Panel Schedules / Phase load calculations

[RJK42]

Hello!!

I'm new to the forums and new to the industry. I was recently picked up by a contractor as a drafter to create new drawings and continue with as-builts. I use AutoCAD MEP to drawing our prints and I am utilizing the tools to implement the built in panel schedules. The scheduling part is nice but I still have to input data for the phase loads. I have been looking through NEC, especially section 220 to see if there are any references to calculations needed for inputting values in the schedules.

Is there a good reference regarding formulas needed in order to calculate these values?? i.e., phase loads, demand factors....

I'm not seeing anything to refer to unless I'm over looking.


Thanks in advance!!

 

[don_resqcapt19]

The loads come from the Article 220 load calculations that should have been done as part of the design process.

 

[charlie b]

I am not sure I understand the question. But here is my stab at provding you an answer.

• If a single phase, line-neutral load is connected to Phase A, then you put the total load taken up by that item in the Phase A column. For example, if you have a 120 volt, 20 amp circuit that supplies five general purpose receptacles, then you put 900 VA (5 receptacles at 180 VA each equals 900 VA) in the Phase A column.
• If a single phase, line-line load is connected between Phases A and B, then you put half the load into each column. For example, if you have a 240 volt, 5000 VA dryer, you would put 2500 VA each in the Phase A column and the Phase B column.
• For a three phase load, you split the load three ways. For example, if you have a 480 volt, 20 HP motor, you first look at table 430.250, see that the full load current is 27 amps, multiply that by 480 and then by 1.732 (the square root of 3), discover that the full load power value is 22,450 VA, divide that number by 3, and put 7484 VA in each of the columns for Phase A, Phase B, and Phase C.

If that does not give you a starting point for your calculation process, can you tell me what you are trying to figure out?

One important point: always do the calculations in terms of VA, not in terms of amps. If you want to figure out the load, in amps, on any given phase, do that part of the calculation only after you have added up everything in terms of VA. This is particularly important if you have a mix of single phase loads and three phase loads.

 

[Smart $]

Hello!!

I'm new to the forums and new to the industry.

Hello!!! ...and welcome.

... I use AutoCAD MEP to drawing our prints and I am utilizing the tools to implement the built in panel schedules. The scheduling part is nice but I still have to input data for the phase loads. ...

Haven't used MEP for over ten years, back when it was called Autodesk Building Systems. As I recall, when the appropriate load information was entered into circuit "properties", the phase loads were automatically computed for [auto-generated] panel schedules...??? There was even load categories to select so the appropriate demand factors were applied...??? The only thing I hated about it was that the schedules were not dynamic, i.e. not automatically updated on additions or changes, having to recreate the schedule for each mod'.

Sounds like Autodesk is slipping hmy:

 

[RJK42]

Thanks!! That answeres alot. In what situations are the phase loads, A,B,C, not equal?? Also when looking at a device, for example a charging station. On the cut sheet of the charging station shows values:

Max output power: 10kW

Maximum output current:
@24V------320 Amps
@36V------230 Amps
@48V------170 Amps
@80V------100 Amps

Utility Requirements
480VAC, 60HZ, 3? (MAX DRAW)-----15 Amps
600VAC, 60HZ, 3? (MAX DRAW)-----12 Amps

EFFICIENCY-----------------------------92%
POWER FACTOR------------------------0.95

Nothing given is in VA. So do I just multiply the Amps X Volts??

15 Amps X .95 (PF) = 14.25A
14.25A X 480V = 6840VA
6840VA X √3 = 20520VA

So, each phase load A,B,C is 20520VA?

Is this correct?? Also, What demand factor should be used regrading a charging station?? Or is this needed??

I'm working with 277-480Y 3?

I am not sure I understand the question. But here is my stab at provding you an answer.

• If a single phase, line-neutral load is connected to Phase A, then you put the total load taken up by that item in the Phase A column. For example, if you have a 120 volt, 20 amp circuit that supplies five general purpose receptacles, then you put 900 VA (5 receptacles at 180 VA each equals 900 VA) in the Phase A column.
• If a single phase, line-line load is connected between Phases A and B, then you put half the load into each column. For example, if you have a 240 volt, 5000 VA dryer, you would put 2500 VA each in the Phase A column and the Phase B column.
• For a three phase load, you split the load three ways. For example, if you have a 480 volt, 20 HP motor, you first look at table 430.250, see that the full load current is 27 amps, multiply that by 480 and then by 1.732 (the square root of 3), discover that the full load power value is 22,450 VA, divide that number by 3, and put 7484 VA in each of the columns for Phase A, Phase B, and Phase C.

If that does not give you a starting point for your calculation process, can you tell me what you are trying to figure out?

One important point: always do the calculations in terms of VA, not in terms of amps. If you want to figure out the load, in amps, on any given phase, do that part of the calculation only after you have added up everything in terms of VA. This is particularly important if you have a mix of single phase loads and three phase loads.

 

[RJK42]

Autodesk AutoCAD MEP does not calculate the phase loads. I had to input the values in:



Autodesk Revit MEP does calculate the phase loads but I haven't had the chance to try it out yet. All of the drawings so far is AutoCAD. I'm waiting for the chance to work with Revit in that respect. It is so much better in my opinion.

Hello!!! ...and welcome.


Haven't used MEP for over ten years, back when it was called Autodesk Building Systems. As I recall, when the appropriate load information was entered into circuit "properties", the phase loads were automatically computed for [auto-generated] panel schedules...??? There was even load categories to select so the appropriate demand factors were applied...??? The only thing I hated about it was that the schedules were not dynamic, i.e. not automatically updated on additions or changes, having to recreate the schedule for each mod'.

Sounds like Autodesk is slipping hmy:

 

[charlie b]

First of all, ignore power factor. Secondly, you have a math error. You multiplied the 6840 times 3, instead of times the square root of 3. Finally, when you get the total VA, you assign one third to each phase.

Take 15 amps times 480 volts times 1.732 (the square root of 3) and you get a total of 12,470 VA. Divide that equally among the phases, and you get 4157 VA on each phase.

Please note that the 15 amps is the ?utility requirement.? That is the rating of the branch circuit. The actual load being drawn by the equipment will be lower than that value. But using this value does give you a conservative result, so it is an acceptable approach.

 

[Smart $]

Thanks!! That answeres alot. In what situations are the phase loads, A,B,C, not equal?? Also when looking at a device, for example a charging station. On the cut sheet of the charging station shows values:

Max output power: 10kW

Maximum output current:
@24V------320 Amps
@36V------230 Amps
@48V------170 Amps
@80V------100 Amps

Utility Requirements
480VAC, 60HZ, 3? (MAX DRAW)-----15 Amps
600VAC, 60HZ, 3? (MAX DRAW)-----12 Amps

EFFICIENCY-----------------------------92%
POWER FACTOR------------------------0.95

Nothing given is in VA. So do I just multiply the Amps X Volts??

15 Amps X .95 (PF) = 14.25A
14.25A X 480V = 6840VA
6840VA X √3 = 20520VA

So, each phase load A,B,C is 20520VA?

PF is mainly involved with determining watts (W) versus volt-amperes (VA), the former being real power utilized, the latter being apparent power, with reactive power being the difference. All your panel schedule load calculations are regarding apparent power (i.e., VA).

(15A ? 480V ? √3) ? 3 = 4157 VA each leg

Also, What demand factor should be used regrading a charging station?? Or is this needed??

I'm working with 277-480Y 3?

I'm not familiar with any demand factor associated with a charging station...

Formal system nomenclature is 480/277V 3? 4W, often shortened to the informal 480/277V. Some include the "Y" after the 480.

 

[RJK42]

One more thing.

One more thing.

Thanks for all the help. Cleared up a lot. I do have one other question regarding the phase loads. I was looking through other drawings we have from other sources and noticed on the electrical panel schedules that some of phase loads are different values that pertain to the same circuit. Why is this and when would I come across different values for each leg??

Thanks again!!!

 

[charlie b]

I can envision a three phase heater that has a small fan that is run by a single phase motor. The heating elements would give you the same load on all three phases, but the motor would show up on only one or two of the phases. So the overall circuit would not have the same load on all three phases. Do you have a specific example in mind?

 

[RJK42]

I don't have an example at the moment. I'll go through our drawings and see what I can find.

Thanks again!!

I can envision a three phase heater that has a small fan that is run by a single phase motor. The heating elements would give you the same load on all three phases, but the motor would show up on only one or two of the phases. So the overall circuit would not have the same load on all three phases. Do you have a specific example in mind?

 

[RJK42]

Hello again!!,

Going back the charger problem. I have 3 chargers on one circuit (277/480). Each charger is 15Amps, so is this correct?:

(15A*3) X 480V X √3 = 28433.34VA total

28433.34 / 3 = 9477.78VA for each phase

So when I come across different loads on once circuit, do I just add the loads up them multiply them by the appropriate voltage?

If I happen to share a circuit with lighting or a motor, would I have to apply a demand factor to the appropriate load before combining them to calculate the total phase load??


Thanks again!!

First of all, ignore power factor. Secondly, you have a math error. You multiplied the 6840 times 3, instead of times the square root of 3. Finally, when you get the total VA, you assign one third to each phase.

Take 15 amps times 480 volts times 1.732 (the square root of 3) and you get a total of 12,470 VA. Divide that equally among the phases, and you get 4157 VA on each phase.

Please note that the 15 amps is the ?utility requirement.? That is the rating of the branch circuit. The actual load being drawn by the equipment will be lower than that value. But using this value does give you a conservative result, so it is an acceptable approach.

 

[Smart $]

Hello again!!,

Going back the charger problem. I have 3 chargers on one circuit (277/480). Each charger is 15Amps, so is this correct?:

(15A*3) X 480V X √3 = 28433.34VA total

28433.34 / 3 = 9477.78VA for each phase

Wow!!! Whatever calculator you are using, you best throw it away and get another one.

(15A*3) X 480V X √3 = 37,412VA or 37.4kVA

So when I come across different loads on once circuit, do I just add the loads up them multiply them by the appropriate voltage?

If I happen to share a circuit with lighting or a motor, would I have to apply a demand factor to the appropriate load before combining them to calculate the total phase load??


Thanks again!!

Convert to VA first, then add 'em up.

Combination loads are one of the pitfalls of trying to do a load calculation via panel schedule. Can't be done via phase load values. If you're looking at doing this often, develop your own spreadsheet.