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Negative return for motorhome - What's the reasons not to use chassis or body return?


MikeVan

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Hi all,

 

New member, first post - hope I don't balls it up! First off, I'm no expert here, just have an understanding of the basics. Hoping for help from someone that knows more than me.

 

So I've been looking into motorhome wiring (self-conversion, Renault Master van) and have been looking at ways to avoid voltage drops. Voltage drop calculations is of course done by taking into account both positive & negative wires.

 

I've checked, and most of the time it seems there'd be LESS of a voltage drop risk over longer distances (say, 8 meters or so, allowing for going up walls, along ceiling, down walls etc) if used the vehicle body as an negative return. This would be a return to a leisure battery, and the leisure battery would be grounded to the vehicle, too.

 

There's some info I found on the difference in voltage drops using a wire return, vs. using a vehicle chassis as a return. Not quite the same scenario as me, though - but some interesting calcs: https://mechanics.stackexchange.com/...attery-neutral

 

BUT - I'm imagining there's some issues with this (using the vehicle body as an earth return)? Things like alternator noise risk of course, but anything else? What about if the inverter was grounded (not as a neutral return, but application-grounded from the inverter body to the van body) - any risks here?

 

Hoping someone that knows more than me can let me know what's that

 

 

Thanks in advance!

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Hi Mike,

 

Welcome to the forum. I think that you are reading too much into the common practice of using a wired return in MH wiring. I do not know of any technical reason for this practice, and I suspect that it is a commercial convenience matter.

 

Many larger motorhomes do not have metal bodies, so any earth return would have to be taken to a chassis earth point. Probably more expensive to install than a parallel wire back to a common distribution point.

 

Panel van conversions, PVCs present there own and similar problems. Consider the lined out roof of a PVC. The lining will probably be screwed to the roof stiffener bars. If fitting ceiling mounted downlighters, you would obviously not put a self tapper through the roof for an earth point. It would have to be taken from a cstom made earth point the side of a roof stiffener. Likewise other units fridge, oven, water/space heating would require the own earth points. The plywood floor is not a good option, so you would be looking at body side stiffeners.

 

I am not sure that I understand your inverter query. May I add that while many people use them, I have avoided using one, other than in an experimental specialist application.

 

Alan

 

 

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Guest pelmetman
MikeVan - 2019-09-16 12:42 PM

 

Hi all,

 

New member, first post - hope I don't balls it up! First off, I'm no expert here, just have an understanding of the basics. Hoping for help from someone that knows more than me.

 

So I've been looking into motorhome wiring (self-conversion, Renault Master van) and have been looking at ways to avoid voltage drops. Voltage drop calculations is of course done by taking into account both positive & negative wires.

 

I've checked, and most of the time it seems there'd be LESS of a voltage drop risk over longer distances (say, 8 meters or so, allowing for going up walls, along ceiling, down walls etc) if used the vehicle body as an negative return. This would be a return to a leisure battery, and the leisure battery would be grounded to the vehicle, too.

 

There's some info I found on the difference in voltage drops using a wire return, vs. using a vehicle chassis as a return. Not quite the same scenario as me, though - but some interesting calcs: https://mechanics.stackexchange.com/...attery-neutral

 

BUT - I'm imagining there's some issues with this (using the vehicle body as an earth return)? Things like alternator noise risk of course, but anything else? What about if the inverter was grounded (not as a neutral return, but application-grounded from the inverter body to the van body) - any risks here?

 

Hoping someone that knows more than me can let me know what's that

 

 

Thanks in advance!

 

No offence ;-) .......

 

But I confess in the last 29 years as a motohome owner :-S .......

 

I have never had a particular issue with voltage drops :-| .........

 

I have always employed folk for that kinda thing :D ........

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