Voltage drop for PV inverter

[ragg987]

I want to fit some solar panels on my garage roof - approx 2.5kWp to supplement the 4kWp I already have on my main house roof. This is facing due south on a 40deg pitch roof. I would also fit a myEnergi Libbi battery and inverter in this space, plus a myEnergi Zappi car-charger. The garage has a small DB with a 32A (6mm) armoured cable in a duct to the CU in the main house, approx 25-30m long. I was thinking of a 5kW inverter to allow more headroom in the peak power input or output to the battery.

 

One of the companies quoting tell me the 32A cable is not suitable as the power drop at 5kW would exceed 1%. I cannot understand why a cable rated for 32A would become a problem - can anyone explain this or am I being given some B*s*?

[SteamyTea]

  On 18/05/2023 at 11:32, ragg987 said:

cannot understand why a cable rated for 32A would become a problem

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Think you answered your own question.

  On 18/05/2023 at 11:32, ragg987 said:

approx 25-30m long

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Run it though a voltage drop calculator and see what comes out.

[ragg987]

A 6mm cable shows a 1.5% drop at 5kW.

 

I guess my question is what mandates the 1% voltage drop limit? Is there a standard someplace for PV inverters, or is this a overheating risk?

[joth]

  On 18/05/2023 at 12:27, ragg987 said:

A 6mm cable shows a 1.5% drop at 5kW.

 

I guess my question is what mandates the 1% voltage drop limit? Is there a standard someplace for PV inverters, or is this a overheating risk?

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The issue is when generating the drop is in the other direction (as that's the direction of power flow) so the garage voltage will rise 1.5% above the grid voltage. This is an issue if you're somewhere with already high grid voltage as it can cause the inverter to cut out if it sees to high a voltage at the output. Which means you loose the benefit of having PV especially on days when lots of people are generating 

This is a bigger problem in UK than in EU as we tend to use the same kit but grid voltage tends to be higher.

 

[ragg987]

Aha - that is really useful thanks - I had not considered the over-voltage in the garage. Grid voltage in our area tends to be steady at about 242v, so well within the 230 -6 +10. But of course no guarantee it will remain there.