Grounded neutral in equipment

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MrKawfey

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Hi, new to these forums and I am not sure if this is the right place to post. If no please suggest the right category.
The question is this: I am working on a piece of industrial equipment (testing machine) that is wired for 208 3 phase input. When this equipment was originally built it had a 3KVA transformer built in to convert to 120V internally. The only components left in the equipment that need 120V are a few small DC power supplies (<100W) and a laptop power supply. All of these are plugged into a power strip that is attached to the 3KVA transformer.

I would like to pull the big transformer out and just grab one leg of the 208 input to feed the power strip. The fourth leg (grounded neutral) of the 208 is bonded to the steel cart that is the frame work for this equipment. Is it ok to tie the neutral line from the power strip to the ground connection in the cart?

If not, is it ok to remove the ground connection to the cart and float it?

Thanks
 
additional info

additional info

Also, not sure if it matters, but there is a fused service disconnect built into the cart and the power to the cart is supplied by regular service cord (not armored or run through metallic conduit).
 
you cannot substitute EGC for the grounded conductor if that is what you are planning. It will work but it is not safe and not to code.

you would need to bring in the N from the 208V supply.
 
Follow up

Follow up

Thanks for the quick reply. I am not on site yet and have nothing to do with the team that is running the 208 power drops to the area. What is the normal procedure, to bring a neutral leg over as the 4th conductor or to bring an earth ground? I would assume that it is an earth ground.
When running 208V three phase wiring throughout a building are you typically pulling a neutral with it? Even if you are not planning a 120V receptacle. Would the neutral just be dead headed at the nearest disconnect or at the 208V receptacle?

Sorry for my ignorance, just want to make sure I can plan out my part safely. My hunch is I should just run a separate 120V lead from the cart for that equipment, but if I can safely get it from the same receptacle it would be easier for the end user.

Thanks again
 
MrKawfey said:
Thanks for the quick reply. I am not on site yet and have nothing to do with the team that is running the 208 power drops to the area. What is the normal procedure, to bring a neutral leg over as the 4th conductor or to bring an earth ground? I would assume that it is an earth ground.
When running 208V three phase wiring throughout a building are you typically pulling a neutral with it? Even if you are not planning a 120V receptacle. Would the neutral just be dead headed at the nearest disconnect or at the 208V receptacle?
Click to expand...

I would say there is no normal. There are plenty of things that are 208 only, and many that are 208/120. The first doesn't require a neutral, and the second does. There are single phase items and three phase items and you have the same issue (120V needs a neutral, 208V does not). 208V circuits are usually done for specific things and the appropriate conductors run. I have specified 208/120V circuits to make provisions when I don't know what equipment will be coming (e.g L21-20 or L21-30). In all cases, you need an equipment ground. You can't use that ground as a current carrying conductor (e.g. can't be a neutral even though it eventually goes to the same bus).

Most laptop and computer power supplies will take a 90V to 250V input. I'm sure you could get DC supplies with a 208V input.

You either need a 5 wire plug and receptacle, or get rid of all the 120V equipment in it.
 
Thanks guys I appreciate the help. I will ask the building guys if they pulled a neutral, but considering I didn't ask them to and just said 208 3 phase, my hunch is they probably didn't. If not I will give the users the option of a small x-former or a separate 120V cord coming out of the cart that they could plug into the wall.

As for the power supplies, they can all handle the 208V, the problem is there is a small gear motor that has a 115V rating. Not sure if it would work with 208V. Not worth replacing it.

Thanks again. I am glad I found this forum.
 
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