Dumb electricity question

[gafftapegreenia]

A clamp meter is nice to have; you can get by with a cheaper non-true RMS meter, the waveform of power services will generally not be irregular enough to make a practical difference.

I respectfully disagree, and this in fact goes against both the words of Steve Terry and Richard Cadena. If you are serious about getting real measurements, you will get a true RMS meter. You also want to make sure it's CAT III rated for the loads you will encounter. Sure, a cheap one might be better than nothing, but I personally would rather buy the right tool the first time. There are Amprobe brand models right around $100. I'd recommend buying one of those before buying the ratcheting Ultimate Focus Tool.


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[MikeJ]

I respectfully disagree, and this in fact goes against both the words of Steve Terry and Richard Cadena. If you are serious about getting real measurements, you will get a true RMS meter. You also want to make sure it's CAT III rated for the loads you will encounter. Sure, a cheap one might be better than nothing, but I personally would rather buy the right tool the first time. There are Amprobe brand models right around $100. I'd recommend buying one of those before buying the ratcheting Ultimate Focus Tool.


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Fair enough, but a averaging meter, is still better than no meter. Realistically, if you calculate you power requirements, you should not be too concerned with having enough power, then you can meter simply to check load balance, so a little inaccuracy over all is not a factor, because you only want to check that each leg has a similar load.

Also, I would not ever buy an Ultimate Focus Tool.

 

[Jay Ashworth]

Am I the only person who thinks that the cost of three current transformers rated for your per leg maximum and the associated monitoring hardware is really not that expensive?

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[JD]

If you are going to read with a regular meter, make sure your dimmers are set to 100% as that is where the waveform is closest to sinusoidal. As the dimmers are lowered, the waveform will become very distorted and your reading may be far from the truth. Also, keep in mind that on three phase power, there are dimmer setting that will actually cause the current flow on your neutral conductor to exceed the current level through any of your three hots. These current flows, called triplen harmonics occur at 180 Hz and may not be represented when metering unless you are using a true RMS meter. Fundamentals of Harmonics | content content from Electrical Construction & Maintenance (EC&M) Magazine

 

[MikeJ]

If I do the math ahead of time, I don't have much need to measure, especially if I know I am well under my limit; an unbalanced load however, can kill a generator, well before tripping the breaker.

 

[garyvp]

If you are going to read with a regular meter, make sure your dimmers are set to 100% as that is where the waveform is closest to sinusoidal. As the dimmers are lowered, the waveform will become very distorted and your reading may be far from the truth. Also, keep in mind that on three phase power, there are dimmer setting that will actually cause the current flow on your neutral conductor to exceed the current level through any of your three hots. These current flows, called triplen harmonics occur at 180 Hz and may not be represented when metering unless you are using a true RMS meter. Fundamentals of Harmonics | content content from Electrical Construction & Maintenance (EC&M) Magazine

Metering is the only way go. Our small community theater has 100 x 3ph mains driving three dimmer packs with face-plate rated at 180 amps (two 120v phases each). Obviously we cannot run them at full power - but we get the 24 channels, and the dimmers run very cool. However, a lot of training and monitoring is necessary, especially for ‘guest’ lighting designers, on how to balance the dimmers vs the grid while enforcing a 20% derating of the power. Had some over-powered shows with some stressed breakers and popped SCRs. I would sometimes meter each feeder with my fluke, but this was cumbersome and not in every techs skill set Finally, we gave up on the math and built and installed a meter box with 7 digital ammeters with dedicated CTs - two for each dimmer phase and one for the neutral, which, as per many folks in this thread, is fascinating to watch when cross-fading (glad the electrician insisted on over-sizing the neutral above the feeders). We also added three voltmeters (came in handy several times when there was a phase drop in our area). Finally, there is a digital thermometer for the main conduit temperature – also interesting to watch during long single scene programs. Everything is housed in 12" box located in the control booth. Total cost of about $1,000 in parts. As they plug and test the grid, a designer/tech can see the loads real time. Note that it still requires a responsible person to check each show out. The head lighting designer can review an entire program in about two minutes and see the loads at each queue and enforce the balancing. Blind without it.

 

[BillConnerFASTC]

Am I the only person who thinks that the cost of three current transformers rated for your per leg maximum and the associated monitoring hardware is really not that expensive?

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Reminds me of what was discussed - not sure if it was installed - for Yale Rep - which had a 400 amp 208 feed for entire building. They added an elevator for ADA and talked about or did - but meters for each leg of the feed so if needed the light board op could pull the master down a little when someone using a wheelchair was late to the show.