Determining cable specification

[LynneC]

Hi there,

I am planning a complete 12V system re-wire of a Fisher 25. I've done extensive research, planning & documentation, & the maths is no problem, but one piece of the puzzle is missing from all the books & magazines I've read - it's this:

Suppose you have the following circuit (simplified as an example):

House battery with 2m cable to +ve bus bar
+ve bus bar feeds two 10W lamps (both 5m away from the bus bar)
Negative return from both lamps to -ve bus bar
-ve bus bar return to battery

All the volt drop examples I've come across are simplified - i.e. "measure the distance from the power source to the appliance, and do the maths" - they don't clearly state what to do if your circuit includes bus bars. When spec'ing the positive cable from bus bar to lamp, is the outward cable run 5m or 7m? If you use 5m & end up spec'ing quite thin cable, might you drop your full 3%/10% voltage allowance through that bit, then drop a further % through the run from the battery to the bus bar, and end up dropping too much across the whole circuit? And when spec'ing the positive cable from the battery to the +ve bus bar, is the outward cable run 2m & the ampage 20W (the sum of the appliance wattages), or what?

I fully intend to get my plans, once complete, reviewed by a proper sparky, but would like to get as far as I can before handing over to him, so appreciate any/all advice.

Cheers

Lynne

 

[john_morris_uk]

In theory you should do two separate calculations - one for the voltage drop from the bus bar and one for the battery to bus bar. In practice, provided that you have adequate size cables to the bus bars from the batteries and the cable run is very short, you can assume that your calculations start from the bus bars. Just make sure that you have oversize cables and short runs to the bus bars though!

There might be a very slight voltage drop to the bus bars from the battery bank, but it will be very small and I suggest that form all practical purposes it can be ignored.

My advice is to not go for the smallest cable you can get away with. Use good quality cable and connections, and you will have a trouble free future on the boat. Small cables and poor quality connectors are storing up trouble.

 

[tobble]

I agree with John - also, I may have misunderstood your post, but if you're saying you'd put a 20A rated cable between the batt and the busbar, don't! That's roughly AWG 14 or 2 square mm.

firstly in the unlikely worst case scenario of turing everything on at once, or even a couple of larger loads, you might be pushing your 20A. Secondly, in the same scenario, as you say, you'll start dropping a lot of V's on that cable.

Given it's only a 2 m run, nice chunky cable won't cost a lot, will work better electrically and will simplify your calcs, because as john says you can basically ignor it.

 

[1955patric]

Hi there,

I am planning a complete 12V system re-wire of a Fisher 25. I've done extensive research, planning & documentation, & the maths is no problem, but one piece of the puzzle is missing from all the books & magazines I've read - it's this:

Suppose you have the following circuit (simplified as an example):

House battery with 2m cable to +ve bus bar
+ve bus bar feeds two 10W lamps (both 5m away from the bus bar)
Negative return from both lamps to -ve bus bar
-ve bus bar return to battery

All the volt drop examples I've come across are simplified - i.e. "measure the distance from the power source to the appliance, and do the maths" - they don't clearly state what to do if your circuit includes bus bars. When spec'ing the positive cable from bus bar to lamp, is the outward cable run 5m or 7m? If you use 5m & end up spec'ing quite thin cable, might you drop your full 3%/10% voltage allowance through that bit, then drop a further % through the run from the battery to the bus bar, and end up dropping too much across the whole circuit? And when spec'ing the positive cable from the battery to the +ve bus bar, is the outward cable run 2m & the ampage 20W (the sum of the appliance wattages), or what?

I fully intend to get my plans, once complete, reviewed by a proper sparky, but would like to get as far as I can before handing over to him, so appreciate any/all advice.

Cheers

Lynne

I would suggest that you calculate the size of the cable from the battery to the bus bar based on the maximum current your switch panel breakers can cope with if all on and taking maximum rated load and then overspec by around 20%. This will minimise any voltage drop between batteries and switch panel and allow for future kit to be added. Don't forget when calculating wire size the total length (positive and negative cable runs) should be used.

 

[LynneC]

Gentlemen, thank you all very much - all received & understood! Will post some pictures of things working when they're all wired in ;-)

Kind regards

Lynne