question

wingingitfulltime avatar image

Multiplus 3K Chassis Ground

Installing MP 3000 in a 5th wheel. Confused over gauge of wire to use on chassis ground lug to RV frame? Have read where it should be same awg of "conductor" wires? Think abyc standard specifies so.


I'm using 4/0 battery/inverter DC cables and have 400ah battery bank (lithium).


What chassis ground wire size is correct?


installation
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5 Answers
Paul B avatar image
Paul B answered ·

The same size as what you used for the AC in or AC out Cables.

Note A Bigger wire size will not harm anything

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Mark avatar image
Mark answered ·

The DC ground cabling should also have sufficient cross section to be able to carry a fault current at least equal to the DC fuse rating (this is commonly overlooked)

Link to Wiring Unlimited book (section 6 is related to grounding);

https://www.dropbox.com/s/pdkngditufe0zfb/Wiring%20Unlimited.pdf?dl=1

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jfro avatar image
jfro answered ·

I to have same question. Use same 4/0 cable to frame? Use same 4/0 cable back to negative buss bar? or use 6awg cable back to frame which is the size of my ground cable on the AC breaker box? Have read and been told all the above answers on different user groups. So confused......


1 comment
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To protect for an AC short to ground you only need to use a ground cable that can handle the max AC current/AC circuit breaker rating.

BUT an inverter (or a charger) has BOTH AC and DC in it and the DC being much lower voltage will operate at much higher currents and be protected by fuses/circuit breakers that are rated accordingly - a 250A or higher DC fuse/circuit breaker is not uncommon.

So if there was a short to ground fault (to the inverter case) on the DC side, then there could be a current flowing through the earth cable (back to the battery negative) as high as the DC fuse/circuit breaker rating.

400A doesn't go well flowing through a 2.5mm AC earth cable...

For normal operation almost anything would do - the issue is if there is ever a fault/short circuit to ground on the DC side.

Pat Davitt avatar image
Pat Davitt answered ·

Actually, the ABYC standard states that the inverter/charger chassis ground wire shall be the same size as the DC Positive cable from the battery, connected to the inverter/charger. And that this ground will run directly to the main ground point of the vessel. The reason is that in some cases, a fault in the inverter/charger has caused the AC ground wire to overheat and cause fires. The larger chassis ground gives the current a safer place to go. And will allow enough current to flow to trip the DC circuit protection device.

Pat

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robertantognoni avatar image
robertantognoni answered ·

Why can't victron just add a strap to the negative DC cable from the chassis? The negative cable should definitely be capable of carrying grounded current in the case of a short. It's totally unreasonable to require an extra 4/0 cable for something that would be extremely rare and likely a manufacturer defect.

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There are very real world reasons it is setup like it is and if you don't understand them you should not be installing such equipment.