# A few electrical questions!!



## mrblitz (Nov 21, 2009)

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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## clive1821 (Mar 27, 2010)

the cables I would use 10mm from the leasure battery and try not to use twin and earth for the mains just use 1.5 or 2.5mm 3 core flex.... dont forget the building is not static so the cables needs to be flex.... dont forget to fuse both ends of the +ve cable from the leasure battery, as both ends supply current, so 50amp fuses... not sure what a sterling unit is... I'm assuming its a mains charger? if so they are usually limited to 20amp... or could be a diode splitter to charge the main battery and leasure batteries.... in either case as long as its ventilated it will be fine.... and locate it in a good posistion where you can get at it.. If i recall the fridge 12v side has the low current side conected to the leasure battery and the hi current side routed to the main battery, and via a relay, the relay is made by the output of the alternator the same supply that goes to the warning light... so the fridge is only supplied with 12v when the engine is running...


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## mrblitz (Nov 21, 2009)

10mm sq?

or 10mm diam.?

from the leisure battery to the distribution board?

Is that what you are suggesting?

The Sterling is a 90amp alternator amplifier that maintains the high output charge from the alternator even after the engine battery is full of charge so that the leisure battery can benefit from this great source of power. This high output is why Sterling suggest 35mm sq (6.5mm diam) cable from the Sterling unit to the batteries.

Does anyone have any reasons why i couldn't install a switch rather than a relay for the 12v power to the fridge?


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## inkey-2008 (May 24, 2008)

A relay does not forget to swich. It may fail at some time so can a switch.

Andy


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## rayc (Jun 3, 2008)

mrblitz said:


> 10mm sq?
> 
> or 10mm diam.?
> 
> Does anyone have any reasons why i couldn't install a switch rather than a relay for the 12v power to the fridge?


The size stated for cables is given in mm2 and this measurement is actually the cross sectional area of the wire inside.

There is no electrical reason that you cannot use a switch rather than a relay. You may though leave it on and flatten the battery. Hampshireman did just that on a day visit to Hamble last year.


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## Fatalhud (Mar 3, 2006)

Hi MB
Forget the online wire gauge calculator
If you go to a shop or wholesalers you will be buying cable in these sizes 
Cross Sectional
Area (mm²)
0.50mm²
0.75mm²
1.00mm²
1.50mm²
2.50mm²
4.00mm²
6.00mm²
10.00mm²
16.00mm²
25.00mm²
35.00mm²
50.00mm²
70.00mm²

The cable for the fridge should be OK at 1.5mm²
Do not use solid cable, use flexible 2 core
If your Sterling unit quotes 140amp then you will need at least 35mm² I would recommend Tri-Rated singles cable as it is very flexible

For the small amount of extra work involved go for the relay, you will forget to switch of the switch one day

Why don,t you wire the inverter direct from the battery with its own fuse.
It will draw around 25amp max, again I would use Tri-rated cable 4mm² should be big enough as its rated at around 40amp or play safe and get 6mm²

http://www.masoncables.co.uk/pdf/Tri-Rated.pdf

Alan H


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