3-phase panel schedule
- Thread starter lizzie14
- Start date
- Status
- Location
- Lockport, IL
- Occupation
- Retired Electrical Engineer
Re: 3-phase panel schedule
I?m not sure I understand the question. We all have our own favorite format for panel schedules. Mine allows me to use three slots for a 3-phase breaker, and to assign whatever load I wish to each of the three phases. I will always take the total KVA of the device and divide by three, and assign that amount to each phase.
My spreadsheet also adds up the load that I have assigned, and tells me how nearly balanced the three phases turned out to be. I have occasionally revised the placement of breakers, in order to more evenly spread out the load. I am aware that the contractor may not use the exact breaker layout that I show on the plans, but I will show a plan with the loads as nearly balanced as can be reasonably achieved.
Is this what you are asking about?
I?m not sure I understand the question. We all have our own favorite format for panel schedules. Mine allows me to use three slots for a 3-phase breaker, and to assign whatever load I wish to each of the three phases. I will always take the total KVA of the device and divide by three, and assign that amount to each phase.
My spreadsheet also adds up the load that I have assigned, and tells me how nearly balanced the three phases turned out to be. I have occasionally revised the placement of breakers, in order to more evenly spread out the load. I am aware that the contractor may not use the exact breaker layout that I show on the plans, but I will show a plan with the loads as nearly balanced as can be reasonably achieved.
Is this what you are asking about?
Re: 3-phase panel schedule
yes my question was what loads you put per phase on the panel schedule. i have always put the total three phase load divided by three to find out the load per phase. another engineer says that i shouldn't divide the load by three, it should be the full load in each phase. that would make the total load for my panel incorrect. can you explain in more detail why we are able to divide the load by three? if you have a 39A load, how many va would i put in each phase of the panelboard schedule?
yes my question was what loads you put per phase on the panel schedule. i have always put the total three phase load divided by three to find out the load per phase. another engineer says that i shouldn't divide the load by three, it should be the full load in each phase. that would make the total load for my panel incorrect. can you explain in more detail why we are able to divide the load by three? if you have a 39A load, how many va would i put in each phase of the panelboard schedule?
- Location
- Lockport, IL
- Occupation
- Retired Electrical Engineer
Re: 3-phase panel schedule
Let me say first that in all calculations related to loads on panels, you need to convert everything to units of VA, then do whatever needs to be done, and only then can you go back and find the equivalent amps. Sometimes it works out the same as if you did the calculation all in units of amps. But you can never be sure, unless you use VA.
You did not give me the voltage rating of your panel. I will assume it is a 208/120 volt panel. A three-phase load of 39 amps, multiplied by ?208 times the square root of 3,? gives you a total of 14,040 VA. That load will be shared equally by all three phases, presuming that it is balanced. That means each phase will supply 4,680 VA. That is the value to assign on each phase, within your panel schedule. You can then, if you wish, divide the 4,680 VA (i.e., that each phase will supply) by 120 volts (the phase to neutral voltage of each phase), and get an answer of 39 amps.
This should not be a surprise. The current in each phase is 39 amps, and the total 3-phase current is also 39 amps. That is because the three are not in phase with each other, and you cannot add 39 + 39 + 39 to get 117. Nor can you start with the original 39 amps as a 3-phase load, divide by 3, and say that each phase has a current of 13 amps.
So you see, if you tried to assign (i.e., within the panel schedule) the entire 14,040 VA to each phase, then the current on each phase would be 117 amps, instead of 39.
Let me say first that in all calculations related to loads on panels, you need to convert everything to units of VA, then do whatever needs to be done, and only then can you go back and find the equivalent amps. Sometimes it works out the same as if you did the calculation all in units of amps. But you can never be sure, unless you use VA.
You did not give me the voltage rating of your panel. I will assume it is a 208/120 volt panel. A three-phase load of 39 amps, multiplied by ?208 times the square root of 3,? gives you a total of 14,040 VA. That load will be shared equally by all three phases, presuming that it is balanced. That means each phase will supply 4,680 VA. That is the value to assign on each phase, within your panel schedule. You can then, if you wish, divide the 4,680 VA (i.e., that each phase will supply) by 120 volts (the phase to neutral voltage of each phase), and get an answer of 39 amps.
This should not be a surprise. The current in each phase is 39 amps, and the total 3-phase current is also 39 amps. That is because the three are not in phase with each other, and you cannot add 39 + 39 + 39 to get 117. Nor can you start with the original 39 amps as a 3-phase load, divide by 3, and say that each phase has a current of 13 amps.
So you see, if you tried to assign (i.e., within the panel schedule) the entire 14,040 VA to each phase, then the current on each phase would be 117 amps, instead of 39.
- Status