# 138 Volts from a 277V System



## TWhitaker (Feb 27, 2018)

I have a challenge with which I need a little assistance. I have a lighting system that I am trying to figure out how they wired it. This is a 480 Volt AC, 3 phase, 4W system. The light fixtures in this building are 120 VAC fixtures and is powered from the 277V panel, not from the 120/208 panel. The voltage across the fixtures are about 138 VAC. The only thing I can come up with is the builder avoided getting a transformer by paralleling the fixtures in groups of 2 across a 277 to ground (or neutral). Am I correct? If so, does anyone knows where I can find anywhere in the NEC that talks specifically about this. My bottom line question is do I continue to use these fixture the way they are wired or try to rerun them for a 120/208 panel - which will be costly for my client (the new owners)? The building has been up and running for about 7 or 8 years but the new owners are saying they go through a boat load if incandescent bulbs. Also, if I continue to use this wiring, can LED bulbs be used? I've seen a lot but not this.


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## MDShunk (Jan 7, 2007)

I think I'd spend an hour finding the head end of this circuit and taking a couple of junction boxes apart to understand better how the original install is wired. Something doesn't compute here. When things don't compute, the original information is often inaccurate or incomplete.


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## macmikeman (Jan 23, 2007)

Tell us about the lamps. I may have a solution.


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## splatz (May 23, 2015)

TWhitaker said:


> The light fixtures in this building are 120 VAC fixtures and is powered from the 277V panel, not from the 120/208 panel. The voltage across the fixtures are about 138 VAC. The only thing I can come up with is the builder avoided getting a transformer by *paralleling* the fixtures in groups of 2 across a 277 to ground (or neutral). Am I correct?


I think I follow you - doesn't seem like a coincidence you're getting exactly half of 277 at the fixture - but that would be series, not parallel, right? If that's the case, when you remove the lamp from one fixture, another one would go out with it, since they're in series. 

If you use two different wattage bulbs, you'd get different voltages on the pair, you'd see bigger voltage drop across the higher wattage bulb. 

I am trying to picture how someone would wire this ... would they basically run a switch loop to the second fixture in each pair? That would make the wiring in the second fixture in each pair look normal, but somewhere you're going to have black and white spliced, or reversed, or something...


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## MDShunk (Jan 7, 2007)

Naturally, if I was there I'd want to put my own two eyes on it, but from the safety of my keyboard my best guess is that this is fed from a transformer, yet undiscovered, and it's just tapped up a little on the high side. -or- these are multi-voltage ballasts, indeed fed at 277, and he's measuring from L-G instead of L-N and getting a screwy reading because secondary bonding is inadequate or missing.


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## micromind (Aug 11, 2007)

If 2 loads of equal impedance are connected in series and 277 volts are applied, each load will have 138 volts. 

If the impedance is not equal, one load will have higher voltage while the other will be lower but the total will still be 277. 

Think of it as a basic 120/240 service with an open neutral.


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## MDShunk (Jan 7, 2007)

micromind said:


> If 2 loads of equal impedance are connected in series and 277 volts are applied, each load will have 138 volts.
> 
> If the impedance is not equal, one load will have higher voltage while the other will be lower but the total will still be 277.
> 
> Think of it as a basic 120/240 service with an open neutral.


Uh-huh, but I'm still going back to my Occam's Razor principle of troubleshooting. When you hear hoofbeats, think horses and not zebras. If someone actually wired it that way, it's a zebra. Not really the first thing that would pop in my mind. I'd still be hunting a transformer I hadn't found.


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