as a
grounding electrode conductor.

Exception: A wire-type equipment grounding conductor installed
in compliance with 250.6(A) and the applicable requirements
for both the equipment grounding conductor
and the grounding electrode conductor in Parts II, III, and
VI of this article shall be permitted to serve as both an
equipment grounding conductor and a grounding electrode
conductor.