Center Tap Delta Service Calc

[lauraj]

I just want to make sure I'm going about this the right way. I'm trying to determine minimum service size.

I've calculated and distributed the volt-amps for each of the loads shown in the attachment. A-phase carries the largest load, so I'm going to use that for my calculations.

My problem is, if I calculate out amps for each load and fill out the panel schedule I get 510 amps on A phase.

I can't figure out how to get 510 amps using the VA values listed in my drawing to calculate amps.

If I separate out the 120 v loads (first 5 loads) and calculate amps from those I get 39,221.8va/120v = 326.8 A

The total VA for the 3-ph loads add up to 25,395.7va. In order to get a total of 510 amps, I have to divide the 25kva by 138v which doesn't make sense to me. (138 v = 240/1.732)

 

[Smart $]

I just want to make sure I'm going about this the right way. I'm trying to determine minimum service size.

I've calculated and distributed the volt-amps for each of the loads shown in the attachment. A-phase carries the largest load, so I'm going to use that for my calculations.

My problem is, if I calculate out amps for each load and fill out the panel schedule I get 510 amps on A phase.

I can't figure out how to get 510 amps using the VA values listed in my drawing to calculate amps.

If I separate out the 120 v loads (first 5 loads) and calculate amps from those I get 39,221.8va/120v = 326.8 A

The total VA for the 3-ph loads add up to 25,395.7va. In order to get a total of 510 amps, I have to divide the 25kva by 138v which doesn't make sense to me. (138 v = 240/1.732)

Can't tell exactly where you are going wrong without knowing the amp values for the loads... or how it is you are getting the 510 amps. Offhand, it seems you may be making your mistake in converting your amp values to va... either that or how you are arriving at the 510 amp total on Line A.

Nonetheless, you are correct on the 138V value. Total three phase VA is equal to line amperes x voltage x sqrt(3). But the total va would be divided by 3 for the amount of va per line. To get the amount amperes per line from the va per line value you would multiply by 3 then divide by the quantity of 240 volts times sqrt(3). This is the same as multiplying by sqrt(3)/240, or 1/138 volts.

 

[lauraj]

Can't tell exactly where you are going wrong without knowing the amp values for the loads... or how it is you are getting the 510 amps. Offhand, it seems you may be making your mistake in converting your amp values to va... either that or how you are arriving at the 510 amp total on Line A.

Each 10 HP motor is 28 A, from motor tables in the NEC.

Heat load is 120 A, 50kw/(240*1.732)

Largest motor adds an additional 7 amps

Add these up, I get 183 amps (3-phase loads only)

327 A (1-ph) + 183 A (3-ph) = 510 A

Nonetheless, you are correct on the 138V value. Total three phase VA is equal to line amperes x voltage x sqrt(3). But the total va would be divided by 3 for the amount of va per line. To get the amount amperes per line from the va per line value you would multiply by 3 then divide by the quantity of 240 volts times sqrt(3). This is the same as multiplying by sqrt(3)/240, or 1/138 volts.

Thank you, I was missing the part that I needed to multiply P again by 3 when I want to go back the other way!


Total P(1-ph) = 1172+1500+16000+15750+4800 = 39221 VA
39221va/120v = 327 A (perfect)

Total P(3-ph) = 3880+3880+16667+970 = 25397 VA
(25397x3)/(240*1.732) = 183 A (perfect)

 

[bob]

Using you figures you have A 64.6 kva B 25.4 kva C 63.3 kva

From this info you have 25.4 kva x 3 = 76.2 kva 3 phase
Phase A 64.6 kva - 25.4 kva = 39.2 kva single phase
Phase C 63.3 kva - 25.4 kva = 37.9 kva single phase

3 phase ampe = 76.2 kva/(.24 x 1.73) = 183.5 amps
1 phase amps = 39.2 kva + 37.9 kva = 77.1 kva/0.24 = 321.3 amps

A & C amps 183.5 + 321.3 = 506.8 amps.
B amps 183.5 = 183.5 amps.