120/240V panel fed from a 208/120 source.

[Ozymandias]

Hi all,

I have a job where I have a 200A fused disconnect with two phases(208/120), the neutral and ground. I will be running feeders a 120' away to a panel which will feed rows of refrigerators.
Is it permissible to use two phases out of the three?
I assume so since that disco fed a panel which is no longer there, and because only two phases are available could I use a 240/120V panel.?
A three phase panel would only have two buses energized. On the 240/120V panel, only 120V circuits would be used from the panel, I understand I would only have 208V available instead of the usual 240V. Thanks for the input!

 

[electrofelon]

Hi all,

I have a job where I have a 200A fused disconnect with two phases(208/120), the neutral and ground. I will be running feeders a 120' away to a panel which will feed rows of refrigerators.
Is it permissible to use two phases out of the three?
I assume so since that disco fed a panel which is no longer there, and because only two phases are available could I use a 240/120V panel.?
A three phase panel would only have two buses energized. On the 240/120V panel, only 120V circuits would be used from the panel, I understand I would only have 208V available instead of the usual 240V. Thanks for the input!

Two phases of a three phase system is single phase, you can use a regular single phase 120/240 panelboard, absolutely. Done all the time.

 

[Fred B]

Hi all,

I have a job where I have a 200A fused disconnect with two phases(208/120), the neutral and ground. I will be running feeders a 120' away to a panel which will feed rows of refrigerators.
Is it permissible to use two phases out of the three?
I assume so since that disco fed a panel which is no longer there, and because only two phases are available could I use a 240/120V panel.?
A three phase panel would only have two buses energized. On the 240/120V panel, only 120V circuits would be used from the panel, I understand I would only have 208V available instead of the usual 240V. Thanks for the input!

Delta or Wye

 

[infinity]

I beleive that all 120/240 volt panels are listed for use on a single phase 208/120 wye system. As electro stated it's done all of the time especially in apartment building construction.

 

[jim dungar]

... could I use a 240/120V panel.?

Yes.
Panels are rated for their maximum voltage(s). Your single phase 208V is less than the maximum Line-Line of 240V, and the 120V Line-Ground rating is also not exceeded.

 

[Ozymandias]

Delta or Wye

Not a 100% but I believe wye

 

[winnie]

If it is a 208/120 source, then it is wye or the equivalent. (In theory it could be a 208V delta plus a grounding transformer but this would be the equivalent of a common wye source.)

If the source were delta, then it would be 240V with 120/240V available by taking the two legs from the center tap coil, and the third leg (the stinger) would be 208V to ground.

Jon

 

[Ozymandias]

Delta or Wye

If it is a 208/120 source, then it is wye or the equivalent. (In theory it could be a 208V delta plus a grounding transformer but this would be the equivalent of a common wye source.)

If the source were delta, then it would be 240V with 120/240V available by taking the two legs from the center tap coil, and the third leg (the stinger) would be 208V to ground.

Jon

Yes, I was concerned about it because it’s such an old building, but once I measured 208/120V at disconnect I am assuming it’s a WYE,.

 

[Ozymandias]

The service is 400A, I was able to clamp on at a gutter and the loads on a 90F day were within 5 amps on all phases. Around 65A.

 

[garbo]

I would never install a 200 amp single phase sub panel due to it could overload 2 of the existing 3 phases. It would be best to install a 120/208 3 phase panel. would not cost that much more for the additional line conductor and a 3 phase panel. Best practice is to look at future needs. we never ran any motor over 2 HP on single phase. If you do place a large load on only 2 of the 3 phases you run the risk of voltage drop on the 2 phase that will cause 3 phase motors to run hotter then normal. Too many customers want the cheapest job but if it was me I would walk away from a job where they insisted on a single phase 200 sub panel. Work safe & STAY HEALTHY.
e

 

[Ozymandias]

I would never install a 200 amp single phase sub panel due to it could overload 2 of the existing 3 phases. It would be best to install a 120/208 3 phase panel. would not cost that much more for the additional line conductor and a 3 phase panel. Best practice is to look at future needs. we never ran any motor over 2 HP on single phase. If you do place a large load on only 2 of the 3 phases you run the risk of voltage drop on the 2 phase that will cause 3 phase motors to run hotter then normal. Too many customers want the cheapest job but if it was me I would walk away from a job where they insisted on a single phase 200 sub panel. Work safe & STAY HEALTHY.
e

Interesting. That’s what made me pose the question, that I could unbalance the loads. The main disco is 400A and each leg under load hovers around 60A. Downstream a 200A fused disco(two legs/phases/208V) feeding the 200A 40 space panelboard that will power 80 refrigerators continuously.

 

[GoldDigger]

The apartment building situation is much better balanced since roughly 1/3 of the apartments use each pair from the three possible pairs.
A single large and unique unbalanced load is troublesome.

Sent from my Pixel 4a using Tapatalk

 

[LarryFine]

Downstream a 200A fused disco(two legs/phases/208V) feeding the 200A 40 space panelboard that will power 80 refrigerators continuously.

I would definitely go with 3ph and use 4-wire MWBCs.

 

[Ozymandias]

I would definitely go with 3ph and use 4-wire MWBCs.

Right, I think for my peace of mind, I will suggest changing the disco to 3 phase and run the last leg to it. Then 3 phase panelboard downstream. Not sure what MWBC is?

 

[synchro]

I would definitely go with 3ph and use 4-wire MWBCs.

Agree. The neutral conductor of the feeder will also experience much less voltage drop if it's a 3ph 4-wire with balanced loads vs. a 3-wire with only two of the phases.

 

[LarryFine]

Not sure what MWBC is?

Multi-Wire Branch Circuits. Two or three hots sharing a neutral.

 

[Ozymandias]

Am I thinking correctly here?
If I have 80 refrigerators at 5 amps average for a total of 400 amps, divided by 3 phases to equal 133 amps per phase?
I’m trying to size the panel board,
Thank you all for the help.

 

[jim dungar]

Am I thinking correctly here?
If I have 80 refrigerators at 5 amps average for a total of 400 amps, divided by 3 phases to equal 133 amps per phase?

You should balance the number of refrigerators then calculate your amps.
80/3 = 27 on the highest loaded leg. 27 x 5 = 135A for the largest conductor/phase.

 

[electrofelon]

I would definitely go with 3ph and use 4-wire MWBCs.

I agree. I did not catch how many circuits would be needed in the OP. OP remember you will need three pole breakers. Technically you can use single poles with a 3x handle tie, and it will be less than half the cost of a three pole however note not everyone makes a three pole handle tie. Siemens does not. They are available for Eaton BR.

 

[Ozymandias]

I agree. I did not catch how many circuits would be needed in the OP. OP remember you will need three pole breakers. Technically you can use single poles with a 3x handle tie, and it will be less than half the cost of a three pole however note not everyone makes a three pole handle tie. Siemens does not. They are available for Eaton BR.

Thanks for the tip, but are you referring to a main breaker at the panelboard or just the individual breakers feeding branch circuits.

I think in the end I’ll have a 3 phase
disco(fused or with a breaker) and travel about a 100’ to the 3 phase panelboard which will either be an MLO or a 3 pole main breaker panel with individual single pole breakers feeding branch circuits.
Sorry for the confusion Electrofelon.