# A few electrical questions



## mrblitz (May 20, 2010)

My long overdue conversion is nearing completion.

I am currently wiring 'er up.

I worked as an electricians mate for a while and will eventually have him come and cast his eye (and his testing equipment) over my work. I worked 100% in domestic and commercial 230v installations and 12v, although similar in many ways, has got me questioning myself. With regards safety and quality of my install so far, i have been very careful to ensure i have installed drop-outs and vents in all the right places, and have made sure that all the right bits are sealed from the living space and combustibles are not where they don't want to be!!

Some background on my system thus far. I have a sterling Alt to Batt. charger to charge my engine battery and leisure bank when on the move and a charger built in to my distribution board to charge my leisure battery when on hook-up. The sterling works as a split charge diode.

Most of the 12v appliances are wired through the three fused switches on my distribution board. All lights through one, all fans and pumps through another, 12v sockets on a third. I may or may not get a seperate 3 gang 15a switch board to split the 12v appliances further. I have a 300w inverter. My fridge is also wired through this board for mains only.

I have thought about it and have decided not to have a fridge relay. I understand all the benefits, but cannot see the point if i have to switch the fridge manually between 230v, gas and 12v why a relay would help in any way. Instead i am going to fit a 15a (or 25a?) illuminated switch through a 15a in-line fuse next to the input selector on the fridge control panel. The fridge is an old Electrolux RM212. I believe it draws 7a. I have just replaced the elements and the gas operation is good.

With regards to the cable used for the fridge...i have calculated that for a 3 metre run from the 12v power source to the 7amp (90w) element i will need to use a minimum of 2.5mm diam. (5mm sq) solid copper 2 core.

This affords me a 3% approx. voltage drop. The max load for this cable in this application is around 37amps (so an online wire gauge calculator tells me). With this in mind, a 15a fuse will operate above the 7a drawn by the fridge and under the max rating of the cable.

If anyone could double check my calculations and give me a thumbs up/down i would be very grateful.

Secondly...i have a question about cable size for the run from the engine battery and leisure bank to the sterling and then on to the distribution board.

I have calculated that with every 12v appliance on at the same time...and my 300w inverter fully loaded i will draw 66amps. An unlikely situation but better to be safe and all that.

The sterling handbook suggests that i use 140 amp rated cable for my 70amp alternator to run from the batteries to the sterling. This is a hefty 6mm diam. if i want to keep the voltage drop to a minimum.

With regards to the cable run from the leisure battery to the distribution board, if my max draw is, say, 70 amps rounded up. Should i double this cable rating too, and use 6mm cable all the way? Or will 4mm cable suffice?

I want to install a fuse along this cable from the leisure battery to the distribution board. On the assumption that i use the 4mm cable which has a max load of 70 amps what fuse should i look to fit? 65 amp??!!!??!?!? potentially larger if i use 6mm cable!!???!?!??

Crumbs.

On a seperate note...has anyone fitted a sterling unit in their motorhome? Where did you install it? I am thinking of fitting mine behind the drivers seat, on the bulkhead wall between the cab and living space. Under the bonnet seems a little out in the open for a £300 unit with no waterproofing! How hot do they get in use? Will i regret fitting it so close to the driving position?

Many many thanks to those of you who have persevered with this post to the end.

MB


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## shortcircuit (May 21, 2010)

Not clear if you intend using "solid drawn" copper cable but you should use flexible.  

Interested to no what type of 4mm cable will take 70amp.  

Whatever cable used the fuse protecting  the cable should be less than the current rating of the cable.


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## Tigatigatiger (May 21, 2010)

shortcircuit said:


> Not clear if you intend using "solid drawn" copper cable but you should use flexible.
> 
> Interested to no what type of 4mm cable will take 70amp.
> 
> Whatever cable used the fuse protecting  the cable should be less than the current rating of the cable.



Agree with you, flexible cable should be used as "solid drawn" will work harden very quickly and break.


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## mrblitz (May 21, 2010)

4mm diam. solid copper core cable has a cross sectional area of 12.5mm sq and will take 70 amps at 12v


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## Tigatigatiger (May 21, 2010)

mrblitz said:


> 4mm diam. solid copper core cable has a cross sectional area of 12.5mm sq and will take 70 amps at 12v



However if you run "fully loaded" the cable will get hot.


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## shortcircuit (May 21, 2010)

Had a quick look at my tables in  the regs and cannot find any 4mm that is rated to take 70amp.  Where have you obtained your info?


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## AndyC (May 21, 2010)

The OP has confused things slightly by quoting cable diameter rather than the more standard CSA (cross sectional area).

Regarding the Sterling B2B charger, I have one, it does get quite warm in use but not hot. In fact it's not in the best location, being inside the underfloor domestic battery box, because it's a bit short of ventilation. However I'll need to find somewhere else for it as I'm about to replace my 3 old batteries with 4 new ones.

Cable sizing for the Battery to Sterling cable will depend on how long a run you need. Mine is about 2m and I've used 10sq.mm. cable which will take 70A. However it's a 24V system, if it had been 12V I would have used 16sq.mm. cable. For fuses I've use the strip link type: VWP - fuse holders 2

AndyC


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## mrblitz (May 26, 2010)

AndyC said:


> Cable sizing for the Battery to Sterling cable will depend on how long a run you need. Mine is about 2m and I've used 10sq.mm. cable which will take 70A. However it's a 24V system, if it had been 12V I would have used 16sq.mm. cable. For fuses I've use the strip link type: VWP - fuse holders 2
> 
> AndyC



Sterling suggest i use 35mm sq cable (6.4mm diam.) from the sterling to the battery banks, as i have a 2m run and a 70 amp alternator.

Should i also use the fuse holders you suggest?  There is nothing in my sterling manual about having to install fuses on the sterling cable runs.
Or are you suggesting i use the fuses on either end of the run from the battery to my distribution board?

Thanks


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## AndyC (May 26, 2010)

mrblitz said:


> Sterling suggest i use 35mm sq cable (6.4mm diam.) from the sterling to the battery banks, as i have a 2m run and a 70 amp alternator.
> 
> Should i also use the fuse holders you suggest?  There is nothing in my sterling manual about having to install fuses on the sterling cable runs.
> Or are you suggesting i use the fuses on either end of the run from the battery to my distribution board?
> ...


I think Sterling are being a bit OT with their cable size recommendations. I suspect they are suggesting that size so that there is the absolute minimum voltage drop. The alternator won't be producing 70A, at least not for very long, if it did it would get seriously hot!

For safety I have a fuse fitted in the supply cable to the Sterling, close to the starter battery, and a fuse in the supply cable from the leisure batteries to the main distribution board, again close to the battery. In my case the Sterling is right alongside the leisure batteries so I haven't fused the supply from the Sterling to them.

I've fitted the fuses for peace of mind, if there is ever damage to one of the cables causing a short, the fuse will blow, isolating the battery and preventing a fire.

AndyC


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## mrblitz (Jun 6, 2010)

One more question...

I have used 30mmsq cable for all of my runs from the sterling to the batteries, and 10mmsq cable for the neg.  I am wiring my battery bank together currently.  I am using 25mmsq cable and fusing with 60a inline fuses.

Is it acceptable to use 25mmsq cable for the positive and 12mmsq for the neg?

On the basis that 10mmsq is rated at over 60a i should be ok i think.

Only i have a bunch of 10mmsq left over and have run out of 25mmsq.

Thanks


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## AndyC (Jun 7, 2010)

mrblitz said:


> One more question...
> 
> I have used 30mmsq cable for all of my runs from the sterling to the batteries, and 10mmsq cable for the neg.  I am wiring my battery bank together currently.  I am using 25mmsq cable and fusing with 60a inline fuses.
> 
> ...


No, the main +ve and -ve cables should be the same size. They have the same current flowing in them.

AndyC


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