three phase wiring with only a N for 120

[chimpera]

So I am researching how to wire a warehouse that will be used with 3 phase machinery. A lot of it will be retrofitted or built from the ground up because its a maker space.

According to Wikipedia:
NEMA connector - Wikipedia, the free encyclopedia

NEMA L14 and NEMA L18 can be used with only a neutral and no ground; allowing both 3 phase 208 and 120 single phase to a machine with only 4 conductors. The problem is that while working on my mill I have had noise problems with limit switches and such.

The real question is can I get away with 4 conductors but wire the machines internally to avoid noise.

I'm aware that the are standards that include L-L-L-N-G. The reason for avoiding these is the extra cost and unavailability of 5 conductor wiring and connectors.

Also I'm not asking for safety reasons because this is a twist lock system and a NEMA standard.

Any insight is appreciated.

Thanks, Ben

[Al_The_Man]

Not sure of your final intentions, but most machinery that is three phase fed has a local 120v single phase transformer in the enclosure that has one side set up as a local grounded neutral, this must be referenced either to Earth Ground through ground rods or to the incoming service ground.
Or is this for general 120v outlets in the shop?
To avoid noise in control power, it is necessary to ground and bond all metallic parts of the system to an Earth Ground point.
http://www.automation.siemens.com/do.../emv_r.pdf?p=1
Al.

[chimpera]

Hi Al,I was hopping you would show up. Can I interpret your response to mean that its ok to have one conductor going to the machine act as both neutral and ground?

Thanks, Ben

[Al_The_Man]

NO NO, you will be contravening the code by using a neutral as ground, the neutral and ground can only come in contact with each other at either the service distribution point or at a local situation I mentioned earlier where a control transformer is set up in an enclosure etc.
In both instances at no point in an installation can you use ground as neutral and vice-versa.
In spite of technically being at the same potential.
Al.

[chimpera]

So NEMA L18 connectors only have a neutral and no ground. If this is never permissible why does it exist? Basically I'm wondering if I can get away with 4 conductors on 3 phase equipment that have 120v controls as is suggested by the existence of the NEMA L18 standard. Thanks, Ben.

[Al_The_Man]

NEMA L18 is L-L-L-N where a circuit is used not requiring a ground, probably not used that often, or for devices that are double insulated as in portable power tools etc that do not require a ground conductor.
So if you require a neutral AND a ground for 120v outlets you cannot use L18.
If feeding some items such as a mill for e.g., to me the best and most common way is to feed it with 3 phase and ground and set up the local 120v 1ph grounded neutral via a 220/120v 1ph transformer as I mentioned earlier.
Al.

[chimpera]

Is your proposed wiring
a.
L
L
L---\_1:1 transformer
G---/

b.
L
L---\_2:1transformer
L---/
G
what about the 120 deg phase difference. it seams like it would produce a very poor 120v supply.

Thanks for taking the time, Ben

[Al_The_Man]

Ans: B, There is not 120deg between two phases when there is no other reference it is 180deg , it is identical to 240v 1ph, So if you hook up a 240/120v transformer across 2 phases you end up with 120v 1ph which is no difference than your domestic 120v supply.
I have an example PDF control circuit from the code I will post when I get back to my other PC.
Al.

[Al_The_Man]

Here is the PDF, ref transf 1T, notice how the secondary X2 is grounded right at the transformer and the neutral taken off at this point.
Al

[chimpera]

Al, that diagram makes almost everything clear. The only thing left is that two legs of 240v three phase are supposed to only have a potential of 208v. If that's correct then I would need a transformer with a ratio of ~1.73:1.
Thanks again

[Al_The_Man]

You should be able to get 208-120v from one of your electrical supply houses.
Allen Bradley and I believe Hammond also make them.
The alternative is to buy a toroidal transformer and modify the 120v secondary winding to suit, very easy to do.
Al.