Three-phase calculation confusion

[Coppersmith]

I’m trying to figure out how big a three phase panel I need for the following:

3 x 2 pole 60 amp breaker powering A/C air handler, MCA 59 amps
3 x 2 pole 50 amp breaker powering A/C condenser, MCA 31 amps

The breakers will be as evenly distributed across the phases as possible.

I know how to do this for a split-phase panel, but three-phase confuses me. Please show me the calculation.

 

[charlie b]

You didn't tell us the voltage. I will use 480/277 for an example.
Add the VA first.

(3 units) x (480 volts - phase to phase) x 59 amps = 84,960 VA
(3 units) x (480 volts - phase to phase) x 31 amps = 44,640 VA
Total = 129,600 VA
Divide by 480 and divide the result by 1.732,
Total = 155.9 amps
I would select a 225 amp panel, to leave room for future loads.

 

[Coppersmith]

The panel will be 240 volts. When I substitute in 240 volts I get the same 155.9 amps.

Is that correct?

 

[topgone]

I’m trying to figure out how big a three phase panel I need for the following:

3 x 2 pole 60 amp breaker powering A/C air handler, MCA 59 amps
3 x 2 pole 50 amp breaker powering A/C condenser, MCA 31 amps

The breakers will be as evenly distributed across the phases as possible.

I know how to do this for a split-phase panel, but three-phase confuses me. Please show me the calculation.

Buy a 100A panel.

 

[Coppersmith]

Something doesn't seem right with that calculation.

If I have three 2-pole circuits drawing 59 amps seems like that means I have 108 amp on each phase. This would be the same if I had 2 three-phase circuits drawing 59 amps.

If additionally I have three 2-pole circuits drawing 31 amps seems like that means I have 62 amp on each phase.
108 + 62 = 170 amps on each phase*. Correct?

Does the 1.732 multiplier come into play here? I didn't think so.



*This is not taking into account that MCA is already 125% of actual load.

 

[electrofelon]

Buy a 100A panel.

Must be a typo or a math error

Something doesn't seem right with that calculation.

If I have three 2-pole circuits drawing 59 amps seems like that means I have 108 amp on each phase. This would be the same if I had 2 three-phase circuits drawing 59 amps.

If additionally I have three 2-pole circuits drawing 31 amps seems like that means I have 62 amp on each phase.
108 + 62 = 170 amps on each phase*. Correct?

Does the 1.732 multiplier come into play here? I didn't think so.

But you are distributing three L-L loads across three phases, AB, BC, CA, so all the fun phase angle stuff rears its head. It works out that the total current on the three phases system is 1.732 X the current of each single phase load

So yes the load is 156 amps, using the MCA figures which as you mentioned "overcount" because you are getting that extra 25% for each unit, not just the largest one of the group. In my experience, HVAC load is usually 2/3 of MCA which would easily fit on a 125 A panel, but that isnt a code approved method of course - Probably barely wont squeeze into a 125 amp panel the legit way, I would just go 200.

 

[oldsparky52]

Something doesn't seem right with that calculation.

If I have three 2-pole circuits drawing 59 amps seems like that means I have 108 amp on each phase. This would be the same if I had 2 three-phase circuits drawing 59 amps.

If additionally I have three 2-pole circuits drawing 31 amps seems like that means I have 62 amp on each phase.
108 + 62 = 170 amps on each phase*. Correct?

Does the 1.732 multiplier come into play here? I didn't think so.



*This is not taking into account that MCA is already 125% of actual load.

Just a technicality, but ... 59*2 = 118.

Another way to look at this is 59*3+31*3 = 270 (single phase line to line amps). Balance this on a 3-phase supply and it's 270/1.732= 156

 

[oldsparky52]

I don't know as much as I think I understand (or something like that), so I have a question for the more educated.

Where is (let's just pick A-phase) in the sine wave where it has the maximum load to the other 2 phases? It appears from just looking at a sine wave drawing that it might be when A-phase is at it's peak (either high or low). Is this correct? If it is incorrect, where in that sine wave would A-phase have it's maximum current draw (let's just use the op's scenario with all fans running)?

 

[oldsparky52]

The panel will be 240 volts. When I substitute in 240 volts I get the same 155.9 amps.

Is that correct?

Yes, it would be the same for any voltage that is inserted. Amps is amps, we are assuming a voltage. If you had given us HP or KVA, then voltage would be a factor on amperage.

 

[oldsparky52]

Something doesn't seem right with that calculation.

If I have three 2-pole circuits drawing 59 amps seems like that means I have 108 amp on each phase. This would be the same if I had 2 three-phase circuits drawing 59 amps.

If additionally I have three 2-pole circuits drawing 31 amps seems like that means I have 62 amp on each phase.
108 + 62 = 170 amps on each phase*. Correct?

Does the 1.732 multiplier come into play here? I didn't think so.



*This is not taking into account that MCA is already 125% of actual load.

I agree with you line of thinking (even though over the years I've been told I'm wrong), so if I was connecting this w/out engineering oversight, I would install a 200 or 225 amp panel.

59*2 = 118
31*2 = 62
118+62 =180 amps per leg.

I know the more educated tell me I'm incorrect, but .... I know I won't be in trouble.

 

[oldsparky52]

I don't know as much as I think I understand (or something like that), so I have a question for the more educated.

Where is (let's just pick A-phase) in the sine wave where it has the maximum load to the other 2 phases? It appears from just looking at a sine wave drawing that it might be when A-phase is at it's peak (either high or low). Is this correct? If it is incorrect, where in that sine wave would A-phase have it's maximum current draw (let's just use the op's scenario with all fans running)?

Edit: One additional question. When A-phase is +120V to neutral, what is the voltage from B and C phases to neutral (yes, I understand these are DC equivalent values and actual peak voltage is higher)?

 

[david luchini]

Edit: One additional question. When A-phase is +120V to neutral, what is the voltage from B and C phases to neutral (yes, I understand these are DC equivalent values and actual peak voltage is higher)?

When the instantaneous RMS voltage A to neutral is +120V, B-N and C-N will be at -60V.

 

[Coppersmith]

Probably barely wont squeeze into a 125 amp panel the legit way, I would just go 200.

Thanks for the calculation. I'm trying to get the exact figure because in addition to sizing the panel, I'm sizing the feeder and then I have to adjust for voltage drop as this panel will be about 200 feet from it's source. Maybe a 150 amp panel would be best. Since 200 feet of pipe, wire, and core drilling is expensive, if I can get this "right-sized", I may be able to bid it at a lower price which may be the difference between doing the job and sitting home.

 

[jrohe]

I’m trying to figure out how big a three phase panel I need for the following:

3 x 2 pole 60 amp breaker powering A/C air handler, MCA 59 amps
3 x 2 pole 50 amp breaker powering A/C condenser, MCA 31 amps

The breakers will be as evenly distributed across the phases as possible.

I know how to do this for a split-phase panel, but three-phase confuses me. Please show me the calculation.

The MCA for the air handler seems really large compared to the MCA of the condensing unit. I suspect the air handler may have electric heating elements. If the A/C condenser is a condensing unit only and not a heat pump, then the electric heat in the air handler should not run at the same time as the condensing unit. This may allow you to use a smaller panel than just calculating off the MCAs.

 

[david luchini]

Thanks for the calculation. I'm trying to get the exact figure because in addition to sizing the panel, I'm sizing the feeder and then I have to adjust for voltage drop as this panel will be about 200 feet from it's source. Maybe a 150 amp panel would be best. Since 200 feet of pipe, wire, and core drilling is expensive, if I can get this "right-sized", I may be able to bid it at a lower price which may be the difference between doing the job and sitting home.

To get an exact figure, you would need to know the full load rating of the units, not just the MCA.

 

[kwired]

I’m trying to figure out how big a three phase panel I need for the following:

3 x 2 pole 60 amp breaker powering A/C air handler, MCA 59 amps
3 x 2 pole 50 amp breaker powering A/C condenser, MCA 31 amps

The breakers will be as evenly distributed across the phases as possible.

I know how to do this for a split-phase panel, but three-phase confuses me. Please show me the calculation.

Air handler with MCA of 59 amps - sounds to me like it probably has electric heat strips in it. If that heat can't run at same time as condenser unit then you only have the air handler load to account for for a common supply.

 

[oldsparky52]

When the instantaneous RMS voltage A to neutral is +120V, B-N and C-N will be at -60V.

Thank you.

So, the 208V we use happens at a different time than when a phase is +120V to neutral. Do you by any chance know when the 208V occurs? Is it when 2 phases are both 104V away from neutral (does that even happen?)?

 

[david luchini]

Thank you.

So, the 208V we use happens at a different time than when a phase is +120V to neutral. Do you by any chance know when the 208V occurs? Is it when 2 phases are both 104V away from neutral (does that even happen?)?

Yes, basically when one is at +104, and one is at -104, the voltage between them will be at 208.

 

[Besoeker3]

When the instantaneous RMS voltage A to neutral is +120V, B-N and C-N will be at -60V.

Sketch out the phasor diagram.....

 

[david luchini]

Sketch out the phasor diagram.....

Ok, I'll bite.....why?