240V 3ph Delta- High leg not used??

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32Lateralus

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Have a remodel of a building. The current system is a 240V 3ph Delta coming off the utility transformer. The High Leg lands on the first panel, but nothing is connected to it and it does not continue onward from that panel. Only the 120V phases are utilized on the system. Is there any issue with this? Doesnt seem right.
 
No issue. No worse that taking 120v from a 120/240V panel.

By the way, line-to-line loads that do not use the neutral could use the high leg.
 
LarryFine said:
No issue. No worse that taking 120v from a 120/240V panel.

By the way, line-to-line loads that do not use the neutral could use the high leg.
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Okay, thanks, thought it was odd having a 3ph system with load on only 2 of the phases. Set off alarm bells. It's a bit different than 120V on a 240V though because you normally connect to the 120V using An or Bn. Both phases are typically utilized. This case only A and B phases are used, C phase is completely empty, 0 load.

Yeah, the high leg just isnt connected anywhere, it lands on the first panels and just stops, nothing in that panel connects to that phase. Everything downstream is single phase. Will look into utilizing that phase if I need it then for 2-pole mech units.
 
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32Lateralus said:
Okay, thanks, thought it was odd having a 3ph system with load on only 2 of the phases. Set off alarm bells. It's a bit different than 120V on a 240V though because you normally connect to the 120V using An or Bn. This case only A and B phases are used, C phase is completely empty, 0 load.

Yeah, the high leg just isnt connected anywhere, it lands on the first panels and just stops, nothing in that panel connects to that phase. Everything downstream is single phase. Will look into utilizing that phase if I need it then for 2-pole mech units.
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Be aware that if you use the high leg with another phase to supply 240 single phase loads the breaker will need to be a 240 rated breaker-not a 120/240 breaker. There is a difference.
 
texie said:
Be aware that if you use the high leg with another phase to supply 240 single phase loads the breaker will need to be a 240 rated breaker-not a 120/240 breaker. There is a difference.
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Good point, thank you. So there is no issue have one of my phases being completely not utilized with 0 load??
 
32Lateralus said:
Okay, thanks, thought it was odd having a 3ph system with load on only 2 of the phases. Set off alarm bells. It's a bit different than 120V on a 240V though because you normally connect to the 120V using An or Bn. Both phases are typically utilized. This case only A and B phases are used, C phase is completely empty, 0 load.

Yeah, the high leg just isnt connected anywhere, it lands on the first panels and just stops, nothing in that panel connects to that phase. Everything downstream is single phase. Will look into utilizing that phase if I need it then for 2-pole mech units.
Click to expand...

There is an advantage to putting all your 120V loads in a single phase panelboard, when using high leg systems. It helps avoid accidental use of the high leg. Using a three phase panelboard for a system like this, you end up seeing about a third of the spaces are empty, and it gives you a false perception that there are plenty of vacant spaces, when they all are off limits for 120V loads. You connect the (now A and C) phases and neutral to the 120/240V split phase panelboard, and only put your phase-to-neutral loads either there, or subpanels that derive from it. The main panel would then only contain your 3-phase loads and your 240V loads that don't use the neutral.

What I meant by "(now A and C) phases" is that the standard changed in 1975 to require it on the B-phase, instead of the C-phase. So an older system likely has a C-phase high leg. It's common that utilities require the high leg on the C-phase in the meter, as many utilities didn't follow the change in the standard, and the meter uses the high leg in a different way than it uses the other two phases.
 
The only potential "gottcha" is that depending on the transformer configuration, there may be a limit to the amount of 120V loads you can have on a High Leg system. It can be as low as 5% of the transformer rating in some cases.
 
Probably not an issue here given that the system is already being used for single phase loading, but Jraef brings up a real issue.

In one common situation you have a small 'stinger' transformer that is added to a large single phase transformer to support a small amount of 3 phase loading in addition to a large single phase load. In this situation having lots of single phase load isn't a problem.

Another common situation is where you have a 3 phase delta transformer with a center tap. These often have a the sort of limit that Jraef describes.

Jon
 
The 120 volt limitation is typical and common on small dry type transformers. But then, as you point out, with POCO 3 phase 4 wire service the opposite is usually true that you have more 120/240 capacity than 3 phase.
 
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