480V, 3 phase delta to 240V with high leg question

[electrofelon]

Thank you. I will review the thread and make use of all the useful comments.
One thing I know from code search is that diversity of electric ovens, electric dryers are conditional based on the load being distributed on 3phase source tables 254 and 255. Not sure how to apply this with a high leg scenario. Granted that the electric range, dryer and condenser are all 230V so those can go on the B phase, but still unsure. Also, this may lead to larger equipment since most of the load will be on 2 out of the 3 phases

hmmm but you said in the OP "I want to bring 3-phase power to the meter bank but 120/240V single phase to each dwelling."

 

[electrofelon]

Is the me

1. The secondary power from the transformer will go to feed a meter bank on each floor.

Thanks for your help

So this is a sub-metering setup?

 

[hillbilly1]

Agreed. But it kinda sounds like this electrician might be a little shady......

Maybe he has a bunch of delta transformers laying around that he wants to get rid of………

 

[tortuga]

3-phase to dwelling unit panels starts to not add up in the calcs , and the metering is a PTIA, stick with 'open wye' 120/208
or use a scott-T if you really want 120/240
(you miss out on 310.12 and 220.82)

 

[LarryFine]

The OP said:
"Transformer will produce three legs 120V not grounded leg 240 direct".

That sounds like like a 208Y/120v system to me.

 

[qcroanoke]

The OP said:
"Transformer will produce three legs 120V not grounded leg 240 direct".

That sounds like like a 208Y/120v system to me.

Or a bad way of saying it.

 

[Eddie702]

@Volt-Amps It might be helpful if you could post some information on the apartments like average square footage, list of appliances, washers, dryers, hot water heaters heating and air conditioning and ranges etc.

That way we could have a better handle on the amount of 120 volt load and the amount of 240 or 208 volt load.

 

[Eddie702]

I was just communicating with the architect, and had the very same question. HE said he believes the contractor does not want to provide 150A panels, he wants to go down to 125A. I believe this is indeed the case to save on cost

So, you suspect he wants 240 volts rather than 208 volts as he is hoping to reduce the panel size based on the voltage?

 

[Volt-Amps]

Actually no, ranges and dryers are 120/240, so a misplaced B phase will fry them, but compressors and water heaters (240 volt) would be ok. But all of those will work on 120/208

Hillbilly,
This is important remark! will the oven need 120V power if it's rated 240V with single point connection? If it does need 120V, wouldn't the transformation be done internally within the over?

 

[augie47]

Most, not all, ovens and dryers need 120v, If can be derived from a 208/120 or 240/120 system

 

[Volt-Amps]

hmmm but you said in the OP "I want to bring 3-phase power to the meter bank but 120/240V single phase to each dwelling."

Yes, to help distribute the loads on 3 phases. From what I learned here on this forum is that single phase equipment rated 240V can be connected to AB, BC, or AC. So for electric oven, condenser, and dryer (all 240V, 1 phase, can be circuited without skipping the B leg.

 

[augie47]

Yes, to help distribute the loads on 3 phases. From what I learned here on this forum is that single phase equipment rated 240V can be connected to AB, BC, or AC. So for electric oven, condenser, and dryer (all 240V, 1 phase, can Yedsbe circuited without skipping the B leg.

Yes, if it's straight 240 with no 120v requirement.
Do note that if B phase is used, the breaker must be "straight rated"(240), no slash rated 240/120 (they cost more)
(Code reference is 240.85)

 

[hillbilly1]

Hillbilly,
This is important remark! will the oven need 120V power if it's rated 240V with single point connection? If it does need 120V, wouldn't the transformation be done internally within the over?

The drum motor is 120 on dryers, but with the advent of inverter drives, don’t have to be anymore. Ovens use combinations of voltages to control the heat output of the burners, cooktop elements can be run on 120 or 240 to reduce or increase heat output.

 

[electrofelon]

Yes, to help distribute the loads on 3 phases. From what I learned here on this forum is that single phase equipment rated 240V can be connected to AB, BC, or AC. So for electric oven, condenser, and dryer (all 240V, 1 phase, can be circuited without skipping the B leg.

You would not bring single phase with one phase the high leg to feed the apartments, just no. Don't even think about it, do not pass go do not collect $200.

 

[Volt-Amps]

120/240V single phase is _slightly_ preferable for a dwelling than 120/208V. But for large buildings it is preferable to present a balanced three phase load to the utility.

The electrician might be thinking that a high leg delta transformer will somehow balance the three phase loading.

I agree with the others, a high leg delta is a bad choice for this application. Most high leg deltas are designed for three phase 240V loading with incidental 120/240V loading. You can in principal make a high leg delta which will function correctly with mostly 120/240V single phase loading, but now you are talking about an uncommon beast.

I concur with using 120/208; but if the electrician is really pushing to get 120/240 then IMHO the simplest approach is to use single phase transformers and put different floors on different 480V legs to balance the loading.

If you want fancy, the 'scott-T transformer' mentioned in post #4 is one possible choice; the other option is 'hexaphase'.

-Jonathan

I believe single phase transformers and alternating between floors is the best course to take.

 

[wwhitney]

I believe single phase transformers and alternating between floors is the best course to take.

Along with doing the actual load calculations for the various apartments on each floor, to confirm that will balance the load among the 3 phases. The easy case is when the number of floors is a multiple of 3, and each floor is identical, then it would definitely be balanced.

Cheers, Wayne

 

[tortuga]

Definitely true if the number of identical floors is a multiple of 3. If the number of floors is 1, 2, 4, or 5, an alternate arrangement to balance the load may be worthwhile.

Cheers, Wayne

Agreed
125A dewlling feeders @ 120/240 vs 150A @ 120/208 and transformer costs being equal go with 240V

 

[Volt-Amps]

So, you suspect he wants 240 volts rather than 208 volts as he is hoping to reduce the panel size based on the voltage?

Yes. My calculations at 120/208 give 139a to 141 amp per unit. So I told the team 150a minimum, 175A recommended for each dwelling.
I had a discussion with the architect and contractor late evening yesterday, the contractor's goal is to cap each dwelling at 125A. I guess it makes a difference when you have 5 floors with 7 units each. He may have a point, but I want to make sure nothing gets fried due to wrong connection being made in the field.

 

[Volt-Amps]

Yes, if it's straight 240 with no 120v requirement.
Do note that if B phase is used, the breaker must be "straight rated"(240), no slash rated 240/120 (they cost more)
(Code reference is 240.85)

Yes, indeed. Another thing I learned from the forum, and have been adding this note on all high leg jobs I did.

 

[Volt-Amps]

So enriching is talking to you guys. You are the best. Thank you!