Service size calculations when combining 1-ph and 3-ph services into one 3-ph service

[CCkk888]

How do I determine size of new service when combining two 3-phase and one 1-phase? I have the peak demands. It?s the 3-phase vs 1-phase calculation I am having trouble with. All are 240/120v; 3-ph @ 48 KW, 3-ph, 3-ph @ 66.8 KW, 1-ph @64.4 KW.

 

[bob]

Just add the demands together but you must be able to balance the 64 kw across the 3 phases. So your total would be
48 KW, 3-ph, 3-ph + 66.8 KW + 1-ph @64.4 KW = 179 kw/power factor. assume 0.80 = 234 kva

Amps = 234/(.24 x 1.73) = 563 amps. Problem is that usually on a delta transformer bank the single phase load is on the
middle transformer and you can not get the load balanced. If the single phase load is all 240 volts then is can be balanced. Can you check and see if this is possible?

 

[jrohe]

How do I determine size of new service when combining two 3-phase and one 1-phase? I have the peak demands. It’s the 3-phase vs 1-phase calculation I am having trouble with. All are 240/120v; 3-ph @ 48 KW, 3-ph, 3-ph @ 66.8 KW, 1-ph @64.4 KW.

You need to calculate this phase-by-phase.

Assumptions:
1. The loads on the 3-phase services are balanced between all three legs (12kW per phase + 22.3 kW per phase)
2. The single phase service is connected to phases A and C since phase B will be the wild leg and the smallest of the transformers
3. The loads on the single-phase service are balanced between both legs (32.2 kW per phase)


This results in the following:
Phase A = 12 kW + 22.3 kW + 32.2 kW = 66.5 kW / 120 volts = 554.2 amps
Phase B = 12 kW + 22.3 kW = 34.3 kW / 120 volts = 285.8 amps
Phase C = 12 kW + 22.3 kW + 32.2 kW = 66.5 kW / 120 volts = 554.2 amps

Then, since you are using the peak demand loads, NEC section 220.87(2) requires you to multiply these values by 125%.

 

[CCkk888]

Just add the demands together but you must be able to balance the 64 kw across the 3 phases. So your total would be
48 KW, 3-ph, 3-ph + 66.8 KW + 1-ph @64.4 KW = 179 kw/power factor. assume 0.80 = 234 kva

Amps = 234/(.24 x 1.73) = 563 amps. Problem is that usually on a delta transformer bank the single phase load is on the
middle transformer and you can not get the load balanced. If the single phase load is all 240 volts then is can be balanced. Can you check and see if this is possible?

The new combined service will be 208/120v. We will be replace motors or use buck/boost transformers for any remaining 240v loads.

 

[kwired]

The new combined service will be 208/120v. We will be replace or use buck/boost transformers for any remaining 240v loads.

Any loads operating at 120 volts will remain same. For non line to neutral loads: resistive loads will decrease in total current unless buck/boost is used inductive loads will increase in current - but will remain at about same VA.

Figure out what the best possible phase balancing arrangement is - and take that total VA and add individual unbalanced load for each phase. Really all you are concerned with is the max unbalanced portion, if you are sized to handle that the other lines will be loaded less.

 

[Smart $]

The new combined service will be 208/120v. ...

Will you be changing the 120/240 1? 3W service over to 208/120 3? 4W or simply keeping it as is for the most part and making it 120/208 1? 3W?

If changing over, you can re-balance the sub, and divide the load across all three line conductors.

If not, you will have an unbalanced condition as previously mention, and will have to compensate.

​

A word of caution on the existing split phase wiring. Any derating of the existing wiring likely considered the neutral as a non-current-carrying conductor. With 120/208 1? 3W feeders and branch circuits, the neutral will be a current carrying conductor. You will have to re-evaluate split phase wiring for possible derating errors. Feeder neutral sizing should be okay, but I suggest you re-evaluate these too.

Calculations

48 + 66.8 = 114.8kW
48 + 66.8 + 64.4 = 179.2kW

Combining without changing 1? to 3?:
114.8kW ? 208V ? 240V ? 0.8pf = 124.4kVA ... ? (208 ? 1.732) = 345.2A [208/120 3? 4W]
64.4kW ? 208 ? 240V ? 0.8pf = 335.4kVA ... ? 208 = 335.4A [120/208 1? 3W]
345.2 + 335.4 = 680.6A on two lines

Combining with changing 1? to 3?:
179.2kW ? 208V ? 240V ? 0.8pf = 194.1kVA ... ? (208 ? 1.732) = 538.9A [208/120 3? 4W]


Don't forget the ampacity of the service conductors must be at least 125% of the calculated values.

 

[Smart $]

You need to calculate this phase-by-phase.

Assumptions:
1. The loads on the 3-phase services are balanced between all three legs (12kW per phase + 22.3 kW per phase)
2. The single phase service is connected to phases A and C since phase B will be the wild leg and the smallest of the transformers
3. The loads on the single-phase service are balanced between both legs (32.2 kW per phase)


This results in the following:
Phase A = 12 kW + 22.3 kW + 32.2 kW = 66.5 kW / 120 volts = 554.2 amps
Phase B = 12 kW + 22.3 kW = 34.3 kW / 120 volts = 285.8 amps
Phase C = 12 kW + 22.3 kW + 32.2 kW = 66.5 kW / 120 volts = 554.2 amps

Then, since you are using the peak demand loads, NEC section 220.87(2) requires you to multiply these values by 125%.

One of us is wrong on the calculations...

 

[topgone]

IDK if you noticed what the OP posted. "All 240/120V". This is a split-phase source, not 3-phase! If the supply is really a 3-phase source, it should either be a 208/120 or a 220/127 - three phase. Smal things that matter much when computing for amps here.

 

[kwired]

IDK if you noticed what the OP posted. "All 240/120V". This is a split-phase source, not 3-phase! If the supply is really a 3-phase source, it should either be a 208/120 or a 220/127 - three phase. Smal things that matter much when computing for amps here.

He later clarified that is will be changed from single phase to 208/120 three phase. However it could have been changed to 240/120 3 phase delta - and would less complicate what happens by dropping to 208 volts for some loads, but also results in one phase being 208 volts to neutral so it can not be used for 120 volt loads.