I have a high delta 3 phase 240/120V main panel rated feeding a tenant to be retrofited (pole mounted type transformer outside). There is no loads on the panel that are useing the 3 poles. Only legs A and C are being used for single and doubles poles. Leg B (high leg) is not being used which i understand why. My question, does the imbalance cause an issue? Does the utility typically care? and for the retorifit should i possibly reconfigure to 240/120 single phase or 208/120 3 phase? Thanks!
High delta 3 phase system. But only two legs are being used
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LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
No, no, and the former (unless you can use the 3ph 240 for load such as HVAC and other motors).
You would need a new utility transformer bank or your own 3ph transformer to receive the latter.
You would need a new utility transformer bank or your own 3ph transformer to receive the latter.
petersonra
Senior Member
- Location
- Northern illinois
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- Semi-retired engineer
I just ran into this situation on the job site. The high leg Delta transformer was actually inside the building but it only had breakers on two of the legs. The electricians figured this was a good reason to use phase b to wire up a bunch of new 120 volt circuits. Turned out there was more than 120 volts. It didn't hurt much of anything when they turned the power on except for a couple of GFCIs which didn't like the higher voltage and failed. The panel now has a large label on it indicating it is a high leg Delta supply. The electrician that did the addition said it's not uncommon in that area for high leg Delta transformers to be laying around because they're not real common anymore and he figures that somewhere in the past somebody had one and just used it rather than buying a new transformer.
It sounds like the kind of thing the people that did the electrical installation at the time the panel board was installed would have done to save money.
It sounds like the kind of thing the people that did the electrical installation at the time the panel board was installed would have done to save money.
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- Tennessee NEC:2017
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- Semi-Retired Electrician
If there any single phase, two-pole 240V loads not requiring a neutral, you could use the "B" or high leg for those, along with A or C, and label it as such. If there was an imbalance, that would help with that. Just make sure there are no 120V loads tied to the high leg & neutral.
ggunn
PE (Electrical), NABCEP certified
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- Austin, TX, USA
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- Consulting Electrical Engineer - Photovoltaic Systems
I see this quite frequently. It isn't a problem.
winnie
Senior Member
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- Springfield, MA, USA
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- Electric motor research
The imbalance can cause an issue but probably not. There are different versions of high leg delta.
In one case you have a large single phase transformer with additional transformer(s) to provide 3 phase power for minor 3 phase loading. If this is what you have then the fact you have zero load on the third leg means a wasted transformer but is otherwise not a problem.
Another common arrangement is intended to supply 3 phase loading such as lots of motors, with a bit of incidental single phase load. This type of transformer would be unhappy with only single phase loading.
I expect the utility is providing the first kind.
How large a service are you talking about? What sort of tenant?
Jon
Thank you. It is 400 amp, restaurant.
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
In that case, you likely have, or can use, three phase loads.
You could either use two panels, one 1ph and one 3ph, or use one larger panel to supply both.
The important thing is to use only A-N-C for any loads that require the neutral.
Unfortuanly no. The A/C's are single phase and the rest of kitchen equipment is single phase but with neutral, so i cant use double poles on the high leg either. I'm just going to use A and C phase since that is all i have.
If your not using the three phase why pay for it? Have them remove the power pot and go on the single phase rate.
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
You can place line-to-line-only loads on A-B or B-C, with the caveat of requiring non-slash-rated breakers.
I.e., 240v, not 120/240v. 277v would also be acceptable, perhaps more available, and perhaps less expensive.
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
If you are only using A and C legs, then it is single phase and you do single phase load calculations, which then apply only to the A and C legs. If you are doing a new service, then you wouldn't even need to install the B leg.
If you have a straight 240V single phase load (no neutral), you can connect it between B and A or C. When you use only 2 terminals of a 3 phase system, then load is fed with single phase power.
There are two significant cautions on this approach: some loads that don't use the neutral still like their 'hot' legs to be balanced with respect to ground, and most double pole 240V breakers are only rated for 120/240V single phase, so you need to find special breakers _or_ just use 2 legs of a 3 pole breaker.
If you don't need 3 phase power, then I agree it would probably be simplest to get rid of it. Since all of your loads are single phase, the only reason you might need to keep the 3 phase is if your load calc requires more kVA than the single phase can handle.
-Jon
Yup. in some cases they have a 3 phase A/C system or some equipment, then through out construction they decide to do single phase A/C system or go to single phase equipment and then you end up with a 3 phase high delta panel operating only two legs.
if its an older place I'd check the equipment nameplates closely, dont just go by the old cord they used. I saved a customer from a expensive upgrade (to 208) by moving a 240V oven and a 240V fryer to the A-B and B-C phases respectively.
They had 3-wire residential range cords on them but were actually '240V' only on the data plate. '240' V straight rated breakers are easy to get around here, Eaton or Siemens.
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