Is it OK to lengthen the line from an emonVS to an emonTX?

Hardware emonTx
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I’m looking at the practicalities of mounting my emonTX and emonVS.

The latter is BIG and - realistically - it’s going to be very difficult to mount it anywhere close to either my consumer unit or the meter - the space is just too tight. I can just about fit the emonTX by the CU.

The best I can do is mount the emonVS about 3m away. As the cable between the VX and TX is (I assume) standard CAT5/6, Is there anything preventing me having a longer cable? Voltage drop?

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Exactly. It’s caught someone out before - there’s a post which I answered, I can’t remember when, it feels like a few months ago. The cable is already using (I think) 2 pairs for the power (Pins 4+5, 7+8). You’d need to check the instantaneous maximum current as the Pi starts up and look up the voltage drop for the particular brand of cable The Shop has supplied with yours, to get an accurate assessment.

[Edit] This was the earlier topic: EmonPi2 only displaying "starting"

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For an emonTx, rather than an emonPi, the only issue might be picking up some additional noise.

For Cat5, the resistance is ~0.1 Ω/m, so for a Tx @ ~100 mA, you’ll lose 0.03 V, which isn’t an issue as the 5 V (nominal) is regulated down to 3.3 V. If you have a Raspberry Pi attached, this would be an issue as at ~1 A for the Pi, you’d get .3 V drop over the cable, so only 4.7 V at the Pi.

Edit: thanks @Robert.Wall - forgot it’s over 2 wires of the cable.

@Robert.Wall - actually, checking, it’s a pair each for the V+ and GND legs, so with the round trip it’s the equivalent of 0.1 Ω/m total drop (0.05 Ω/m each way). Maybe it needs a boost converter!

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All of which means it’s OK for any sensible length of cable with an emonTx on the end - you’ve got about 1.2 V you can afford to lose to voltage drop, so about 12 m maximum, do your sums if it’s anything else.

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The CAT6 cables we currently use are 24 AWG - part number ERT-601K.

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That’s 88 mΩ/m per single core (Ref: AWG - Wire Gauge Sizes: Current Ratings, Charts, Measurements, and Conversion Guide) , so two cores in parallel but also two in series still comes to 88 mΩ/m of cable run.

I make that in excess of 100 m for an emonTx, so noise would probably be a bigger problem than voltage drop. But for 1.8 A starting current for an emonPi requiring 5 V, it’s a totally different matter.

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I’d got my units in a twist, so it’s 120 m :+1:

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For completeness, looking here https://raspberrypi.stackexchange.com/questions/99961/powering-the-pi-4-safe-voltage-levels-and-current-requirements for more about the RPi itself – hence answering the original question but for the emonPi2, and taking the number given there of 4.7 V being the minimum guaranteed starting voltage, and 1.8 A as the maximum starting current, and given the power supply output voltage can be 5 V - 2%, with a further 0.9% for load regulation on top, you have 4.856 V out of the emonVs (worst case). This leaves 156 mV available for voltage drop in the cable and connectors, so at 158 mV/m voltage drop, you can have just 1 m of cable - exactly the length supplied.

For anyone contemplating using something other than an Ethernet patch cable, 2 cores of 24 AWG is approximately 0.4 mm², so using the approximations and assumptions above, using 1.mm² for the power would allow a length of about 2.5 m, 2.5 mm² a length of about 6.25 m, and 4 mm² a length of about 10 m.

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Thanks for the discussion and info.
In my case, it’s a bare emonTX5 - no Pi - so I should be OK with 3m.