single phase vs 3 phase

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ggunn said:
No such thing as single phase 208/120V. If it is three wire single phase and 208V phase to phase, phase to neutral is 104V.
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I had a situation this summer where a client wanted 120/208 single phase outlet (NEMA 14-50). Building service was 120/240 3 phase. I tried to talk them into the 120/240 single phase NEMA 14 receptacle. They insisted it needed to be 208 line a line not 240. So yes, there is no way to get 120/208 single phase out of a single phase transformer, but there is by tapping off a 120/208 3 phase system. In this case I just installed a 120/208 phase transformer and tapped two legs off of it.
 
electrofelon said:
I had a situation this summer where a client wanted 120/208 single phase outlet (NEMA 14-50). Building service was 120/240 3 phase. I tried to talk them into the 120/240 single phase NEMA 14 receptacle. They insisted it needed to be 208 line a line not 240. So yes, there is no way to get 120/208 single phase out of a single phase transformer, but there is by tapping off a 120/208 3 phase system. In this case I just installed a 120/208 phase transformer and tapped two legs off of it.
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One way you could get the 208V circuit from a 120/240V high leg delta service, is to use a 277/480V panelboard with a single pole breaker on the B-phase. Can't do that with standard 240V panelboard, because few (if any) single pole breakers are straight-rated, and most (if not all) require 120V to ground.
 
electrofelon said:
I had a situation this summer where a client wanted 120/208 single phase outlet (NEMA 14-50). Building service was 120/240 3 phase. I tried to talk them into the 120/240 single phase NEMA 14 receptacle. They insisted it needed to be 208 line a line not 240. So yes, there is no way to get 120/208 single phase out of a single phase transformer, but there is by tapping off a 120/208 3 phase system. In this case I just installed a 120/208 phase transformer and tapped two legs off of it.
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Carultch said:
One way you could get the 208V circuit from a 120/240V high leg delta service, is to use a 277/480V panelboard with a single pole breaker on the B-phase. Can't do that with standard 240V panelboard, because few (if any) single pole breakers are straight-rated, and most (if not all) require 120V to ground.
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Not sure why Electrofelons customer wanted 208 when he had 120/240, but wouldn’t a relatively cheap buck/boost XF work?
 
Carultch said:
Granted, split-phase is really produced by a different method than a time delay or phase delay, but in the ideal case, it is mathematically identical to a phase shift of 180 degrees.
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In the ideal case, of course, but any blip on the waveform exposes it as an inversion rather than a phase shift.

INCOMING! :D
 
ggunn said:
In the ideal case, of course, but any blip on the waveform exposes it as an inversion rather than a phase shift.

INCOMING! :D
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Indeed. I have that covered in Post #12 below:

Residential Split Phase 180deg Phase Difference

Hi, all. Got a question regarding residential split-phase systems. After looking into how the split-phase systems are derived I keep seeing it mentioned that the Lines are 180degrees out of phase from each other. I understand that in a 3-phase system the Lines are 120deg out of phase (and that...
forums.mikeholt.com
 
electrofelon said:
If you only needed 120 between one line and the grounded conductor that would work. In practice I think it would be too hard to coordinate the connections.
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You're proposing that if the equipment needs 120V N-A and 208V A-B (and not 240V A-B for some reason) and doesn't care about N-B voltage, you could buck B only to be 88V N-B? That seems to be allowed by 210.9.

Cheers, Wayne
 
electrofelon said:
If you only needed 120 between one line and the grounded conductor that would work. In practice I think it would be too hard to coordinate the connections.
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Ok.sorry, I’m confused..
a buck boost with two wire 240 input, two wire output @208V?
 
wwhitney said:
You're proposing that if the equipment needs 120V N-A and 208V A-B (and not 240V A-B for some reason) and doesn't care about N-B voltage, you could buck B only to be 88V N-B? That seems to be allowed by 210.9.

Cheers, Wayne
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Yes theoretically, although I think in most situations this wouldn't be practical.

Another kinda related interesting situation is where you have 120/240 three phase and someone wants a three phase plus neutral cord and plug connection. A L21 doesn't have a specific designation for high leg location, and I'm not aware of one that does.
 
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