Determining power factor to use for load calculation from utility bills

[philly]

I am looking at a years worth of utility bills trying to come up with an existing load for a load calculation based on NEC 220.87. The monthly bills provide an "On-Peack Actual Demand" value in kW which appears to represent the maximum kW demand for the given billing cycle (month)

Obviously in order to convert this to a maximum kVA value and ultimately an maximum load amperage a power factor has to be known. In previous bills I've reviewed a minimum power factor for each period was provided which I typically used to convert the max kW to a kVA value.

With the absence of a power factor value is there a typical "minimum" value that is assumed/used when trying to determine max kVA/Amps from utility bills? The facility is likely classified as an industrial facility with several motors, machinery, etc... so I'm not sure if there are different power factors used for this case depending on facility type?

 

[FionaZuppa]

I am looking at a years worth of utility bills trying to come up with an existing load for a load calculation based on NEC 220.87. The monthly bills provide an "On-Peack Actual Demand" value in kW which appears to represent the maximum kW demand for the given billing cycle (month)

Obviously in order to convert this to a maximum kVA value and ultimately an maximum load amperage a power factor has to be known. In previous bills I've reviewed a minimum power factor for each period was provided which I typically used to convert the max kW to a kVA value.

With the absence of a power factor value is there a typical "minimum" value that is assumed/used when trying to determine max kVA/Amps from utility bills? The facility is likely classified as an industrial facility with several motors, machinery, etc... so I'm not sure if there are different power factors used for this case depending on facility type?

what size kva is the xfrmw, how many panels does that serve, what size panels are they? extrapolate back. its not actual PF for you, but should tell you what poco thinks it might be for what that xfrmr services.

 

[iwire]

what size kva is the xfrmw, how many panels does that serve, what size panels are they? extrapolate back. its not actual PF for you, but should tell you what poco thinks it might be for what that xfrmr services.

Notice the OP pointed directly to the code section they are trying to comply with. Can you help them with that?

220.87 Determining Existing Loads. The calculation of a
feeder or service load for existing installations shall be
permitted to use actual maximum demand to determine the
existing load under all of the following conditions:

(1) The maximum demand data is available for a 1-year
period.

Exception: If the maximum demand data for a 1-year period
is not available, the calculated load shall be permitted
to be based on the maximum demand (measure of average
power demand over a 15-minute period) continuously recorded
over a minimum 30-day period using a recording
ammeter or power meter connected to the highest loaded
phase of the feeder or service, based on the initial loading
at the start of the recording. The recording shall reflect the
maximum demand of the feeder or service by being taken
when the building or space is occupied and shall include by
measurement or calculation the larger of the heating or
cooling equipment load, and other loads that may be periodic
in nature due to seasonal or similar conditions.

(2) The maximum demand at 125 percent plus the new
load does not exceed the ampacity of the feeder or
rating of the service.

(3) The feeder has overcurrent protection in accordance
with 240.4, and the service has overload protection in
accordance with 230.90.

 

[iwire]

With the absence of a power factor value is there a typical "minimum" value that is assumed/used when trying to determine max kVA/Amps from utility bills? The facility is likely classified as an industrial facility with several motors, machinery, etc... so I'm not sure if there are different power factors used for this case depending on facility type?

Do you have access to the meter? Some of them display the PF.


That said, what part of 220.87 requires you consider PF?

 

[FionaZuppa]

Notice the OP pointed directly to the code section they are trying to comply with. Can you help them with that?

there's only two scenarios. poco has accounted for PF in billing, or they havent. if they have then poco has the answer. if not then i have proposed a method that would yield a rough PF #. as for your ask to "help", sure. gots to put on that thinking cap . or, get a recording clamp on meter that can sample every 15min for 30day. if what i said makes no sense, then so be it, wait for others to chime in.

 

[philly]

That said, what part of 220.87 requires you consider PF?

I agree there is no specific requirement to consider power factor however when load calculations are performed they are performed in kva. Weather we are sizing a transformer in kva or converting a kva value to amps for a feeder calculation we have to use a kva value and not a kw value. If only a kw value is known then a power factor needs to be known to calculate a kva value. ( unless you assume a pf of 1 which is not realistic.)

When monitoring with a meter I guess you can take the maximum demand currrent directly from the meter without having to necessarily worry about kw, or kva and power factor. However in a case of looking at utility bills it does not show a maximum current demand but rather only shows a maximum kw demand. This kw value needs to be convetrd into a kva value in order to do any useful load calcs. My question is what power factor to use when only a max kw is given on a utility bill in order to convert it to a kva value?

 

[iwire]

Weather we are sizing a transformer in kva or converting a kva value to amps for a feeder calculation we have to use a kva value and not a kw value. If only a kw value is known then a power factor needs to be known to calculate a kva value. ( unless you assume a pf of 1 which is not realistic.)

Where in 220 does it require the PF to be considered?

 

[jrohe]

Do you have access to the meter? Some of them display the PF.


That said, what part of 220.87 requires you consider PF?

Where in 220 does it require the PF to be considered?

:thumbsup:

 

[FionaZuppa]

Where in 220 does it require the PF to be considered?

Note: The unit values herein are based on minimum load conditions and 100 percent power
factor and may not provide sufficient capacity for the installation contemplated.

why would you not use kVA or a monitoring system?

 

[iwire]

why would you not use kVA

Because I do not have to and neither does the OP.

What you posted from the NEC is just a tip, a suggestion, a friendly reminder, not a requirement.

The calculations in article 220 result in an oversized service or feeder and therefore the PF even if poor is not going to really be an issue.

Don't believe that? Ask a utility engineer and they will tell you that the real world loading is about 40 to 50% of the NEC calculated load.

Furthermore the NEC admits this as well by letting us use conductors with ratings below the calculated load for dwelling units.

or a monitoring system?

We do, the company I work for owns three recording meters and we use them for this very purpose.

 

[philly]

Where in 220 does it require the PF to be considered?

I agree that requirement doesn't existing in 220.

Maybe I'm missing something here then. Let me give an example.

I'm trying to determine the existing load on a 208V switchboard that is fed from a dedicated service entrance. The service entrance is being removed and the existing switchboard will be re-fed from an upstream 208V switchboard that if fed from another utility service. I therefore need to determine the feeder size in order to size the breaker and feeder at the upstream switchboard in order to re-feed this existing switchboard.

The utility info that I have is fore the service entrance that is currently feeding the switchboard that will be re-fed. The utility bills show a maximum peak load of 94.4kW over the past year. So in order to come up with the new feeder size I need to determine an ampacity value in order to determine the new feeder breaker rating and conductor ampacity. Obviously breaker ratings and conductor ampacitys aren't rated in kW so this kW needs to be converted to current.

We all know that this 94.4kW value will have different current values associated with it depending on the power factor of the circuit. For instance @ pf=1 current=262A, @pf=.9 current=291A and @pf=.8 current=327A. So obviously the power factor will determine the existing load current which then has to have a factor of 25% added to that per 220.87.

So are we saying here that since power factor isn't referenced in 220 that we can ignore it and just convert the max demand kW to an ampacity value using A=kw/(208V * 1.732) ignoring any power factor (which would represent a unity power factor). This does not seem realistic since there aren't any facilities that I am aware of that operate at a unity power factor.

 

[iwire]

So are we saying here that since power factor isn't referenced in 220 that we can ignore it and just convert the max demand kW to an ampacity value using A=kw/(208V * 1.732) ignoring any power factor (which would represent a unity power factor). This does not seem realistic since there aren't any facilities that I am aware of that operate at a unity power factor.

Please see my post above about my thoughts on that.


Even in the example load calculations in the NEC annex they assume 1pf Perhaps not realistic but not going to result in an overload is it? Part of 220.87 is a requirement to calculate the existing at 125% giving some additional headroom on top of very conservative NEC load calculations.

 

[FionaZuppa]

What you posted from the NEC is just a tip, a suggestion, a friendly reminder, not a requirement.

The calculations in article 220 result in an oversized service or feeder and therefore the PF even if poor is not going to really be an issue.

NEC is full of these extra print "tips".
if the NEC has no regard for PF directly, but does indirectly by way of the over-engineered math, then why even mention such "tip"? the tip is meaningless if the NEC math is, as you say, "...and therefore the PF even if poor is not going to really be an issue."

and btw, i replied with that quote because it was said that 220 doesnt mention PF, which is false :thumbsup:

 

[philly]

Please see my post above about my thoughts on that.


Even in the example load calculations in the NEC annex they assume 1pf Perhaps not realistic but not going to result in an overload is it? Part of 220.87 is a requirement to calculate the existing at 125% giving some additional headroom on top of very conservative NEC load calculations.

I see where you are coming from with this and also see that they use 1pf in the annex examples.

I agree that the NEC load calculations are conservative and will result in an oversized service/feeder when you start stacking individual load calcs which are each conservative, but what about in a situation where we are not looking at conservative calcs per say but actually looking at real "measured" load information such is the case here. Do you still think the 125% gives enough extra room where power factor can essentially be ignored and used at unity? Would you feel comfortable using a 1pf for this case knowing that that the combination of the "maximum demand" (in-frequency occurrence) and 125% give you enough of a safety margin?

I appreciate the help!

 

[wbdvt]

Having worked in a utility for awhile, I can offer several things which may or may not be of help:

1. The suggestion of looking at the meter may be worthwhile as sometimes they do show the power factor if the utility programmed the meter to display that.
2. The fact that there is no pf on the bill leads me to believe that the tariff they are under does not have a pf penalty so no pf monitoring required
3. Sometimes the tariff will contain a minimum pf that is assumed so it may be worthwhile to review the tariff

 

[philly]

and btw, i replied with that quote because it was said that 220 doesnt mention PF, which is false :thumbsup:

out of curiosity where in the NEC did you find that tip?

 

[iwire]

NEC is full of these extra print "tips".
if the NEC has no regard for PF directly, but does indirectly by way of the over-engineered math, then why even mention such "tip"? the tip is meaningless if the NEC math is, as you say, "...and therefore the PF even if poor is not going to really be an issue."

I have explained my position clearly, you are free to disagree with it.

and btw, i replied with that quote because it was said that 220 doesnt mention PF, which is false :thumbsup:

Tomato, tomato the tip is located within 220, it is not part of 220. It is not code.

You are free to consider it however you want.

 

[iwire]

out of curiosity where in the NEC did you find that tip?

220.12, 220.40. 430.22 mentions power factor, 430.120 in notes to table 430.250 maybe more.

 

[Ingenieur]

Looks like 325 to 400 A
pick the next size up from 325 (checking V drop)
like was said you have 25% which is like a 0.8 pf

how consistent is the month to month demand?

 

[electrofelon]

Besides PF, note that there is no requirement to consider lack of perfect balancing, nor whether the load is continuous or not. Personally, I find only 25% adder beyond the demand figure to be cutting it kinda close compared to "regular" NEC load calcs.

Most utility bills will have a VAR field which could be used to get average PF, but of course that wouldnt give you the PF at the period of max demand.