Delta vs Wye

[Myself248]

Forgive me, DC pro, AC noob, trying to learn enough about AC to help a local hackerspace set up submetering.

Our service is 240v 3-phase delta with a split-leg to provide a neutral and 120v for office loads.

Coming off the pole there are just 3 wires, so I guess the neutral counts as derived, but it does tie to earth ground at the panel. Should I worry about metering the neutral current? (It occurs to me that I could throw my clamp-meter on the neutral and get a feel for how much is flowing there. It shouldn't be much!)

Is there any math or reconfiguration required to do delta rather than wye? I've seen some other meters that claim to only support wye, and I don't understand why. The Omnimeter-II UL page says "There is not an electrical system, up to 240 volts, in common use that this meter will not support." [emphasis mine] So is this whack-ass delta thing considered common? Is it supported?

Any other clue y'all feel like imparting, please do. Thanks!

[Jameson]

Hello there.

Good to hear you are coming over to the AC side.

Your description is a bit confusing, but it sounds like you have what we would call a 3-phase 4-wire system. This system will require 3 CTs and will not require a CT on the neutral. Let us know if you do not have 3 hot wires and a neutral available to you. It sounds like if you measured from any hot to any other hot you would measure 240 volts, or from any hot to neutral you would measure 120 volts. Is this correct? If not, please let us know how many hots you have, and if you have a neutral.

Here is a 3-phase 4-wire (Wye) wiring diagram for our Omnimeters: http://www.ekmmetering.com/information/ ... ml#answer8

Any of our Omnimeters (v.3 and v.4) will work well with this system.

Here is some good info about 3-phase systems and the differences between Delta (3-phase 3-wire) and Wye (3-phase 4-wire): http://en.wikipedia.org/wiki/Three-phase_electric_power

If you have any questions, please let us know.

[Myself248]

I have three hots, a neutral, and a ground. Phase-to-phase voltages are 240 every which way. Neutral-to-phase voltages are 120, 120, and 208 volts. (Measured 124, 121, 211, but close enough!)

I'm pretty sure this is the system described on the last page here:
http://www.programmablepower.com/suppor ... bution.pdf

also known as:
http://en.wikipedia.org/wiki/High-leg_delta

Also, I have to correct a previous statement: There are 4 wires coming in from the pole.

So my questions are:

Does the meter need to know whether it's connected to a delta or a wye system? Is the current on the phase conductors the same either way for the same amount of power being used? It would seem to me that it would have to be, but as I said, AC is still not intuitive for me.

Does that get weird when you throw a neutral into the mix? I measured about an amp on the neutral. That suggests to me that some imbalance in the 120/120 side is leaking power on the neutral that the phase CTs might not measure. Or would the meter be able to infer it based on imbalance between the phases? Either way, it's small enough I'm not worried about it, this question is mostly academic. (I'm pretty sure that since a 120/120 domestic system is meterable using only the hots, that this would be too, but I want to check.)

Thanks again!

[Jameson]

Thanks,

Yes, this is a 120/208 3-phase 4-wire system.

Any of our Omnimeters will work with this system. You will only require 3 CTs. (you will not need a CT on the Neutral). If you connect your meter to your system as shown in the wiring diagram the meter will self configure to this system (you do not need to make any settings in the meter or set any DIP switches, just connect it and go).

Here is a video version of the wiring diagram: http://www.youtube.com/watch?v=DeKiZddR ... r_embedded

Is the current on the phase conductors the same either way for the same amount of power being used?

I think you are asking if the current will be the same on all hot legs. In the case of a 3-phase 4-wire system, you can have wildly different currents on each leg. For example, if you connect a 120 volt hair dryer from Phase A to Neutral, you will only sense the current used by the hair dryer on Phase A.

Blondel's theorem describes why you only need N-1 CTs for a given system. N equals the number of conductors. Don't you just love wikipedia?: http://en.wikipedia.org/wiki/Blondel's_theorem

Here are the 4 AC systems commonly used in the world and the number of CTs used for each:

* Use 1 CT for single phase 2-wire systems (one hot and one neutral, with or without ground).
* Use 2 CTs for a 120/240V three-wire system (two hots and a neutral, with or without ground).
* Use 2 CTs for 3-phase 3-wire systems.
* Use 3 CTs for 3-phase 4-wire systems.

I appreciate your questions, and agree that this stuff can be a bit of a mind bender. If you have any other questions, please let us know.

Best regards,