Measuring PV more than 30 metres from CU


My PV inverter connects into a secondary CU in an outbuilding, which then runs to the main house CU about 30m away before going into the grid. I’d like to monitor the PV generation and the grid connection for export. What’s the best way to deal with this distance? Can I run a CT clamp over that distance with an extra long wire? I doubt it. There is wifi at both locations (different networks though).


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Presumably, you want to know PV production without the load in the outbuilding?

I have not run a c.t. over that distance, but with care - routing the cable to avoid other cables that might carry and radiate interfering signals - it is certainly worth a try. See the article in ‘Learn’ for details.

Otherwise, the ‘off the shelf’ solution would be an emonPi in the outbuilding, sending via WiFi into your LAN.

Thanks Robert - yes I’m wanting to measure PV generation without the loads in the outbuilding. There’s an ASHP out there which will use a load of power … and in fact I will want to monitor that later as well.

I’ll start with an extended CT and see how it goes.

If there are more than two circuits to be monitored in the outbuilding, or even if only two, then slightly less ‘off the shelf’ is an emonTx with an esp8266 added on.
Two cables is still likely to be the more economic option, but it’s getting close.

I ran them over 30 metre currently somewhere around 50 - 60 metres though this was for a sun limiting inverter but it uses the same CT as oem devices do

I run the distance using CAT5/6 wire and connect multiple pairs the the CT as two wire and for each to each side side twisted pair ( this will will reduce interference) then i connected multiple twisted pairs together this increases the size of the transmission wire which reduces line loss. also as robert said i avoid crossing any electrical lines but if that would be difficult you could use a shield cat 5/6 and ground the wire to absorb and dissipate any interference

you probably could run multiple CTs with one wire just use solid core Cat6 as it it has plastic wire divider to reduce noise interference between the pairs

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Provided the c.t. has an adequate VA rating, conductor resistance will have no effect, as the c.t. is a current source. Even if it’s a voltage-output c.t, the effect will be minimal as the conductor loop resistance will be small compared with the recommended 10 kΩ maximum source impedance for the ADC. (Cat 5 is 2.567 Ω per 100 ft per leg, so about 5 Ω total loop resistance - that wouldn’t concern me.)

The most important things are to use twisted pairs properly, and screening and earthing. As the ‘Learn’ article points out, you must not use a pre-wired “extension” cable because the input is actually the sleeve of the input connector, which is normally connected to the cable screen.

Success! I’ve used a single twisted pair in a Cat5 cable, with no screen, over a distance of 35m. The Cat5 runs alongside a mains line for about 10m and all seems OK.