North America 120 Volt Power

Has anyone here got a well tuned system running on the North American single split-phase power? If I understand correctly is seems that the Europeans receive a single 240 line that they monitor, while in North America we need to monitor two 120 volt lines that are 180 degree out of phase. 

I am assuming that by using two CT's, one on each line, I should able to simply sum them to get my total current and therefore my apparent power usage. 

If I wanted to get to real power and power factor, I would need to monitor the voltage. Since there is only one voltage input on the emonTX should I simply use that, or would accuracy be increased by simply using the reciprocal voltage for the second CT? 

I guess in theory since each 120 volt line would have different loads on it is likely that they have different power factors on each side. 

Has anyone looked into this? Or am I overthinking I and should just use the two CT's and ignore voltage all together?

Robert Wall's picture

Re: North America 120 Volt Power

This is a regular question. If you do a search through the forums, you should find comments by people who have done it. If my memory serves, the voltage balance of the two lines is good, so you can measure the voltage of one and treat it as being accurate for the other.

Yes, you're correct. Almost all UK domestic consumers receive a single phase 240 V supply, with the neutral at or very near earth potential. Some Continental European domestic customers receive a 3 phase 230 V supply (with the phases 120° apart), again with the neutral at or near earth potential.

Whether you want to measure voltage depends on what you want to do, and the accuracy you want. If your system voltage is constant and your power factor is too, you can measure current and multiply by the correct fiddle factor and call it power, and it'll be correct. If either or both of those assumptions are not true, then depending on how far your voltage strays from the value you assume, and the same for power factor, then that's how inaccurate your measure of power will be. Long-term, you could probably choose values that meant the average would work out reasonably correct. But nothing is guaranteed.

In summary, I believe the way this has been done successfully is to monitor one line voltage and two line currents, and treat the line voltage as being representative of both lines. You can then have real power, apparent power and power factor for each line, and by adding the powers the total consumed power.

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