Multiple switchboards and VT? Plus CT cable limitations

Hi all,
I’m in Australia and have a 3 phase feed to my house. I have 3 switchboards, a main board where my meter is, and this feed to sub boards, one in my house and one in my shed.
I have solar which is connected on a single phase on the house sub board.
I have plans to replace the existing solar with a much larger 3 phase setup connecting to my shed sub board.
Which leads me to how to monitor it all.
Initially I’m thinking one unit in the main board monitoring the mains, the feed to the shed, and maybe the feed to the house.
A second unit in the house sub board to monitor the solar, and possibly the main feed to the house if not done at the main board plus some circuits going in to the house.
In the future if the new solar gets put in, i will have a 3rd unit in the shed.

The main question, do i need to have 3 VT for each unit, or can one device collect this information for all devices

Also, what are the cable length limits to get from a CT back to the device, eg, can i have the device in the house sub board, and have a CT on the back of a power point on the other end of the house a 40m (130’) cable run away.

To answer the VT question first, while you can use three VTs, most folks use one to measure three-phase, and I would recommend you try that first. IoTaWatt can derive voltage references for the other two phases. The variation in power measurements on those phases would be proportional to any voltage difference from the referenced phase.

Using a single VT (derived reference), the economy of connecting the second to the same VTs becomes less of an issue. However the answer is yes, you can connect a VT to multiple units as long as you are strict about preserving the same polarity in the extensions and splitters. I recommend using inexpensive splitters usually sold to CCTV users.

Your initial plan sounds good, but you might just consider monitoring only at the house and shed if the main just forks to those two loads. If you upload to a server like Emoncms or influxDB, you can add the two together there for total usage/import/export.

Extending a CT lead 130’ is a long way. If you use 22Ga CAT cable, the resistance should not be a factor, but there could be some noise. Others have done long runs but no reports of results. I’m assuming they are working ok.

Re the Sharing of the VT I didn’t mean connecting one VT to two devices.
I meant connecting it to one device and the second device using the data of the VT from the first device. So device 2 has no VT plugged in, just a data feed from device 1 via wifi or a shared db to do the calcs.

Running cables is difficult currently both in terms of cable lengths and in some cases the need for excavators and concrete saws which is why I’m looking at multiple devices, and hoping the VT can be avoided at every device.

It’s not just the rms voltage. The VT provides the basic heartbeat of the IoTaWatt. As each CT is sampled (about 760 times per AC cycle), so too is the voltage sampled. Each current sample requires a voltage sample. Where you see the voltage as 240V, IoTaWatt sees it as 50 cycles per second that go from 0 to 340V and back to 0 in a sine wave, sampling all the way. Phase correction is measured in microseconds.

It needs a VT signal.

Power monitors that don’t use a voltage/phase reference only measure apparent power (VA), which is not what you are billed for. To get real-power (Watts) you need that VT.

Using three VTs is not only expensive, it requires three plugs each on a different phase. That’s why IoTaWatt offers derived reference to do three-phase with one VT. Try it, I think you’ll be satisfied with the results.

A minor pointer on billing in some countries. You can often be billed on both Apparent power ( kVA ) and energy ( kWh ) - if you are being billed on both you certainly need to know both.

Thanks :slight_smile:

I have a mate outside of Sydney who has 3 Phase. He has 2 boards, a Main / House / single phase Solar and a sub board in his garage where he has the 3 phase Solar. He uses 2 IotaWatts and has a single derived VT in each. The boards are probably about 25m from each other. The 3 phase loads he measures are a sauna in the garage and his aircon. All other loads are single phase.

There is no method AFAIK of making an IotaWatt “aware” of a neighbour device. They are standalone.

How far is your house board from the main? Unless you need specific circuit monitoring i.e. a HWS in the shed you might be able to get away with one IW in the house provided you can get 3 CT circuits back to the main to get your “mains” (total) current readings.

Bob, I’ve never tried it but would CAT 5/E cable work for distance CT connection? 8 cables in 4 twisted pairs should help with noise. and seems tailor made for a 3 phase with a possible VT direct from the main board.

IMO cat5 (or 6) is a good choice. I talk about it in this post Ecs1030 ct sensor

I don’t think the VT needs to be out there unless the mains cables between the meter and house are causing a significant voltage drop.

I wanted to monitor power consumption on a cottage remote from the main house, so I put a CT on the cottage active mains cable (direct from the meter box / grid connection) and cut the CT clamp lead in two and spliced it to a disused telephone cable that runs back to the main house where I spliced the other end of the telephone cable on to the length of the CT clamp cable with the 3mm stereo plug. Works fine for a 30 metre run, the telephone cable was 4 core so I joined them to make it a two conductor cable.

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The house to main is around 20mt cable length but getting the cable there will involve an excavator, and in just over week I’ll need to pull up 150mm reinforced concrete too. Considering machine hire, and conduit costs, plus dealing with a 100-150mm “crawl” space under the house, I’m happy to just buy another unit.
Roof is a true flat clip lock roof with exposed beams inside so the roof cavity is practically non-existent and combine this with the ceiling being asbestos, I’m not going to send anything that way.
The VT requirements got me thinking… but I’ll save that for another thread.

Thanks everyone, I think I’m going to ultimately have 3 devices. And 9 vt’s as there may be voltage differences between the phases that I want to monitor, especially when my current solar is on the phase that only has my shed and the lounge aircon on it