Howto measure 480V WYE 3-phase reference voltage?

1

I have my IoTAWatt devices and CTs, and I reviewed the installation plans with my electrician…

We have 1600amp 480Y/277V main service, split into 4 feed legs, for which I have 12 400amp CTs.

One point we’re not clear on, is how we’re supposed to setup the voltage reference transformers on a 480Y/277V WYE 3-phase service.

Should we install three 277v to 120v transformers, and then plug in the three 120v to 9v reference voltage transformers to that?

How do we tell Iotawatt that we’re running 408Y/277V service under the voltage reference? I didn’t see anything in the manual about this.

Is there some other recommended way to get a reference signal for 480v WYE service?

2

…ohh, and also, is it fine to put a Y-splitter on the reference transformers and feed them into multiple IOTA watts?

In our main 480Y panel we’re using 2 iotawatts (one box for the 3*4-split main inputs, and one for the load outputs), and in main 208Y panel we’re using 3 iotawatts (15 circuits three phase 208Y to subpanels).

3

Hello Dave,

The good news is that you are not on the bleeding edge. The manufacturing plant where IoTaWatt are produced has 480V/277V three phase and we have an IoTaWatt monitoring the main panel.

I see the AX-27750-H is currently available on ebay. It’s also availabl;e as an ELCO ETR12-277 from Amazon, and several electric supply online (google it).

As in the pictures in the post above, they mount to a standard hole in a wiring box. The secondary (12V) are just wire leads, so you need to attach a cord with 5.5mm x 2mm barrel plug (like the other VTs).

You can use three, or just one for derived reference. Yes, you can split the transformer output with a commercial CCTV splitter cord. It is imperative that you observe consistent polarity when splitting, and the commercial splitters pretty much insure that as they are made for DC where polarity is less forgiving.

If you share AC reference between IoTaWatt base units, it is critical that each base unit have it’s own USB power supply.

The above transformer is already in the VT tables as AX-27750-H. I would however recommend calibrating after installation as with limited experience I don’t how much variation there is.

4

That helps, but didn’t exactly answer my question.

We have to put in a 277v to 120v transformer to create a 120v output for the USB power supply anyway… because in this case it’s easier than running conduit from another 120v panel.

Can I use the 120v->9v reference transformer in the 120v outlet, and then just “calibrate” it up to the 277v supply voltage?

Or do I have to use this alternative AX-27750-H 277v reference transformer?

5

That should work with respect to measuring the voltage but be aware that every transformer introduces some phase shift, and so you will have the compound shift of two transformers. IoTaWatt corrects the phase shift of both the VT and CT according to the measured shift defined in the configuration tables. If the shift of the 277V/120V transformer is small, you could probably ignore it.

My rule-of-thumb is that shift is inversely proportional to the weight of the transformer. So as long as it weighs at least a few pounds, you should be OK.

6

Thanks for the info…

For others, I see the code here contains the set of currently pre-configured VT options…