Open delta load calc

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Npstewart

Senior Member
Speaking with another designer I work with, he has a difference of opinion on load calcs for an open delta service.

He is saying for open delta services you should have two seperate load calcs, one for single phase loads, and one for three phase loads. So basically you do the load calcs and add the demand ampacity together at the end. Is this correct.

Typically with open deltas I have a minimum of two panels, one three phase which I feed all my three phase loads from, and then I feed a single phase panel from the three phase panel using the A&C legs, and I feed all my 120V single phase loads from there. I know I could technically just use the A&C spaces in the three phase panel but I always run out of spaces. Anyway when im done, I only have one load calc and I calculate the required service size using a voltage of 240-3.

If I did it like the designer suggests, I would have two load calcs one three phase and one single phase, one would have a voltage of 240-3 and one would have a voltage of 240-1, the result would be the sum of the amps.


Thanks guys.
 

Smart $

Esteemed Member
Location
Ohio
A lot depends on how the system is balanced.

I believe the typical open delta service is for where the major load is 120/240, with little 3? loading. In this case, yes the AC xfmr would need likely be sized larger than the AB. Yet it is not totally unfeasible to have a balanced overall 3? loading.
 

bob

Senior Member
Location
Alabama
If I did it like the designer suggests, I would have two load calcs one three phase and one single phase, one would have a voltage of 240-3 and one would have a voltage of 240-1, the result would be the sum of the amps.
The larger transformer will carry 100% of the single phase load and each transformer will carry 58% of the three phase load.. Since you have loads with different angles, you can not add them directly. However if you did, it would give you an approximation of the total.
 

Npstewart

Senior Member
Maybe im missing something here. I only have one transformer, its a 240-3 open delta transformer....My question is basically referring to the load calc itself.
 

jghrist

Senior Member
Maybe im missing something here. I only have one transformer, its a 240-3 open delta transformer....My question is basically referring to the load calc itself.
I've never heard of an open delta transformer. I thought they were all made up of two single-phase transformers. Not saying there isn't one made, just that I've never run into one.
 

Hameedulla-Ekhlas

Senior Member
Location
AFG
Me neither.
Welcome back Mivey


I think open delta is a 3phase transformer bank using 3Ph to 1Ph or 3Ph to 2Ph

"If one of the transformers from a delta connected bank is removed, the remaining two are said to be open delta connected. This can be done, and the remaining two transformers will still transform the voltages in all three phases and supply power to all three phases of the secondary mains.

The open delta connection is often used where an increase in load is anticipated. The third unit is added when the load grows to the point at which it exceeds the capacity of the two transformers. Furthermore, if one transformer of a three-phase bank should become defective, the defective transformer can be removed and the remaining two transformer continue to render service to at least part of the load."
 

Hameedulla-Ekhlas

Senior Member
Location
AFG
Speaking with another designer I work with, he has a difference of opinion on load calcs for an open delta service.

He is saying for open delta services you should have two seperate load calcs, one for single phase loads, and one for three phase loads. So basically you do the load calcs and add the demand ampacity together at the end. Is this correct.

Typically with open deltas I have a minimum of two panels, one three phase which I feed all my three phase loads from, and then I feed a single phase panel from the three phase panel using the A&C legs, and I feed all my 120V single phase loads from there. I know I could technically just use the A&C spaces in the three phase panel but I always run out of spaces. Anyway when im done, I only have one load calc and I calculate the required service size using a voltage of 240-3.

If I did it like the designer suggests, I would have two load calcs one three phase and one single phase, one would have a voltage of 240-3 and one would have a voltage of 240-1, the result would be the sum of the amps.


Thanks guys.
An open delta using two 100kVA transformers would have rating of 86.6 of the sum of the kVA of the transformers being used.
If two identical kVA rated transformers are added to a third, or phantom, transformer, giving a combined 300kVA, the rating of the transformer bank would be 57.7% of the 300kVA
 

Smart $

Esteemed Member
Location
Ohio
An open delta using two 100kVA transformers would have rating of 86.6% of the sum of the kVA of the transformers being used.
Please note minor correction.

If two identical kVA rated transformers are added to a third, or phantom, transformer, giving a combined 300kVA, the rating of the transformer bank would be 57.7% of the 300kVA
I'm uncertain what you mean by adding 2 to a 3rd. If anything it would be adding a 3rd to 2 in existence. You would not have 3? power with just one.

That said, with 3 100kVA bank of xfmr's delta connected, you would have 300kVA of 3? power available.
 
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Hameedulla-Ekhlas

Senior Member
Location
AFG
Good to be home. Motel living get old.We get that. The OP was saying a single transformer enclosure, not two transformers banked together. The enclosures I have seen were either one or three windings.
yeah you are right. I was just talking regarding his question. He might mean 3phase transformer bank.
 

Hameedulla-Ekhlas

Senior Member
Location
AFG
Please note minor correction.


I'm uncertain what you mean by adding 2 to a 3rd. If anything it would be adding a 3rd to 2 in existence. You would not have 3? power with just one.

That said, with 3 100kVA bank of xfmr's delta connected, you would have 300kVA of 3? power available.
please note minor correction
Good point and thanks.

That said, with 3 100kVA bank of xfmr's delta connected, you would have 300kVA of 3? power available
I have mentioned that

An open delta using two 100kVA transformers would have a rating of 86.6% of the sum of the kVA of the transformer being used.If two identical kVA rated transformers are added to a third giving a combined 300 kVA, the rating of the transformer bank would be 57.7% of the 300 kVA

200kVA * 86.6% = 173 kVA
300kVA * 57.7% = 173 kVA
 

Smart $

Esteemed Member
Location
Ohio
...

I have mentioned that

An open delta using two 100kVA transformers would have a rating of 86.6% of the sum of the kVA of the transformer being used.If two identical kVA rated transformers are added to a third giving a combined 300 kVA, the rating of the transformer bank would be 57.7% of the 300 kVA

200kVA * 86.6% = 173 kVA
300kVA * 57.7% = 173 kVA
I'm just not getting what you are saying.

If you have three 100kVA transformers, they would be configured closed delta and provide the total rated power of 300kVA.

Please try to restate your point another way, and just maybe I'll see what you are getting at.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Engineer
You can determine the 3-phase kVA rating of an open delta in one of two methods:

#1 - typically used when first creating the open-delta
take 86.6% of the sum of the two transformers, for example: (2) 100kVA units are connected in open-delta yielding .866 x (100+100) = 173kVA 3-phase
or
#2 - typically used when (1) transformer is removed thereby creating an open-delta
take 57.7% of the full 3-phase kVA rating, for example: (1) 100kVA unit is removed from an existing 300kVA bank yielding .577 x (300) = 173kVA remaining capacity.
 
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Smart $

Esteemed Member
Location
Ohio
...
#2 - typically used when (1) transformer is removed thereby creating an open-delta
take 57.7% of the full 3-phase kVA rating, for example: (1) 100kVA unit is removed from an existing 300kVA bank yielding .577 x (300) = 173kVA remaining capacity.
If this is what Ham' was trying to get at, I can understand this ;)
 

Hameedulla-Ekhlas

Senior Member
Location
AFG
You can determine the 3-phase kVA rating of an open delta in one of two methods:

#1 - typically used when first creating the open-delta
take 86.6% of the sum of the two transformers, for example: (2) 100kVA units are connected in open-delta yielding .866 x (100+100) = 173kVA 3-phase
or
#2 - typically used when (1) transformer is removed thereby creating an open-delta
take 57.7% of the full 3-phase kVA rating, for example: (1) 100kVA unit is removed from an existing 300kVA bank yielding .577 x (300) = 173kVA remaining capacity.
thanks for helping jim dungar.
 
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mivey

Senior Member
yeah you are right. I was just talking regarding his question. He might mean 3phase transformer bank.
Yeah. As strange as it might be, I supposed he could be calling a 2-pot bank one transformer. Then again, someone might actually make a 2-winding pot. Maybe we will find out.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Engineer
Yeah. As strange as it might be, I supposed he could be calling a 2-pot bank one transformer. Then again, someone might actually make a 2-winding pot. Maybe we will find out.
It is not really that strange, even NEC 450.3 talks about a "transformer" being two or more single phase units operating as a unit.
 

mivey

Senior Member
It is not really that strange, even NEC 450.3 talks about a "transformer" being two or more single phase units operating as a unit.
And that term is specifically identified for its use in that section. To continue with this use even after jumper, smart$, & bob started discussing the different line-line windings seems strange to me.
 
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