Delta High-Leg Mystery
Delta High-Leg Mystery
(OP)
I'm working on a tenant improvement project that was initially visited, documented, and photographed by another engineer. His notes indicate that the building and the tenant space are supplied by a 240/120-volt, three-phase, delta high-leg system. A photo of the tenant's exterior disconnect switch revealed that the three phase conductors serving tenant's panelboard were tagged as black, blue, and red. The absence of an orange tag on the "B" phase conductor had me suspicious, but it wasn't the first time I had encountered an installation that didn't comply with the NEC's high-leg marking requirements. I contacted the utility and they confirmed that the facility is indeed being provided by a 240/120-volt, three-phase, delta high-leg system.
Here's the part that is completely throwing me off: a photo of the tenant's panelboard shows a huge lineup of single-pole circuit breakers connected to all three phases. No doubt they're connected to lighting and receptacle loads. I could see fluorescent lighting with universal voltage, electronic ballasts operating on a 208-volt circuit, but surely any 120V cord-and-plug equipment connected to the "B" phase would have been damaged. I suppose that autotransformers could have been used to step-down the voltage from 208V to 120V where needed, but it seems highly unlikely. It's also, as far as I know, prohibited by the NEC to use the high-leg for phase-to-ground loads in the first place.
Any ideas?
Here's the part that is completely throwing me off: a photo of the tenant's panelboard shows a huge lineup of single-pole circuit breakers connected to all three phases. No doubt they're connected to lighting and receptacle loads. I could see fluorescent lighting with universal voltage, electronic ballasts operating on a 208-volt circuit, but surely any 120V cord-and-plug equipment connected to the "B" phase would have been damaged. I suppose that autotransformers could have been used to step-down the voltage from 208V to 120V where needed, but it seems highly unlikely. It's also, as far as I know, prohibited by the NEC to use the high-leg for phase-to-ground loads in the first place.
Any ideas?






RE: Delta High-Leg Mystery
SceneryDriver
RE: Delta High-Leg Mystery
Its possible that the universal voltage ballasts are connected between 240V legs, along with a lot of other equipment. By itself, that would be OK, but would require a 2 pole breaker.
It wouldn't be the first time I've seen non-ganged single pole breakers installed to feed line to line loads. Its possible that whoever did the wiring just left handle ties off. Not unlikely given the other workmanship issues in evidence here.
RE: Delta High-Leg Mystery
RE: Delta High-Leg Mystery
I jumped to the conclusion regarding improper use of single pole breakers since the OP mentioned the improper labeling of the high leg. Maybe I'm just becoming too cynical about the state of workmanship these days... ;) Handle ties would be an easy fix if they really need to be multi-pole breakers. If the breakers were improperly used though, I'd probably start going through everything with a fine tooth comb. No telling what else one might find.
SceneryDriver
RE: Delta High-Leg Mystery
Bill
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"Why not the best?"
Jimmy Carter
RE: Delta High-Leg Mystery
RE: Delta High-Leg Mystery
Or
You may have 208 Volt supplied loads on the wild leg.
Bill
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"Why not the best?"
Jimmy Carter
RE: Delta High-Leg Mystery
My high-leg delta rules are a little rusty. Is it legal (or even a good idea)to serve 208v single phase loads from the wild leg to neutral? jmbelectrical mentioned that it's (now?) illegal to do that, but was it OK at some earlier time?
-SceneryDriver
RE: Delta High-Leg Mystery
RE: Delta High-Leg Mystery
Another possibility is that 240 Volt loads may have been fed from two breakers but the handle ties were not installed.
We have given you a number of possibilities to check but the bottom line is you or someone must do some on-site investigating before any of us know for sure what is in use.
Let us know what you find when you are able to do a site inspection.
Bill
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"Why not the best?"
Jimmy Carter
RE: Delta High-Leg Mystery