I am taking a supply, max 60a, possibly 45a, from a house intake position to an outbuilding 120m away.

It is possible that PV panels will be installed on the outbuilding in the future so I am sizing the sub main accordingly. As I understand it, the cable should be sized for 1% VD. Is this calculation based on the size of the PV install or the rating of the sub main (45 or 60 amps).

Thanks.

A 60A submain feeding the outbuilding from the house is calculation 1.
If the solar panels can only give say 20A, the same submain is calculated for your 20A feed back to the house at 1pc VD- calculation 2.

At 120m, the cable is going to be painful.

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Lack of planning on your part doesn't make it an emergency on mine....

Every man has to know his limitations- Dirty Harry

I agree with Pete - worst case from the PV inverter's point of view is all generated power being exported back along the submain to the grid (any load in the outbuilding will reduce the PV current, and so reduce the voltage drop (rise) as the PV inverter sees it). I presume the 1% should be from the inverter to the origin of the installation though - so leave some for the final circuit as well as the submain.
- Andy.

The PV would be for use in the house as well as the outbuilding.
I'm looking at a 35mm2 cable.

That's fine - the inverter only has to "push" against the supply voltage - any local loads help the situation, rather than hinder.

Say there's perfect 230V at the supply terminals. The 1% rule (2.3V) should mean that the PV inverter shouldn't have to generate at much more than 230V+2.3V = 232.3V to push its power into the grid.

If supply is high, say 250V, then then inverter would have to generate at about 252.3V.

If the supply is at its high limit, 253V then in theory the inverter would have to boost to 255.3V, but they're normally limited to 253V maximum - otherwise local loads would be exposed to a voltage higher than permitted. Hence the need to keep the voltage drop between the inverter and supply as low as practical.

Say there was local load, equal to the PV output, half-way between the PV inverter and the supply terminals, with the supply at 230V.
No current would be imported from the grid, so the voltage at the half-way point would be the same as at the supply terminals (230V) and only half-the voltage drop between the PV inverter and the load (as it's only half the cable) - so v.d. = 1.15V meaning that the PV inverter would only have to generate at 230+1.15V = 231.15V.

A bigger load would drag the half-way point to a lower voltage, making life even easier for the inverter.

If the house load were connected at the intake position, it would make less difference to the voltage the inverter was generating against.


Ignoring the final circuit for the moment,
2.3V at say 16A (max for G83/1) and 120m would suggest a cable with v.d. characteristics of <1.2mV/A/m

Presuming the outbuilding will have lighting (again ignoring the final circuit for a moment), v.d. should be < 3%, so 6.9V at 45A for 120m would suggest <1.28 mV/A/m

so 35mm2 looks about right. (OK it's a fraction short by the tables, but there's room to adjust for a lower conductor temperature, which should compensate).

- Andy.

Thanks for the replies and Andy, thank you for that detailed response, very helpful.

I would do two calculations to determine the cable size.

Firstly assume no PV and full load in the outbuilding, select the cable such that that voltage drop does not exceed 5% which should be fine for an outbuilding, even with a light. (the 3% drop for lighting is a recomendation and not a requirement)

Secondly, assume no load in use and full PV production of 16 amps which is the maximum permitted for small/simple systems.
Select the cable such that the voltage rise does not exceed 1.1%

I would justify the use of 1.1% and not 1% on the grounds that the published data assumes heavily loaded and therefore very warm cables. In this case the cable will be lightly loaded and the voltage drop therefore a bit less.
There is also a little wiggle room due to the inverters being rated in watts, not amps. If the voltage at the cut out is 230 volts, and the inverter is producing 16 amps, then slightly more than 1% rise is of little concern.
If the voltage at the cut out is 250 volts, then voltage rise is more important, but under these conditions the inverter output current will be not 16 amps, but under 15 amps.

Use whichever is the larger cable size.