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Greg46

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Hello All,
I’ve been asked to install a cable 170m in SWA, this is to be pulled through ducting. The cable is straight from the meter to an isolator in a beach hut. As the property is so small with little electrical appliances, I have accounted for 45A max current and deduced a 35mm2 SWA. Is this correct, I’ll just fit a 40A RCD on the CCU. I only ask because if I apply 3% volt drop I end up with a cable between 70 and 90mm2. This seems massively ott. Any comments would be great as although I’ve been an electrician for 10yrs+ it’s mostly been on the commercial side and I’ve not really had much to do with domestic supply cables. Thanks very much in advance.
 
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Volt drop it taken from the source of supply to the furthest point of your final circuits so for lighting this is 3%. On submains a rule of thumb figure is often quoted at 1% to give you a 2% drop on the final lighting circuits but being a beach hut it is probably negligible. 35mm cable is going to exceed the volt drop limit.
 
Change the protective device to 32A if the final loading is only small.
 
If you only have fixed lighting that is designed for a voltage range that allows lower than 230V then you could stay with the 5% volt drop and also take account of the lower operating temperature and extended length to give more leeway on the volt drop.
I hope if you have 45A demand that your RCD would be capable of carrying more than 40A. I would expect far more problems with volt drop and cable length in a commercial setting than in a domestic.
 
Hi - not sure I've understood correctly, but perhaps consider fitting an RCD to match the upstream fuse rating (or more), rather than picking one with 40A rating. One less thing :) .
 
First thing to clear up, as others have pointed out.

Do you mean 3% volt drop or diversity, as the latter doesn't make much sense.
 
Sorry my terminology was bad, your correct. I went for 40A to oversize it really. The measured load will not be above 30A for sure. So my question is really.. what size cable would you use, an I correct in thinking 35mm2 is adequate? Thanks again
 
I have amended it so it says volt drop. I would have thought dropping to 30/32A is the best option.
 
Using 35mm² SWA underground and with an operating current of 40A you would be just outside the Vd for lighting after correction for operating temperature, but assuming unrealistically 30°C for the ambient temperature. If your design current is less, say 38A, then you would just be within at 6.78V dropped.
However it is up to you to ensure your calculations do ensure compliance for this installation.
 
How big is this beach hut and what are they growing? I would expect it to draw more than a few amps unless they have as shower installed?
 

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