Adding neutral wire to RPC output for 110V loads, some NEC/NFPA questions

I've seen mention of it in a few other places but I haven't found a detailed answer for running a neutral along with the center-tapped delta output from a RPC. The plan is to add outlets on the machine instead of running extra leads to the walls or putting in transformers. A 240 Delta to 208Y transformer is cost prohibitive at this point.

The current setup: 5HP American Rotary panel with a 3HP idler. Shop equipment is all 1 - 1.5 HP stuff but I may add something larger later and want to future-proof the wiring now.

The plan: Run a SO 5-wire cable from the RPC to the machine in use. Connect things using twist-lock NEMA plugs/receptacles. I've gone over the requirements for conductor sizing and overcurrent protection and understand the math but don't know what exceptions/special cases apply here. The calculations will also depend on what solution I use for the 110 outlets so I need to sort that out first.

The questions:

* NEMA L21 devices are spec'd for 120/208Y. Is there any issue with using them for 240 high leg--other than the marking being wrong? Should I go up to L22 for higher voltage rating?
* Since it's not hardwired, does this negate the NEC tables for dedicated motor circuits and just use the standard ampacity and derating tables?

I've got more questions but they also hinge on what connector to use and what NEC tables apply.

Thanks!

The RPC is indeed a high leg system, so the neutral would be only useful for the supply pair of wires. The high leg is not suitable for use with it.

I don't know of a technical reason why you could not supply it, so long as you follow the protocols for a high leg system.

As a general rule, though, I would not do that with the intent to use it for internal control power. It is much "cleaner" to supply local power in the machine through an internal transformer. That way the machine relies only on the main power wires, and everything is internal, with less chance of miswiring and the like.

NEC requires that wiring etc be "approved for the purpose". Any part that does not carry a listing for the voltage or amperage invlved is therefore not "approved for the purpose", and is an NEC violation. Use correctly rated plugs and receptacles.

NEC ONLY applies to parts in the fixed power system up to the receptacle. Once you get to a plug and external cables, machines, etc, you transition to UL requirements and OSHA.

CrankyOldMan said:

. A 240 Delta to 208Y transformer is cost prohibitive at this point....

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If your 120 volt requirements at each machine are modest, you don't need an expensive three phase tranformer.
All you need is a step down, 240 to 120 volts. The primary goes to the two utility legs at the machine, the
secondary has one end grounded and becomes your neutral. Fuse the secondary appropriately.

How much 120 do you need?

Thanks for the quick feedback! Sounds like what I'm thinking about could be done but shouldn't, hence the lack of a clear explanation of how to do it that way. I didn't realize the change from NEC to UL at the plug, thanks for clarifying that. Looks like it may be better to just bite the bullet and run separate 110 outlets next to the 3-phase outlets, or just hard wire everything.

Once I wrapped my head around the RPC making a high-leg delta it both cleared up some confusion and added new questions. I would definitely only use the neutral with the supplied legs and not the manufactured leg. The machines are all post-war era, manual mills and such, so the only controls would be the motor starter circuits. The 110 would be for work lights, DROs and the like. If anything needed more in-depth controls I would go with a 24VDC system.

jim rozen said:

If your 120 volt requirements at each machine are modest, you don't need an expensive three phase tranformer.
All you need is a step down, 240 to 120 volts. The primary goes to the two utility legs at the machine, the
secondary has one end grounded and becomes your neutral. Fuse the secondary appropriately.

How much 120 do you need?

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Theoretically not very much but an external NEMA 5-15 is an invitation to plug in 15A worth of equipment. A 2 kVA transformer is a big chunk of change and goes a long way towards a 240D->208Y system transformer. That was part of the thought process of bringing in the neutral with the 3-phase: enough amps to supply an outlet. Running the power cords into the electrical panel and terminating them on blocks would be a decent workaround and allow a much smaller kVA transformer, effectively making them part of the machine.

Not sure what you are now suggesting, but two cords on same machine seems like even less of a good idea than running the neutral in.

I see that you can get a 250VA transformer, sufficient for a light or two and a DRO, for 60 bucks from Digikey in the US.

JST said:

Not sure what you are now suggesting, but two cords on same machine seems like even less of a good idea than running the neutral in.

I see that you can get a 250VA transformer, sufficient for a light or two and a DRO, for 60 bucks from Digikey in the US.

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I was thinking that if I went with a conduit drop I could just run the 110 wires alongside the 240 wires--accounting for fill, current carrying conductor derating, etc.--and feed the outlets from inside the enclosure. Just adding a handy box to the exterior (like a power strip) and running a bit of SJ cable to the outlets on the wall would be easier yet. Definitely wouldn't want to have two power cables going into the main enclosure.

If I went with the smaller transformer I would use cable glands on the lamp and DRO power cables and land them inside the enclosure. It may be even easier to find lights rated for 120-277 and just run them directly off the 240. I'm pretty sure most LED drivers are spec'd for global voltages and a DRO with 240 should be even easier to find.