Existing Calculations & Power Factor

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solarEI

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I'm trying to understand the KW/KVA/PF relationship in the real world and i'm hoping you guys can give me a hand :) I understand PF=KW/KVA...here is the scenario:

I'm working on an addition to an existing 120/240V, 1phase service. I used an IDEAL 400A ac clamp meter and got the following:
Panel A = 62A
Panel B = 3A (almost entirely receptacle loads)
Panel C = 69A
Panel D = 22A
(these readings are the larger of the two phase loads feeding the panel).

With the panels totaling 156A; 156A x 240V = 37440 (W or VA?, shouldn't it be W since that is the real power?) (I don't want to get into the nitty gritty details, i know there are other factors to consider such as loads out of phase, etc...i don't care about the exact calculations, i just want to understand what's going on.)

The utility meter had a current demand reading of 42KW, I'm trying to determine if the difference in the ultility reading vs. the panel readings are due to PF or possibly just loads switching on and off?

If the utility meter and the panel readings (37440) are both in W, where do you get the VA to determing the power factor (I hope i'm not getting ahead of myself)?

Do you even need to consider the PF in this type of situation? What is the acceptable tolerance for a negligable PF (i.e. +/- .05) for calculation puropses?

With equipment nameplate ratings, are the manufacturers assuming the PF is negligable, I would expect a PF<1 in the testing conditions?

With KW=KVA*PF is it better to calculate the loads, services, etc...using KVA since it will be than KW?

I'm sure I'm brain f***ing this but I want to make sure I understand these relationships and what's going on. Thanks in advance!!

-solarEI
 
Your first calculation gave you volt-amps. You multiplied volts x amps.
Utility demands readings are usually an average over 15 or 30 minutes. You probably read the highest average since the last meter reading. It would probably have no relationship to your instantaneous readings, unless you were lucky enough to measure simultaneously.

If you were lucky enough to read simultaneously with the utility demand, your number would be higher beause you are measuring va and the utility is measuring watts, which as you correctly showed, is less than va.

Power factor can be important with bad pf loads, you will find vars equal to or greater than watts. You should do your calculations in va or kva, which will give you actual amps as you used in your first calculation.

Jim T
 
Real power in Watts can be measured by (Er * Ir) using cl/amp-probe, or using nominal voltage VA * PF * Sqrt(phases).

If you measure volts at the same time as cl/amps, (Er * Ir) = Real power in Watts.
 
Thanks guys, I think this is slowly starting to come together. Since I didn't measure the voltage, is there anyway to derive the PF from the info I do have?

If the measured loads are all in VA, and the calculations are done in VA, at what point does the PF even contribute to the service or feeder calculations?

Other than power quality issues, is the PF just taken into consideration for utility metering purposes, so they are billing based on real power (W) vs. the total power (VA)?

Thanks again,

-solarEI
 
In real world terms,
PF is the electricity borrowed to make magnetic fields and are returned to the utility un-consumed as the alternating current...alternates... 60 times a second

W- is the electricity consummed and paid for that does real work- make lights, heat, etc.

VA is how many "amps" will be needed to deliver both the W and PF since they cannot be directly added together- if you want to know why, you need to leave the real world and join us in the theory discussion that will surely follow what I just wrote!!!!
 
solarEI said:
Thanks guys, I think this is slowly starting to come together. Since I didn't measure the voltage, is there anyway to derive the PF from the info I do have?

If the measured loads are all in VA, and the calculations are done in VA, at what point does the PF even contribute to the service or feeder calculations?

Other than power quality issues, is the PF just taken into consideration for utility metering purposes, so they are billing based on real power (W) vs. the total power (VA)?

Thanks again,

-solarEI
Click to expand...

There is no way to calculate the pf from the data you have. If the load is large enough to justify, the utility company might have a meter that measures kW and kVAR, which can then be used to calculate kVA and pf. Unfortunately, it does not sound like this load would have that information from the meter. It would only measure the kW-hours and the peak kW demand for the previous month(s) as mentioned already. The measurement you took is VA at one point in time.

If you are doing service and feeder calculations, then you want to deal in kVA so you can size your equipment for the total or apparent power that will be flowing through them. Billing is done in kW-hours. For customers with poor power factors, some utilities charge a penalty for that since they are providing kVA, but only making money on kW-hours, so the penalty provides some balance to that lost revenue and hopefully makes the customer correct their poor power factor.
 
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