How to determine the kVA of a panel given the Amps per phase

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silver_bullet

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Hello,

I was provided the following information for a 208/120V 3ph 4W Panel,

Phase A: 6.5
Phase B: 7.8
Phace C: 13.6
Neutral : 7A

Phase A+B+C=27.9A

How do I calculate the kVA for this panel? The loads connected are all single pole. Would the calculation be the following:

kVA = (0.120X1.73)(27.9) = 5.8kVA ?


I used 120V since all loads connected are single pole (1P).

Thanks for your time,
 
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Sorry, but your basic concept is off base. To start with the confusing part, A + B + C is not 27.9, but rather has a magnitude of just 7.0. That is because the three phase currents can not be added as though they were simple numbers. They are not. They are phasors. You can start by saying (not quite correctly, but it is a starting point) that phase A current has a phase angle of 0 degrees, that the phase B has a phase angle of 120 degrees, and that the phase C has a phase angle of 240 degrees. Since the loads are not perfectly balanced, those phase angles will be slightly different than those numbers. What is an exact thing is that the total of the A, B, C, and N will be zero at all points in time. Kirchhoff's Current Law would have told you that.

So you cannot add amps to amps and expect to get amps. What you can do is add VA to VA and expect to get VA. In your case, which has the very special limitation that all loads are single phase, you can take the phase A amps of 6.5, multiply by 120 volts to get the phase A load in VA, and do the same for phases B and C, and add them up. Here again, in this special case, the result will be mathematically the same as the answer you obtained. Mathematically the same, but arrived at though a process that will not always give you the right answer. You should always add things up in units of VA, and only convert to amps as the very last step.
 
Thanks Charlie for refreshing my memory. Can't believe I forgot that fact.:happyno:

Also if there was a mix of 208V single/three phase circuits and some 120V, would you use 208V in your calculation for all 3 phases for the whole panel to be on the conservative side if given the exact same information from the electrician?
 
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Given this particular scenario...

6.5A × 120V = 780VA
7.8A × 120V = 936VA
13.6A × 120V = 1632VA
........................3348VA

(6.5A + 7.8A + 13.6A) × 120V = 3348VA

It is when you also have line-line and 3Ø loads that you cannot add the amps.

Another aspect is that panel kVA is not indicative of imbalanced loading as exhibited by your figures. When determining conductor size/ampacity and OCPD ratings, one should use the line current with the greatest magnitude.
 
silver_bullet said:
Also if there was a mix of 208V single/three phase circuits and some 120V, would you use 208V in your calculation for all 3 phases for the whole panel to be on the conservative side if given the exact same information from the electrician?
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I am not sure where you are going with this. If you are getting current values from an electrician, then obviously the panel is built, it has a feeder coming in to it, and there is a feeder breaker at the other end of that wire. So the design process is long since completed. What, then, is your role?

If you are starting with a new design project, and if you are given a set of loads (in units of amps), some single phase 120, some single phase 208, and some three phase 208, then what you do is,
  • Add up all the single phase 120 current values, and mutiply the sum by 120,
  • Add up all the single phase 208 current values, and mutiply the sum by 208,
  • Add up all the three phase 208 current values, and multiply the sum by 208, and then multiply that result by 1.732,
  • Add up the three values calculated above,
  • Divide that sum by 208, and divide again by 1.732.
  • You now have a design value for the current that will be drawn on each of the three phases (assuming you can balance the loads closely enough). That is the value you use as a design basis for selecting the panel rating, the feeder to the panel, and the feeder breaker from the upstream panel. Be sure to add some margin for future load growth. Most of my clients have asked for 20% above the calculated load.

 
This is an existing panel and I am sizing a UPS for this panel, an electrician provided metering information so that I could examine the load on the panel to size the appropriate UPS rating.

basically in the final design, the feeder connected to the panel will be re-routed to the maintenance bypass input of the new UPS and the output of the UPS will feed the panel.
 
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silver_bullet said:
This is an existing panel and I am sizing a UPS for this panel, an electrician provided metering information so that I could examine the load on the panel to size the appropriate UPS rating.

basically in the final design, the feeder connected to the panel will be re-routed to the maintenance bypass input of the new UPS and the output of the UPS will feed the panel.
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Unless you want to be adding additional breakers/fuses, it's probably best to size the UPS for the existing feeder rating and its supply breaker/fuse. Tell us those values and then perhaps we can provide better guidance.
 
silver_bullet said:
This is an existing panel and I am sizing a UPS for this panel, an electrician provided metering information so that I could examine the load on the panel to size the appropriate UPS rating.

basically in the final design, the feeder connected to the panel will be re-routed to the maintenance bypass input of the new UPS and the output of the UPS will feed the panel.
Click to expand...
Not sure that the total kVA is of much use. A UPS would have to be rated an the maximum current drawn on one phase.
 
Besoeker said:
Not sure that the total kVA is of much use. A UPS would have to be rated an the maximum current drawn on one phase.
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Seems to me the largest VA will be 1632 VA so that would be what you need but, if there is any updates in the future then sizing a UPS for the 1632 VA may be a bit short sided. As someone stated size the UPS to the max available by the conductor size feeding the panel. If you are pretty certain no changes will happen then go with the 1632 VA
 
Dennis Alwon said:
Seems to me the largest VA will be 1632 VA so that would be what you need but, if there is any updates in the future then sizing a UPS for the 1632 VA may be a bit short sided. As someone stated size the UPS to the max available by the conductor size feeding the panel. If you are pretty certain no changes will happen then go with the 1632 VA
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With respect, I disagree.
Phase C is 13.6A thus the UPS output has to be rated for at least this. The imbalance is an irrelevance.
That makes it 4.9 kVA minimum without allowances for increases in future loading.
 
With equal respect, it also depends on the extent, if any, to which the UPS has both a per line limit and an overall power limit.
Transformers and generators can tolerate some single phase overload. Can any UPS units do this?
 
GoldDigger said:
With equal respect, it also depends on the extent, if any, to which the UPS has both a per line limit and an overall power limit.
Transformers and generators can tolerate some single phase overload. Can any UPS units do this?
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The per line limit, although it is not normally stated that way, comes from the kVA rating.
A 5kVA 208V 3-phase unit can deliver about 14A. If that is required on one phase, it can be delivered on all three. It doesn't matter if the other phases aren't loaded to that level.
 
Besoeker said:
The per line limit, although it is not normally stated that way, comes from the kVA rating.
A 5kVA 208V 3-phase unit can deliver about 14A. If that is required on one phase, it can be delivered on all three. It doesn't matter if the other phases aren't loaded to that level.
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Agree. A 5kVA, 208V, 3-phase transformer is the best fit but I doubt if you can buy this off the shelf. I usually see 6kVA three-phase units available.
 
Besoeker said:
Quite so. The same considerations about line current would apply. Apart from overload capacity.
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I was mostly replying to the comment about 5kVa not being a common size - which is generally true for three phase transformers in North American markets they typically go from 3 to 6 kVA.

I really don't know what is common sizes for UPS.
 
Besides using a standard size UPS, you have to make sure the rating is based on output capability. The unit will have some losses, and therefore the input power will be slightly greater.

Also remember KVA does not equal KW if a power factor is involved.
 
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