Volt Drop Calculations

[Themightyimps]

Evening all, just want to run a query past you if I could. A 3 phase supply has been installed on a site and I have been asked to 'just run a cable in'. This proposed supply would initially be to an existing 63A CU that needs replacing and is currently run from a domestic CU on site. It is approximated that the run from new 3 phase to said CU is 100m and multicore SWA would be buried. Consulting 4E4A table I have values of 75 and 96A for 16mm and 25mm respectively giving Mv/A/M of 2.5 for 16mm. Using the formula for VD 2.5x63x100÷1000 I get a VD of 15.75 suggesting 25mm+ will be required. Could anybody advise on permissable % drops for SWA. Thanks in advance

 

[Fleeting]

You need to use Table 4D4A.
For a submain with regard to volt drop you must also consider the volt drop of all final circuits from the consumer unit. What protective device do you have for the submain.

 

[Andy™]

same % regardless of cable type. dont forget final circuits too, max out submains & finals circuits and what you are supplying may have its voltage too low

 

[Themightyimps]

Submain currently has nothing and burnt out neutral bar so would be replacing.

 

[Themightyimps]

It's a bit of a nightmare, current submain @63A is supporting 4 x 16A to individual boards in vans on quite long runs (max approx 50m) plus fence lighting etc

 

[Fleeting]

Your volt drop is calculated from point of source to the furthest final circuit. What you need to consider is are the four 16A sockets being used at capacity for example is 10A adequate because if you allow for full load your cable just for the submain will be 70.0+.

 

[Themightyimps]

Your volt drop is calculated from point of source to the furthest final circuit. What you need to consider is are the four 16A sockets being used at capacity for example is 10A adequate because if you allow for full load your cable just for the submain will be 70.0+.

I've been on site before and frankly just wanted to walk away from it, I made safe what was there at the time (increased cable size etc as it had all been done in 2.5). Problem is there are these four hook up supplies basically for sockets and lights in each caravan so not drawing a massive amount unless they plug in bloody heaters and now he is wanting a new circuit putting from the submain for a 'shed' that will need sockets and lights meaning another (non-RCD) garage board. Using a radial I suppose I could get away with 20A and 6A for lighting. Its pushing the 63A close is my thought?

 

[Fleeting]

Four hook ups over a three phase supply so 32A on one phase isn't too much of an issue so 63A isn't a necessity. I rewired a caravan park a few years ago five 16A socket outlets on a single phase supply, the main switch has a BS1361 40A fuse and it has never blown. You have four sockets and a three phase supply.

 

[Themightyimps]

Four hook ups over a three phase supply so 32A on one phase isn't too much of an issue so 63A isn't a necessity. I rewired a caravan park a few years ago five 16A socket outlets on a single phase supply, the main switch has a BS1361 40A fuse and it has never blown. You have four sockets and a three phase supply.

You're probably right mate, I'm over -analysing, I'm going to have to get a calculator out and check the furthest supply volt drop and go from there Why can't I say no to these jobs lol

 

[Fleeting]

Don't need a calculator just use this.
https://www.clevelandcable.com/cable-calculator/

 

[Themightyimps]

Don't need a calculator just use this.
https://www.clevelandcable.com/cable-calculator/

Cheers mate, as I thought it's suggesting 25mm giving a 5% drop at 100m to the submain.

 

[Fleeting]

Cheers mate, as I thought it's suggesting 25mm giving a 5% drop at 100m to the submain.

You can't use 5% submains are often based at 2% to allow for the final circuits. Don't you have another 50m on top of that.

 

[Andy™]

Cheers mate, as I thought it's suggesting 25mm giving a 5% drop at 100m to the submain.

like i said earlier, dont forget your final circuits. lighting is 3%, if youve used 5% on the submain then youve got problems...

 

[Themightyimps]

You can't use 5% submains are often based at 2% to allow for the final circuits. Don't you have another 50m on top of that

Yes I do, 25mm won't cut it, I was just giving what the calculator said. The client is in for a shock