120/240 High Leg Service

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Volt-Amps

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Regards,

I have a load calculations question:
1. Incoming service voltage is 120/240, high leg, 3 Phase.
2. But all my loads are single phase (bldg with 4 residential units).
3. I populate the panel schedules (A, skip B, C, and so on).
4. Since column B remains blank with 0 loads, I end up with total load on A and C legs.
5. Now when I convert kVA to Amps, do I divide by Sq. Rt of 3 (1.732), even though it's high leg, no 3 phase loads and nothing on B leg?

Thank You
 
Volt-Amps said:
Regards,

I have a load calculations question:
1. Incoming service voltage is 120/240, high leg, 3 Phase.
2. But all my loads are single phase (bldg with 4 residential units).
3. I populate the panel schedules (A, skip B, C, and so on).
4. Since column B remains blank with 0 loads, I end up with total load on A and C legs.
5. Now when I convert kVA to Amps, do I divide by Sq. Rt of 3 (1.732), even though it's high leg, no 3 phase loads and nothing on B leg?

Thank You
Click to expand...

4 residential units and no 240V loads?
 
180624-0739 EDT

Volt-Amps:

Most likely your source is two separate transformers. One transformer is a 240 V center tapped unit used to supply all single phase 120 V loads, and probably all single phase 240 V loads.

A second 240 V transformer is connected to one end, not the center tap, of the first transformer. If a very large three phase load existed, then a third transformer would be added to form a closed delta. I would suggest that never would a three phase transformer be used, and possibly there are none made with the center tapped winding.

Thus, as you are going to use this source it is nothing more than just a split phase single phase source.

Suppose the transformer were a 50 kVA unit, then each half of the secondary can be loaded to only 50,000/240 = 208 A. This is based on the logical assumption that the entire 240 V winding is wound with the same size wire.

If your loads are 100 A 120 V and 60 A 120 V, then the maximum 240 V load is 108 A. Which puts 208 A on 1/2 of the secondary and 168 A on the other half.

If you add three phase loads, then some single phase loads need to be reduced, and depending upon whether it is an open or closed delta, and how the transformers are connected, then the single phase load may need redistribution.

.
 
Just to add, 240v loads that have no neutral connection can be connected to the high leg and either other line.
 
LarryFine said:
Just to add, 240v loads that have no neutral connection can be connected to the high leg and either other line.
Click to expand...

And just to add, you must be careful not to overload the transformer driving the high leg. It can be significantly lower in capacity than the one or ones driving the other two legs.
 
My favorite way to do this is to use two panels; a typical 1ph with neutral, and a 3ph without it.
 
Volt-Amps said:
Regards,

I have a load calculations question:
1. Incoming service voltage is 120/240, high leg, 3 Phase.
2. But all my loads are single phase (bldg with 4 residential units).
3. I populate the panel schedules (A, skip B, C, and so on).
4. Since column B remains blank with 0 loads, I end up with total load on A and C legs.
5. Now when I convert kVA to Amps, do I divide by Sq. Rt of 3 (1.732), even though it's high leg, no 3 phase loads and nothing on B leg?

Thank You
Click to expand...
Is the building being converted from some other usage to residential usage?

Since it is delta system - the one side of the delta that has the center tap neutral - is exactly the same thing as a stand alone single phase transformer. If you were to open the primary line(s) supplying the other side(s) (open vs full delta) you are still left with basically same single phase source as if you weren't subscribed to three phase supply. One must make sure you still have enough available kVA, as the load is all on one side of the delta now.

Only time you can convert VA to A using the 1.732 factor is when all three phases are carrying equal load. Unbalanced loads you can only calculate the balanced amps that way. Since the high leg has zero amps the balanced portion is zero. 0 x 1.732=0, now calculate your unbalanced load on unbalanced segments using V/A and no additional factors.
 
LarryFine said:
True, but most 2p and 3p breakers are so rated.
Click to expand...

3-pole yes but not 2-pole. I don't know any suppliers around here that stock 240v 2-pole breakers. They are special order.

I just skimmed over this thread but if this service is only servicing residential units and not any 3-phase loads I would change it to 1-phase.
 
ActionDave said:
3 pole breakers yes. 2 pole, not so much most of the off the shelf ones are slash rated 120/240. Not that anybody notices. I didn't till I joined here.
Click to expand...
Big box stores won't have 240 only breakers. Supply houses often won't have them in stock - unless maybe they have enough industrial places in their client base that regularly use them. Just one plant that regularly uses them - they will suggest the plant order extras to keep in their own stock.

Been a long time now since I inquired about a QO 2 pole - seems it was going to be over $100 when a 120/240 standard breaker was only maybe $12 or so.
 
I did preliminary design changing it to single phase

I did preliminary design changing it to single phase

But owner wants to keep existing service. I ran my calculations without 1.732 because it didn’t make since to me to do, but obviously there is more for me to learn about this 120/240 high leg services.
I am the only electrical engineer at this small firm, unfortunately no senior engineer is here with me to learn from, I feel I can learn a lot from contractors here and experienced engineers on this site than any other source.
For example, the very first thing I would want to know is probably if the existing service is open delta or closed delta. How do I determine that?
 
180623-2123 EDT

Volt-Amps:

Count the number of transformers. Two and it is open delta, three and it is closed.

If neutral is earthed, and it almost certainly is, then the wild leg should read about 240*cos 30 = 240*0.866 = about 207 between the wild leg and neutral or earth. But this measurement won't tell you whether it is open or closed.

If you put a large single phase load singularly across each pair of phase wires, then you might be able to tell which pair of wires was not powered from a secondary because of the difference in source impedance.

Another method would be to load the wild leg to neutral and see which wire from the single phase center tapped winding had a change in current.

.
 
Volt-Amps said:
But owner wants to keep existing service. I ran my calculations without 1.732 because it didn’t make since to me to do, but obviously there is more for me to learn about this 120/240 high leg services.
I am the only electrical engineer at this small firm, unfortunately no senior engineer is here with me to learn from, I feel I can learn a lot from contractors here and experienced engineers on this site than any other source.
For example, the very first thing I would want to know is probably if the existing service is open delta or closed delta. How do I determine that?
Click to expand...
If source is two transformers it is open delta, if it is three transformers it is closed delta. Either open or closed can have different sized units depending on how it was designed to be used. Lots of 120/240 single phase load but only limited amount of three phase load - the 120/240 unit may be large in comparison to the other(s). If originally set up for a heavy amount of three phase loads - all three very well may be same size.

If you aren't going to have any three phase load at all it really won't matter to you if it is open or closed delta - only thing that will matter is the side of the delta that has the 120/240 - it is all that will see load as you described things in the OP.
 
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