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

MikeVan · Sep 15, 2019

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...nge.com/questions/57206/when-to-use-chassis-and-when-to-wire-to-battery-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!

raynipper · Sep 16, 2019

I'm no expert here but the vehicle often used to be the return path as all the motoring queries online about bad earth show. Corrosion will always be a worry. Heavy loads like an inverter will be prone to welded joints failing where a solid dedicated heavy wire would be less susceptible.

Ray.

MikeVan · Sep 16, 2019

Thanks Ray, good thoughts.

I think I'm going to test voltage drop across a long wire (something like 4.5mm2) and from some bare metal on the body, see the difference in drop. Wonder how to test things like noise - Perhaps wire a stereo using van body as neg return and see if it picks up any electronic clicks or alternator buzz etc? Hmmm

Mrplodd · Sep 17, 2019

Road Vehicles have, for as long as I can remember always used the "earth return" system. I.e. the vehicles metal body served as the "return" side of every circuit. Common sense really, saves a lot on wiring!

The issue you may have with a self build is that a lot of your electrical equipment will STILL need to be connected to vehicles metal to complete the circuit.

On a different note (and sincere apologies if you are already aware) but ALL vehicles have an absolute maximum weight (shown in its VIN plate) If you are doing a home conversion DONT use domestic type cupboards etc, they are made of chipboard or MDF and are FAR too heavy. Have a look at what motorhomes and caravans have their internal furniture made out of, its VERY thin and lightweight to try and keep the overall weight down p, you will need to do the same.

If you are aware then please ignore the above lecture!

Oh, and welcome to MHF!

Andy

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

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