Connecting to two different circuits at different times

[jpmcgee]

I'd like to monitor both my electric water heater and my HVAC. Both are 240v 3-wire and are both fed off the same breaker box. I don't need to monitor each one at the same time. I'd like to get a single OmniMeter I and two sets of CTs. About once a month, I want to move the meter from one set of CTs to the other.

Do I need to also connect the power/voltage reference leads to each specific circuit, or is it OK to leave them connected to the water heater circuit when I want to monitor the AC circuit (or vice versa)?

Just FYI, high accuracy is not necessary. I'm feeding this data into a thermodynamic model of the house. Lots of other numbers are known only to 10% at best. So getting numbers that are a few percent off wouldn't matter at all.

Thanks, Pat

[Jameson]

Hello Pat,

Yes this is quite possible.

I would recommend you get 1 Omnimeter and 1 set of split-core CTs.

http://www.ekmmetering.com/ekm-metering ... s-cts.html

The split-core CTs make it very easy to move the meter and CTs from one location to another. Or if it seems easier to you 2 separate sets of CTs will also work.

The power/voltage reference wires only need to come off of the correct phase. So anywhere you can tap into Line 1 or Line 2 will work. If the water heater circuit is close to the AC circuit, you should not have to move the power/voltage reference wires (just move the CTs).

I would say you can expect to have full accuracy (0.5% accurate) when using the meter on each of these 2 systems.

Best regards,

[jpmcgee]

Thanks. I'll go ahead with the two sets of solid-core CTs so I can run the wires to the outside of the breaker box once and not have to worry with opening the box when I want to switch.

Pat

[Nevland]

You might want to consider interposing a couple of DPDT switches, From a safety stand point and CT longevity. It is important to keep the unused installed CT's terminated. Very high voltages are generated when CT's are not terminated. Also the small screws used on the V3 or V4 meter are not made foe easy interchange. A side pulse is that they could be both on at the same time and then would be additive.

Bill

[Nevland]

Another thought. If you have a v3 or V4 meter and using singles phase, you could obtain another CT and connect to the unused 3 rd phase.
This would give the amps used. Hooking up the voltage would then give watts used, but would make total watts used inaccurate. I think you would have to subtract the watts form the additional CT to get the correct total.

Bill

[jpmcgee]

Hi Bill,

Interesting point about the wire and screws. I can connect the CTs to barrier strips (Cinch 4-140) in sets of two , then run jumpers from the OmniMeter to the other side of the barrier strip. That way all the connects / disconnects are between the jumpers and the barrier strips.

For the set of CTs not connected to the meter, would it be better to short them out or put a resistor across each? 10 ohms or so?

The two circuits I'm monitoring are both 240v 3 wire. I don't understand your point about the additional CT for the 3rd wire. Could you explain a little more fully, please?

Thanks,
Pat

[Jameson]

Bill does make a good point above. CTs should never be left on a line with current running through them without either connecting them to the meter CT inputs or shorting out the White CT Wire to the Black CT Wire. A direct short will work, we use wire nuts or our Lever Nuts for this.

Thanks Bill!