Question regarding 220.87

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philly

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When verifying existing loads on an existing feeder for the addition of new loads per 220.87 can the 30-day continuous recording requirement be accomplished by daily spot checking of existing meter displays over a 30-day period? To me the word "continuous" would not allow that and would require a continuous recording?

Also when trying to measure the total load on existing SWBD if it is not practical to monitor main on SWBD can the monitoring process happen by setting up (3) meters on outgoing feeders from SWBD and adding the cumulative currents (same time stamp) to arrive at the total current? Depending on power factor of loads on each feeders wouldn't the current angles for each feeder be different and not follow linear addition (would require phasor addition)?
 
philly said:
When verifying existing loads on an existing feeder for the addition of new loads per 220.87 can the 30-day continuous recording requirement be accomplished by daily spot checking of existing meter displays over a 30-day period? To me the word "continuous" would not allow that and would require a continuous recording?

Also when trying to measure the total load on existing SWBD if it is not practical to monitor main on SWBD can the monitoring process happen by setting up (3) meters on outgoing feeders from SWBD and adding the cumulative currents (same time stamp) to arrive at the total current? Depending on power factor of loads on each feeders wouldn't the current angles for each feeder be different and not follow linear addition (would require phasor addition)?
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The first question, I don't see how spot-checking satisfies the 15-minute requirement.

Second question
I don't see why scalar addition shouldn't work, but it depends on how different the power factor is for each of the three feeders. If you have concerns and assuming your logger can also measure power factor, compare it through the three data sets. If the power factor is close enough, my opinion there is little value in extracting phase angles from the data. At the end of the day, the 220.87 requires the feeder to be sized for 125% of max average demand plus the new connected load. Any variation in phase angles is unlikely to cut into that 25% cushion.
 
philly said:
... Also when trying to measure the total load on existing SWBD if it is not practical to monitor main on SWBD can the monitoring process happen by setting up (3) meters on outgoing feeders from SWBD and adding the cumulative currents (same time stamp) to arrive at the total current? Depending on power factor of loads on each feeders wouldn't the current angles for each feeder be different and not follow linear addition (would require phasor addition)?
Click to expand...
If the phase angles of the feeder currents are each different then the magnitude of their vector summation can only be less than their scalar summation, because the vectors aren't aligned with each other. And so using a scalar summation of RMS values may not be exact, but if anything it will overestimate the actual current. Also the angles would have to be substantially different to make any significant difference in the result. To take a familiar example, the magnitude of the vector sum of two equal RMS currents at 60 degrees apart is 1.73 times each one instead of 2 times, or a 13.5% reduction.
 
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