Panelboard Sizing for Mechanical Loads

[petersonra]

430.26 Feeder Demand Factor. Where reduced heating of the
conductors results from motors operating on duty-cycle, intermittently,
or from all motors not operating at one time, the
authority having jurisdiction may grant permission for feeder
conductors to have an ampacity less than specified in 430.24,
provided the conductors have sufficient ampacity for the maximum
load determined in accordance with the sizes and
number of motors supplied and the character of their loads
and duties.

You are still back to asking for special permission.

 

[nhee2]

You are still back to asking for special permission.

agreed. but the question was 'Find me somewhere it says you can do anything other than what is described above.'

430.26 allows it (with permission from AHJ).

 

[drktmplr12]

agreed. but the question was 'Find me somewhere it says you can do anything other than what is described above.'

430.26 allows it (with permission from AHJ).

and where in the design process does the AHJ chime in?

 

[drktmplr12]

If we are talking about a very large industrial facility (>2,500kVA) I would say there is value in approaching the building department prior to getting into detailed design and requesting permission after discussing the load, and probably making references to IEEE 141-1993 and/or a similar facility for which you have connected and demand data

 

[petersonra]

agreed. but the question was 'Find me somewhere it says you can do anything other than what is described above.'

430.26 allows it (with permission from AHJ).

You are always allowed to ask for special permission if you want to do something the code does not allow. Does not mean you will get it.

 

[electrofelon]

I agree that is what the code says, but I have never installed an MCC where the rating even equaled the sum of all of the connected loads. These were industrial installations where there was no actual interlocking to prevent some loads from running at the same time as other loads....it has always been based on the design engineer making a demand factor guess based on some knowledge of the process operations.

Yeah I have heard many times on this forum over the years engineers say MCCS are almost always installed non-code compliantly. I think there is a pretty wide belief by inspectors that if it's engineered by a PE it's good, even though there are only a few cases that is actually allowed per NEC.

Unfortunately it is a fact that NEC load calcs are usually horrible and grossly overly conservative. It seems they could get a bit more realistic with this. I routinely use my own judgment if it's a job that's not getting inspected. I recently connected about 8 200 amp panels and 3 100 amp panels to a 600 amp generator feeder (not legally required). It's a building and client I am very familiar with and I'm very familiar with the loading.

 

[JoeStillman]

Make sure you are not using the MCA (minimum circuit amps) as the loads. It's kind of a pain, but in circumstances like this, where there are a lot of mechanical items, it makes sense to break out all the fans, compressors, heaters etc. using RLA for compressors and FLA for fans. Make sure you are using the kW heater rating for your actual voltage (i.e. 208 vs. 240). Add up the actual connected loads and do the 430 Part V maths.

Also, non-coincident loads may be significant. Use the larger of a heater or compressor for example.

But arbitrary diversity factors have no basis in the Code.

 

[electrofelon]

Make sure you are not using the MCA (minimum circuit amps) as the loads. It's kind of a pain, but in circumstances like this, where there are a lot of mechanical items, it makes sense to break out all the fans, compressors, heaters etc. using RLA for compressors and FLA for fans. Make sure you are using the kW heater rating for your actual voltage (i.e. 208 vs. 240). Add up the actual connected loads and do the 430 Part V maths.

Also, non-coincident loads may be significant. Use the larger of a heater or compressor for example.

But arbitrary diversity factors have no basis in the Code.

Yeah I was going to mention that: just adding up MCAs over counts quite a bit. One time I was able to get all my roof HVAC on a 200A roof panel (had an existing 200A feeder I didn't want to upgrade) by throwing the MCAs away and going individual motors.

 

[petersonra]

Yeah I was going to mention that: just adding up MCAs over counts quite a bit. One time I was able to get all my roof HVAC on a 200A roof panel (had an existing 200A feeder I didn't want to upgrade) by throwing the MCAs away and going individual motors.

MCA has little to do with actual load. It is about the ampacity of the conductors. Why would you add them up to figure any load?

 

[electrofelon]

MCA has little to do with actual load. It is about the ampacity of the conductors. Why would you add them up to figure any load?

I have found actual load is pretty close to 66% of the MCA every time I have measured it.

The advantage to adding up MCAS for a feeder or service is just simplicity, otherwise you've got to dig into the units and get all the individual data.

 

[petersonra]

I have found actual load is pretty close to 66% of the MCA every time I have measured it.

The advantage to adding up MCAS for a feeder or service is just simplicity, otherwise you've got to dig into the units and get all the individual data.

That sounds about right. But you end up with a wildly inflated number for total load by adding up mca. If you have mca won't you have fla too?

 

[electrofelon]

That sounds about right. But you end up with a wildly inflated number for total load by adding up mca. If you have mca won't you have fla too?

Yeah but you have to usually go physically look at the unit. Sometimes it doesn't make that much of a difference and it's not worth doing, sometimes it does.

 

[zmogilev91 @gmail.com]

I guess the question is, is applying diversity factors allowed to electrical loads based on engineering judgment? I’ve seen this done broadly on many large projects, but never thought it was an issue.

I agree with your co-worker.

The code requires conductors serving motors or groups of motors to be rated for the connected NEC motor load + 25% largest motor +100% noncontinuous non-motor load + 125% of continuous non-motor load.

Find me somewhere it says you can do anything other than what is described above.

I agree. Then another question arise: does contingency and future growth have to be included in 125% of non-continuous load? Many thanks for your response.

 

[petersonra]

I agree. Then another question arise: does contingency and future growth have to be included in 125% of non-continuous load? Many thanks for your response.

That is a design decision and not a code issue.