240V split phase -> 120V outlet via transformer 1

[textex]

Noob. I am trying to cost-optimize a branch circuit for a remote light display with a circuit distance of about 500 feet. US NEC applies. The remote lights will be commodity 120v equipment*, up to 1500W power required. My hope is to wire this with a 240V (split phase) buried branch circuit and a small transformer. Compared to a typical 120V branch circuit, this configuration allows 4x distance with the same voltage drop and power. Under these assumptions, 12 awg copper (~$.705/foot currently) is acceptable for the 240 circuit, versus 4 awg AL (~$1.90/foot) for a conventional 120v circuit.

My question has to do with what the NEC allows. Is it permissible to use an adequately-rated small transformer to serve a receptacle load? (If not, is it okay if the loads are hard-wired?) I could use an isolating 240-120V step-down transformer with 3-conductor branch circuit (with EGC), referencing one of the secondary sides to the primary (split-phase) neutral, with the EGC unbonded. This seems straightforward, but probably not cost effective.

Hopefully, I could use 2-wire cable (with EGC) and an autotransformer (such as Hammond 170J). I think (but not sure) this would be wired with one of the split phase legs straight through to the receptacle "hot" as well as a one of the outer terminals on the autotransformer. The other split phase leg would go to the second outer terminal. The receptacle neutral would wire to the center of the transformer. The EGC would be unbonded. My understanding is that the neutral would float with respect to the ground, but be close. Can this be made NEC compliant, if not for receptacle then for hard-wired loads?

*It may be possible to use all 240V compatible loads via European to Nema 6-15 plug adapter but...Nah

 

[cranky108]

I think many of use can tell you what you can do, but the NEC may not allow.
The NEC was written to keep people who likely don't know safe.

 

[jghrist]

Why can't you use a multi-wire branch circuit (see NEC 210.4) to serve two sets of lights with a common neutral? You will need to use either a 2-pole breaker or two single-pole breakers with a handle tie. This ensures that the two sets of lights are on opposite legs of the 120/240 V service. No need for a transformer. The neutral current will be the difference between the current in the two sets of lights.

 

[jraef]

I agree, same number of wires, same net outcome, no transformer needed.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[textex]

Yes, this is a great strategy if I can plan out and balance the load, perhaps as a hard-wired application. I am concerned about having receptacles. If somebody plugged in something that gets close to the receptacle max load unbalanced, the voltage drop would be catastrophic. Putting aside the practical necessity of such a system, I'm curious whether it's NEC compliant. Besides, the MWCB would require 3 conductors, while the strategy I was curious about was to use 2 conductor cable.

 

[itsmoked]

With pumps far away on big properties we frequently use 240 to 600V step up and then step down transformers on the opposite ends. Your wire can get a lot smaller then.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[LionelHutz]

If you're going to introduce transformers then why not step up to 480V or 600V and then step down again at the other end?

An isolation transformer with X1 & X2 feeding line and neutral with X2 grounded would be the appropriate solution.

I also believe that once you introduce a transformer you may need new earth grounding rods.

 

[FacEngrPE]

If you use a long feeder (any voltage) with an isolation transformer, the load side of the isolation transformer needs a separately derived neutral (see the NEC for details). If you use an auto-transformerthat might not trip in the event of a fault to earth, as the connection between the earthing rod and the circuit is 500 ft away.