NEC 220.87

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hhsting

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Let’s say I have electrical utility bill for plasma facility which draws blood of one year worth of data on certain switchgear that tells me each month kWH use and demand kW and I take the maximum peak kW from that bill and apply 220.87 with my new load to determine load calculation to see if switchgear is overloaded.

My question is data in utility bill is demand kW not demand kVA so can one still use utility bill with kW demand?


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I believe 220.87 demand is referring to KW.
Per 220.87 (2) you are allowed up to 80% of this KW in reference to the maximum amperage of the switchgear.
Just curious, what is your max KW, the max size of your gear in amps and the voltage of that gear?
 
It appears that you are aware that the monthly KWH is irrelevant. Take the peak KW, apply (i.e., divide by) what you believe to be a reasonable power factor, and that gives you the existing load in units of KVA. Add 25% to that, and subtract the result from the switchgear's rating (which you will have to convert from amps to KVA). That will tell you what spare capacity (if any) the switchgear has remaining.
 
charlie b said:
It appears that you are aware that the monthly KWH is irrelevant. Take the peak KW, apply (i.e., divide by) what you believe to be a reasonable power factor, and that gives you the existing load in units of KVA. Add 25% to that, and subtract the result from the switchgear's rating (which you will have to convert from amps to KVA). That will tell you what spare capacity (if any) the switchgear has remaining.
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What you say makes sense, just the code book doesn't say to use KW or KVA. Using KVA is certainly more conservative. I hate the usage of KW as it barely has any meaning to me. Is there something the code book that supports the usage of KVA other than the fact that it makes sense?
 
It's there, but it's carefully hidden. The exception under 220.87(1) specifically mentions KW. However, the process described in 220.87(2) tells you to compare the total load (including the new load being added) to the AMPACITY of the feeder or service. You can't get to "amps" without going through KVA.
 
charlie b said:
It's there, but it's carefully hidden. The exception under 220.87(1) specifically mentions KW. However, the process described in 220.87(2) tells you to compare the total load (including the new load being added) to the AMPACITY of the feeder or service. You can't get to "amps" without going through KVA.
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Wow, I get it now. Why does the code book insist on working in riddles. Sigh.
 
hhsting said:
Look at the definition of ampacity Article 100 where does it say kVA?


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It probably doesn't say which is probably why you could argue the KW method with your AHJ.
Then the AHJ will argue back with Charlie's interpretation which also holds water.

If the NEC code book defined things clearly, there would be no purpose or need for this message board!!! How ironic.
 
hhsting said:
Look at the definition of ampacity Article 100 where does it say kVA?
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It doesn't, and it doesn't have to. This is not about NEC language, but rather about physical science. As I said earlier, 220.87(2) tells you to compare the total load (including the new load being added) to the AMPACITY of the feeder or service. The rating of the service switchgear under discussion is a known thing. To verify that you are not overloading the switchgear, you have to know the amps associated with the peak measured load (with 25% added for conservatism). You measure in units of KW, but you can't convert that result to amps directly. You have to first determine the equivalent KVA, by applying the power factor. Then you use the "physical science" equation that KVA divided by volts and then (for 3-phase systems) divided again by the square root of 3 will give you amps.
 
charlie b said:
It appears that you are aware that the monthly KWH is irrelevant. Take the peak KW, apply (i.e., divide by) what you believe to be a reasonable power factor, and that gives you the existing load in units of KVA. Add 25% to that, and subtract the result from the switchgear's rating (which you will have to convert from amps to KVA). That will tell you what spare capacity (if any) the switchgear has remaining.
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Where would one find reasonable power factor? Which code? Would POCO have reasonable power factor?
 
hhsting said:
Where would one find reasonable power factor? Which code? Would POCO have reasonable power factor?
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Putting a meter on it for a while would be your best bet reading the KVAR, KW and KVA.
I'm not sure there is any code on power factor as it varies from plant to plant, but .8 power factor would be a good conservative guess in my opinion.
 
electrofelon said:
When SCL has given me meter data, it has several fields such as "KVAR/day" and "KVAR usage",
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If the slash between KVAR and day is intended to mean that the one is divided by the other, it would me meaningless. KW, KVA, and KVAR are all units of the rate of using (or supplying) energy. Dividing KVAR by time is like talking about your car's speed in units of miles per hour per hour. So my guess is that the slash is an inappropriate short hand intended to mean "times," and not "divided by." Multiplying KVAR by Day would yield a measure of energy, as would the phrase "KVAR Usage." That said, neither of these would be useful in determining the existing load, as addressed in 220.87
 
charlie b said:
If the slash between KVAR and day is intended to mean that the one is divided by the other, it would me meaningless. KW, KVA, and KVAR are all units of the rate of using (or supplying) energy. Dividing KVAR by time is like talking about your car's speed in units of miles per hour per hour. So my guess is that the slash is an inappropriate short hand intended to mean "times," and not "divided by." Multiplying KVAR by Day would yield a measure of energy, as would the phrase "KVAR Usage." That said, neither of these would be useful in determining the existing load, as addressed in 220.87
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Yes their wording is odd and not technically correct. They call demand "KW usage". Which also technically doesn't make sense.

This brings up a good question though: if a utility penalizes for PF, how is this calculated and charged? Would it be some sort of average PF figure, or average PF during the highest demand? It seems the latter would be the best thing for getting amps from demand.
 
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