120/240V 3ph Delta Questions

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32Lateralus

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So I am unfamiliar with this system as all my design time so far has been on 208V and 480V systems. I have been asked to take a look at a building that has a 120/240V 3ph delta system, with a '3-pot bank' which seems to mean 3 individual pole mounted xfmrs making a closed delta with a high leg. Is this all correct to start?

My understanding is (assuming B as high leg):
A-N: 120V
B-N: 208V
C-N: 120V
All L-L is 240V

Now where I am uncertain is can 3 phase loads such as motor loads be connected to this system as long as they are rated 230V 3ph?

When calculating 3ph load would the voltage be 240V*sqrt(3) ~ 415V
So a 10A, 230V 3ph motor load.... 10A * 415V ~ 4150VA.... and is this distributed equal on the phases so 1383VA per phase?

Let me know if there is anything I am missing
 
32Lateralus said:
Let me know if there is anything I am missing
Click to expand...
So I am unfamiliar with this system as all my design time so far has been on 208V and 480V systems. I have been asked to take a look at a building that has a 120/240V 3ph delta system, with a '3-pot bank' which seems to mean 3 individual pole mounted xfmrs making a closed delta with a high leg. Is this all correct to start?

My understanding is (assuming B as high leg):
A-N: 120V
B-N: 208V
C-N: 120V
All L-L is 240V
Click to expand...
sounds right.

Now where I am uncertain is can 3 phase loads such as motor loads be connected to this system as long as they are rated 230V 3ph?
Click to expand...

Yup three phase (or single phase L-L) loads that do not have a neutral are fine. The two disclaimers are there may be some VFD's that need a balanced wye supply, and two pole L-L loads that use the high leg will need straight not slash rated breakers.

When calculating 3ph load would the voltage be 240V*sqrt(3) ~ 415V
So a 10A, 230V 3ph motor load.... 10A * 415V ~ 4150VA.... and is this distributed equal on the phases so 1383VA per phase?
Click to expand...

No, voltage is 240. 3 phase KVA = I x V X 1.732.
 
Your all good so far. I use amps * 415 also for three phase loads. For single phase 240 you still use 240.
The one thing I'll add is that when you have limited 120V single phase, you can use any of the legs for L-L 240V single phase loads, like LED lighting, you just need to make sure you use straight rated breakers.
I usually do a standard single phase 120/240 panel for all the 120V stuff like office plugs.
Then a three phase panel for all the 240V loads, single or three phase.
 
electrofelon said:
sounds right.



Yup three phase (or single phase L-L) loads that do not have a neutral are fine. The two disclaimers are there may be some VFD's that need a balanced wye supply, and two pole L-L loads that use the high leg will need straight not slash rated breakers.



No, voltage is 240. 3 phase KVA = I x V X 1.732.
Click to expand...
Right, I kinda added a non-existent step, youre saying it's not 415V its 240V 3ph. The math is right but for technicality leave it as 10A * 240V * sqrt(3)

Is that what youre saying?
 
415V is just a handy shortcut to convert Amps to VA for the balanced 3ph delta load.
Say i have a 60A sub panel and I want to know how many VA it can power
60A X 415 = 24900 VA
 
32Lateralus said:
So I am unfamiliar with this system as all my design time so far has been on 208V and 480V systems. I have been asked to take a look at a building that has a 120/240V 3ph delta system, with a '3-pot bank' which seems to mean 3 individual pole mounted xfmrs making a closed delta with a high leg. Is this all correct to start?

My understanding is (assuming B as high leg):
A-N: 120V
B-N: 208V
C-N: 120V
All L-L is 240V

Now where I am uncertain is can 3 phase loads such as motor loads be connected to this system as long as they are rated 230V 3ph?

When calculating 3ph load would the voltage be 240V*sqrt(3) ~ 415V
So a 10A, 230V 3ph motor load.... 10A * 415V ~ 4150VA.... and is this distributed equal on the phases so 1383VA per phase?

Let me know if there is anything I am missing
Click to expand...

You may want to peer at the utility's service drop and confirm that all the phase conductors are the same size. That's a sign they expect higher load on the high leg. A smaller high leg means the service was sized for less three-phase load and it may be problematic to add a lot. The latter would probably be open delta instead of closed. I've seen cases where the customer owned service conductors are all the same size on an open delta, so don't go by those.
 
jaggedben said:
You may want to peer at the utility's service drop and confirm that all the phase conductors are the same size. That's a sign they expect higher load on the high leg.
Click to expand...
I think of a 240V high leg delta service as being two different 3-wire services that share 2 wires, so 4 wires total. Namely a 240V 3-wire delta 3-phase and a 120/240V split single phase.

From that point of view, how does one determine the ratings of the 2 separate subservices? I understand that they can be built with different transformer combinations, sometimes for when the majority of the load is 3 phase, and sometimes for when the majority of the load is single phase.

Cheers, Wayne
 
A word of caution: Art 240.85 cautions you for use of a "standard" (slash-rated 120/240) 2 pole breaker on this system where one pole would see 208 volts to ground. Straight rated 2 pole 240v breakers are available but more uncommon.
 
Back in the days of T8 fluorescent lighting there were some 120-277 "multi-volt ballasts" that expected one leg to be a neutral, and did not play well on 240 lighting circuits. Now with LED lighting most take 240V L-L just fine.
Also if you do 240V lighting single phase you need 2 pole contactors and or switches as there is no neutral.
 
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