120v meter with 3 phase power?

[BioHazard]

My shop is wired with a 3 phase 400A 208V Wye system. I'd like to buy some cheap used kilowatt meters like utilities use and monitor the electrical useage of a few seperate circuits. Problem is 3 phase meters aren't cheap.

The things I want to measure are only 120 volt. Would it be possible for me to use a single phase, 120v kwh meter like this one to measure a single circuit on my 3 phase system? Will it read correctly?
http://www.hialeahmeter.com/eawame.html

[Ronmar]

How large are the 120V loads you want to monitor?  If it is 120V and the loads are under 15A, could you use a killowatt?

[WStayton]

Hi, Guys!

  Let me start by saying that what I don't know about three phase far exceeds what I do know!  <smile>
 
  Having said that, isn't a single phase of three phase absolutley identical to a single phase of converntional 120/240, aside from the differances previously noted as to the cleanliness of the waveform, etc.?

  So, I would think, if it is otherwise idnetical to a 120/240 phase, you could measure it with something designed for 120/240 except that I'm not sure if there is a "neutral: available for a "Y" or "Delta" - one of those is just the three phases hooked in a ring, no???  Should be (always dangerous!) delta, but that isn't even slightly guaranteed!  <grin>

  Rcavictim is pretty conversant in "three phase" - what say you, will a killawatt work on one phase of three phase power???

  My two cents which is worth less than that!

Regardz,

Wayne Stayton

[BioHazard]

Having said that, isn't a single phase of three phase absolutley identical to a single phase of converntional 120/240, aside from the differances previously noted as to the cleanliness of the waveform, etc.?

That's what I thought too, although somebody is telling me that it won't work, though neither of us can explain why.  The loads are a bit too big for a kill-a-watt, up to 30 amps or so. The wiring is 3 hots and a neutral, so it seems like it should work? Of course, I only want to measure 1 hot and 1 neutral....

[BioHazard]

The "120 volt wiring diagram" shows a pretty simple setup with just one hot wire and a neutral.
http://lib.store.yahoo.net/lib/eofcorp-store/240V.pdf
Can anyone think of a reason why this wouldn't work on one leg of my 3 phase system?

[bschwartz]

From what teenie tiny bit I understand about 3ph, you can use 1 leg at a time for 120, but NOT 2 to make 240.

[WStayton]

bschwartz:

  With "single" phase you have just TWO sine waves of voltage going back and forth between +120 and -120, for a net of 240 when you pick up both sides that are exactly (180 degrees) out of phase.

  With three phase, you have THREE each, single phases going back and forth between +120 and -120 just the same as in 120/240, but they are 120 degrees out of phase, not 180 like with the two phase, so when you go across two of them you don't get 240 volts net, but something like 210 volts net.  Most things designed to work on 240 volts will work on 210 volts, but that isn't universally true and there may be some loss of power and/or efficiency. 

   However, if you have three phase available, you are dumb to use 210 volt two phase because of the increased efficiency of three phase.  A ten HP 240 volt motor is something like 200 pounds in a continuous duty version, while a three phase version is more like half that, so if three phase is available, almost nobody uses 210 two phases just because of the cost of the additional copper. 

  Also, changing direction of rotation is usually a hassle with 240 two phase, having to get inside the unit and swap about 10 wires around, while with a three phase unit, you can just interchange any two of the incoming phases and, presto, change of direction of rotation!  <grin>

   I found this out the hard way, when a three phase transformer blew out and I couldn't be there when the power company installed a new one and inadvertently exchanged two of the phases - made everything in the whole installation run back words!  It was relatively easy to fix, but I wasn't certified to open the high voltage end of the transformer, so I had to go around and swap two of the incoming leads to each panel in the installation to get things running, in the right direction, again!!! That was one time that being the dumb s#!t at the bottom of the ladder was NOT labor saving - at least not MY labor! <grin>

  Again, I hope that my clarity or lack thereof has not further confused the issue!

Regardz,

Wayne Stayton

[BioHazard]

LOL, yeah, before I moved into my shop there had been a tweaker living in there for a while doing meth. Apparently meth makes you think you are an electrician! He basically rewired the whole place, all the while, not knowing he had 3 phase power! There were no ground wires, and the neutral was usually way overloaded...*sigh*