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Phase demand loads and current in panel schedule

Phase demand loads and current in panel schedule

The panel schedules need to indicate demand load and current per phase. This could be done by adding two more rows under "Total Load" (which shows total calculated load per phase) and "Total Amps" (calculated amps per phase).  The demand values are important part of the electrical design process and prevent overloading of the single phase on the panel. The values could be named "Demand Load" and "Demand Amps."

24 Comentarios
Martin__Schmid
Autodesk
El estado se ha cambiado a: Gathering Support

Hi @oduday can you elaborate how you would calculate demand per phase?   

 

Making some assumptions to try to understand... and you can point to the error in my understanding:

 

As an example, if the connected receptacle load is

Phase A: 11kVA

Phase B: 10kVA

Phase C: 9kVA

Total: 30kVA

 

Referring to the OOTB demand factors (100% of the first 10KVA, 50% of the remaining), the resulting demand per phase would be:

Phase A: 10.5 kVA

Phase B: 10 kVA

Phase C: 9 kVA

Total: 29.5 kVA

 

However, looking at the panel in aggregate, the demand ends up being 20kVA (100% * 10kVA + 50% * 20kVA)... so the demand load and current is about 2/3 the connected vs. looking at the phases individually.  However, one can't simply take such a factor (the 2/3) and apply to each phase because someone could do this:

 

Phase A: 15kVA

Phase B: 15kVA

Phase C: 0 kVA

 

Resulting Demand:

Phase A: 12.5kVA

Phase B: 12.5kVA

Phase C: 0 kVA

Total: 25kVA

 

The above 2nd scenario likely bad design, and computing the load per phase isn't going to solve that for you.. but something else helping you see the total load balance per load classification might (?).

 

Perhaps it isn't receptacles primarily that you have concern about... whatever additional info/example you can provide can help us understand more about this request.

 

 

oduday
Contributor

Hello Martin,

I understand your point. We are bombarded by the local building department requests to indicate a demand load individually per each phase. Currently, in Revit, we added the calculated values of each phase electrical load multiplied by the total demand factor. It does not reflect the real situation but helps us during the permit review process with panelboards and switchboards. 

I understand that Revit's panel balancing option gives a great result but it is could be applied during the original design phase only. Later, when construction starts and more loads added, there is no way to re-balance the entire panel. It will involve the major work from the Electrical Contractor to move circuit breakers inside the installed panelboard per new "re-balanced" layout. This also applied to the existing conditions where improvements are being made. Again, re-balancing helps mathematically relocate loads inside the panel to have each phase calculated load values as close as possible. It does not allow to show the impact of the load types in each phase.

The idea is to evaluate the load for each phase in relation to the transformer, feeder conductors or feeder circuit breaker. The electrical load types are not distributed evenly between phases. One phase could have more receptacle loads, another phase could have more lighting or kitchen equipment loads. Evaluating these loads individually for each phase helps prevent circuit breakers, conductors or transformers to be undersized.

Everybody understands that the sum of the phase demand loads will not be equal to the total panel demand load. There is no requirement to have a total panel demand load to be determined based on the sum of the phase demand loads. These are different values. In some way, it reminds me of school math where three pipes filling the basin, but in this case, the panel is a basin.  The amount of water running in each pipe is not equal and we want to make sure that pipe (conductor) and valve (circuit breaker) are properly sized to let the safe amount of water run through. Current Revit calculations for panelboards and switchboards average these three pipes into single pipe value. This works for panel or switchboard "total" calculations but not for each individual phase.

I hope I described the issue. It would be great to see this happening for panels and switchboards calculations.

Please let me know if you have any further questions.

oduday
Contributor

Panel Schedule SamplePanel Schedule SampleSwitchboard SampleSwitchboard Sample

Martin__Schmid
Autodesk

Hi @oduday - thanks for the detail.. this helps describe the why, but doesn't help us understand how you would expect this to be computed.  

 

Does the building department have guidelines on how they expect you to compute this?  It may be that all the data you need is readily available, and a dynamo script could compute this for you... or it may be that the data is aggregated and stored in the model in such a way that this isn't possible.. but we'd need to understand more about the how in order to have an understanding of what improvements would be necessary to make this easier.

oduday
Contributor

I assume that it could be done by taking, for example, phase A, calculate its total electrical load, evaluate the demand for this individual phase using load types connected to it, and determine the phase demand load using a multiplier of the total phase electrical load and phase demand load.

Building departments have no guidelines about it. This issue was brought multiple times in Revit forums by different people. 

For AutoCAD project, we are doing it in Excel:Excel Panel ScheduleExcel Panel ScheduleExcel Load Calcs including PhasesExcel Load Calcs including Phases

scbunker
Collaborator

@Martin__Schmid, I'm not sure I'm following what exactly your question is in your first response. But I'll throw in my 2¢ (in more detail than we've discussed in the past) on this issue and maybe it'll help.

 

You are correct that someone could put everything on phase A and B and nothing on phase C. I want to point out, though that this isn't necessarily bad design - it could be design constraint. Suppose the designer must (for a some legitimate (albeit unfortunate) reason) add load to an existing single-phase panel that's fed from a three-phase panel; you could end up with that scenario and not be able to do anything about it. There are lots of reasons that certain scenarios can occur that aren't necessarily bad design. We don't live in a perfect world and we're not perfect ourselves.

 

It's important to note that 'total panel kVA' or 'total panel Amps' is mere shorthand. We don't really care about the total current on a panel. We care about the total demand current per phase. To say that a panel "has 160 amps demand on it" is just a shortcut. Sure, if the panel is perfectly balanced panel, this would mean 160 demand amps per phase. But such a panel is rare, as connected load per load classification must be perfectly balanced. 

 

Both companies at which I've been employed calculate demand per phase (in Excel, for AutoCAD jobs) for this reason. In fact one of those companies refused to do panel schedules in Revit for years precisely because it wouldn't do this calc. Admittedly, in new construction it's rarely an issue because we have control over the entire design. But when adding load to an existing system, you really have to pay attention to this when the panel is heavily loaded. I have on more than one occasion had to move load from one phase to another because one phase was overloaded in demand -  but not necessarily connected - load.

 

Example with made-up numbers to suit my purpose: Suppose I have an existing 200A 208/120V panel with the following known demand (however I obtain the information):

 

Phase A: 22 kVA / 183 A

Phase B: 21 kVA / 175 A

Phase C: 22 kVA / 183 A

Total: 64 kVA / 178 A

 

Now suppose I need to add a 1.7 kVA single-phase lighting load to that panel. That's about 14 A connected 18 A demand. Adding 18 A to 178A - pretty close but doable and I've done it before when I didn't have a better option. But I cannot add this load to phases A or C. It would overload those phases in demand, but not in connected. Revit only gives you that bottom line in demand. It doesn't tell you which phase will support the load. Phase B isn't necessarily the lowest phase in connected load. 

 

You ask how the calc is to be done, but you seem to answer your own question because you do calc it. Below is the panel schedule calcs (in Excel) from one company I've worked at:

FoxitReader_2019-07-31_20-22-21.png

This is from a real new-construction job that I designed. Demand load is calculated per-phase. Notice that the lowest phase connected isn't the lowest phase demand. In fact the phases are exactly reversed when comparing relative load connected vs demand. If I relied on the per-phase information that Revit gives me (connected only) to determine which phase were most lightly-loaded, I'd actually be picking the most heavily-loaded. This demand information is mapped in Excel to whatever upstream panel is feeding it and the calcs are redone. The panel total kVA and amps are just shorthand and mostly meaningless without also having the per-phase information.

 

I've seen @oduday's Revit solution suggested on other forums and blogs when I've researched this issue. It may get past the permit reviewers, but it's not an accurate calc. I think Revit should be able to calc this correctly - firms have been doing it for a long time in Excel.

 

Hopefully that answered your question and gave you useful info without being too long-winded (or with too many math errors) Emoticono feliz

 

 

 

 

oduday
Contributor

Thank you for support. You precisely described my point. Everybody figured out how to do it in Excel, but Revit for many years does not implement this critical feature. It should be easily done with some programming on Autodesk side. At the minimum, it would be nice to have the "Demand Factor for Phase *" parameter introduced in the next generation of Revit. 

Martin__Schmid
Autodesk

Thanks for the details from both of you.. very helpful.

 

The basis of my original question is founded the detailed example @oduday provided that has a load summary with 'Calculation A' showing:

  • Total connected receptacle load of 31022 VA
  • First 10,000VA computed @ 100%
  • Remaining 21022VA computed @ 50%

'Calculation A' 

Recepts.png

This seems to imply that the receptacle total demand load is 20511 (10000 + 21022 * 0.5).. but the detailed computation summarized in 'Calculation B' indicates that the receptacle demand is: Phase A @ 10310, Phase B @ 8882,  Phase C @ 10760

 

'Calculation B'

Rec.png

 

If everything is computed per phase, then what is the relevance of 'Calculation A'? 

 

oduday
Contributor

Calculation A is a panel schedule the way we issue it to the building department. Calculation B is a hidden area with formulas inside the Excel file that makes all calculations for a panel schedule, including phase connected and demand load summaries.

2019-08-01 11_30_03-Window.png

scbunker
Collaborator

@Martin__Schmid, in my opinion, Calculation A is mostly irrelevant. Notice the calc example I provided you doesn't do that calc. Any calculation concerning the entire panel (which is either a sum or average of the three phases) is merely shorthand to make quick glances and conversation easier. Thus, in my opinion, the only total-panel value that is of any real use is total demand (whether kVA or amps) in order to get a feel or idea of how heavily loaded the panel is, assuming it's roughy balanced. I don't even think total connected is really useful. I like the calcs that my former company did which is why I provided that example for you.

 

@oduday's panel schedule reports each load class for the overall panel and perhaps some may want this to get an overall idea of how the panel is loaded but isn't really useful, IMO. I wouldn't show that calc on any panel schedule I built whether in Excel or Revit (had I the option). 

scbunker
Collaborator

@oduday wrote:

Thank you for support. You precisely described my point. Everybody figured out how to do it in Excel, but Revit for many years does not implement this critical feature. It should be easily done with some programming on Autodesk side. At the minimum, it would be nice to have the "Demand Factor for Phase *" parameter introduced in the next generation of Revit. 


No problem, @oduday. I've told Martin before that I don't understand why the engineering community hasn't been more vocal about this issue. So thanks for posting!

oduday
Contributor

I agree with you. Your panel schedule load summary looks more informative. With you permission, can I steal this idea and data format for our company standard?

scbunker
Collaborator

@oduday:, sure, go ahead!

 

An aside to both of you: the person who developed that Excel sheet (who I presume was the company owner - he's a bit of a brainiac and used to teach engineering classes) knew that total panel calculations don't really make sense. That's why (I think) the only formula spelled-out in the calcs is the bottom-line, total-panel calculation. It's just defining what that number is exactly in case anyone scratches their head over it. I think that's probably unnecessary since that's such a common calc.

Anonymous
No aplicable

@oduday

 

I was discussing this yesterday with a co-worker, and have been trying to push this through to revit ever since. I see you have added "Demand Load" and "Demand Amps" into a Revit panel schedule. What type of parameter are you using to do this? Are you just using a text parameter? Are you using excel or a circuit schedule to do this for you, and then imputing it using a text parameter? I feel like i am getting fairly close to getting this added, but not sure it is possible without the help of autodesk programmers. It seems that I can get the total into a circuit schedule, but I can not get that information into a Panel schedule in the appropriate area, as it is looking for electrical equipment where you have the Demand load and demand amps. 

 

Thanks,

 

Ryan

oduday
Contributor

Ryan, we used the calculated value with the estimated demand factor applied per each phase values.

Anonymous
No aplicable

@oduday 

 

So, what are you pulling the calculated value from?

 

Ryan

oduday
Contributor

Revit has "Apparent Load" and "Current" parameters for each phase. I am combining these values with "Estimated Demand Factor" for calculated phase values. I understand that it does not represent the actual phase estimated demand load values but it makes it as close as possible to the phase demand loads.

Martin__Schmid
Autodesk

hi @scbunker and @oduday 

 

Revisiting this.. I see that your receptacle demand is taken per-phase (e.g., so a panel w/ exactly 10kVA per phase will be 30kVA/(208(sqrt3)) = 83.3A (thus, no derating).  Is it correct to interpret that since each phase is not derated, that the feeder/service (e.g., the OCPD) would similarly NOT be derated?

 

How would you compute the demand on a series of clothes driers per (220.54).  Assume each is 208v/1p (L-L), 2kVA, each on a dedicated circuit.

 

Thus, Phase AB has 2 driers, BC has 2 driers, and CA has 2 driers... result is that each phase sees 4 driers.. but the 3-phase OCPD for the feeder would see 6.  Would you apply a 75% factor to the OCPD (and/or the feeder), or would you leave it at 100%... and would you expect all engineers (at least those governed by the NEC) to follow suit?

 

example.png

 

ecd.png

scbunker
Collaborator

@Martin__Schmid

 

Actually, the demand factor is not determined per phase, but is rather determined for the whole panel and then applied per phase.  I haven't taken the time to review my previous comments to see whether I let you astray; I'm sorry if I did.

 

I have Excel files from two different firms. Although the formulas they are using are slightly different, they're actually the same thing. As in the calculate the exact same way.

Example 1:

A = if(A+B+C <= 10, A, A / (A+B+C) * 10 + 0.5 * A - A / (A+B+C) * 10)

 

Example 2:

A = if(A+B+C <= 10, A, (10+(A+B+C-10) * 0.5) / (A+B+C) * A)

 

The second example makes most sense to my brain. The expression (10+(A+B+C-10) * 0.5) / (A+B+C) is determining the demand factor that is applied to the receptacle loads on the panel (this would be the percentage that the current Revit panel schedules report). That demand factor is applied to each phase. The first example is somehow the same formula rearranged because it calculates exactly the same, but I haven't taken the time to wrap my head around the logic behind how it was developed.

 

So in your example of 10 kVA of receptacle load per phase for a total of 30 kVA on the panel: total receptacle demand on the panel would be 20 kVA, divided to 6.67 kVA per phase. This jives with how I understand the code. At first glance, it may not makes sense because changing one phase can affect the others. For example, here's 10 kVA per phase:

10 each.png

And here is phase C increased to 20 kVA:

unbalanced.png

What may not make sense at first glance is that increasing load on phase C actually decreased demand load on phases A and B. But it does make sense if you think of it as first determining the receptacle demand load for the panel, then applying that demand load to each phase.

 

The same would be the case in your example of multiple dryers on different phases. Determine the demand factor based on the total number of dryers, then apply that demand factor to each phase. So, to use your example, total connected load for A, B, C would be 4000, 4000, 4000, the total number of dryers on the equipment is 6, so the demand factor applied to each phase is 75%, and demand for A, B, C would be 3000, 3000, 3000.

 

If you delete the dryer on circuit 6 in your example, total connected would be 3000, 4000, 3000, demand factor would be 85%, and demand would be 2550, 3400, 2550. 

 

Hopefully that makes sense.

 

 

scbunker
Collaborator

An aside on why it would make sense for an increase of load on one phase could lead to a decrease of demand load on the other phases:

 

The purpose of the demand factor for receptacles is to try to capture the fact that not all the receptacles are going to be used at once. But whether a receptacle is used is not dependent upon what phase it's connected to. They're just taken as a collective whole. The more receptacles you have, the less likely it is that many of them will be used at once. So the demand factor for all of them decreases. That makes sense in my head, at least.

 

An aside to the aside: the statement that it doesn't matter what phase receptacles are connected to is not necessarily universally true. I worked on a project once that had several dozen sets of loads with identical kVAs that happened to be grouped into three circuits each. Although on paper the loads were identical, the way I knew the loads would be used would very likely result in one of the three circuits being used more than the other. Had I simply placed these loads in order of appearance on the panel (ABC, ABC, ABC...), one of the phases was likely to be a problem. Therefore, I placed each instance of the groups so that they were rotated on the phases (ABC, BCA, CAB). Just looking at the identical kVAs of each individual load, it didn't make sense to do this, but I knew it was necessary. This is just an engineering judgement situation that code and software can't really account for.

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