Grounding an INVERTER - Don't undersize your grounding wire! | Ask The Expert with NIGEL CALDER

[@BoatHowTo]

We will probably make another video about this to explain the concept in more detail. For now, here is some explanation clarifying the comments: We ground AC systems primarily for shock prevention. We ground DC equipment for a variety of reasons. Regardless of the reason, for both AC and DC systems we must think about the worst-case fault current the grounding conductor may be called upon to carry before a circuit breaker trips or a fuse blows, shutting down the fault current. In the case of AC systems, the grounding conductor is the same size as the ‘live’ wires (hot and neutral), or one size under. It’s the same for DC systems - a grounding conductor is the same size as the DC positive and negative conductors or, in some circumstances, permitted by the ABYC to be one size under. Consider a short circuit from the DC positive side of an inverter to its case. The DC positive conductor will be protected by a high current fuse with a rating as high as several hundred amps. In the event of a short circuit to the case, the grounding conductor needs to be able to carry hundreds of amps, without melting down, long enough for the fuse to blow and shut things down. And yes, this is a large conductor! Which is why it is frequently omitted by inverter installers. The inverter will still function fine, but with reduced safety. The AC grounding side is also typically tied to the case. So now if we get a DC positive short circuit to the case, the AC grounding conductor, which is far smaller than the DC negative conductor, will be asked to carry the fault current. It will melt down along its entire length long before the DC positive fuse blows. Luckily, for all those installations without a properly sized DC grounding conductor, a DC positive short to the case is a very rare event. The case grounding conductor goes back to the DC main negative bus, or whatever serves this purpose on the boat. Note that the full fault current still has to get back to battery negative without melting anything down, so there has to be a conductor back to battery negative from the DC main negative bus that is at least as large as the inverter’s grounding conductor. - Nigel

[@joesmith1142]

perfect, thank you for taking the time to elaborate. that reply just cost me a couple hundred dollar in copper lol....

[@jimh.5286]

Thanks for the response!

[@joesmith1142]

Another question hopefully you can address in an updated video. My inverter will be about two feet from my lithium house bank that feeds it. The positive and negative busbars for this lithium bank are the same ones that will feed power to the inverter. You mention a DC positive/case fault only needs to get back to the battery negative. Is it then sufficient to just connect this case ground directly to the same negative bus bar feeding its DC negative input with the same gauge 4/0 wire, instead of running it all the way back to the engine/earth busbar thats near the engine block? My DC negatives all eventually connect to this engine block earth/ground but with smaller gauge wire.

[@kankama1]

Thanks for that, but where do you send the ground if you have a boat like mine? I have a composite catamaran with outboards, no metal through hulls but I do have an HF dynaplate. Do I just send the erath back to the negative on the battery switch?

[@josephcrawley3095]

I assume you go back to battery negative but I'm not sure if you guys addressed that in this video?

[@BoatHowTo]

see our pinned comment above

[@jimh.5286]

I see several aspects of boat inverters that make grounding issues rather complicated. First, we're dealing with potential faults of both a low-voltage DC circuit, as well as a high voltage AC circuit. Secondly, we're dealing with two hazards, fire hazard and potential electrocution hazard. And just to make things even more interesting, we're sometimes dealing with small wood or fiberglass boats that have no existing ground connections to the water.

[@jimh.5286]

My understanding of grounding the housing of AC devices, 120 v. for example, is to prevent an electrical fault to the housing from electrocuting someone who touches the live housing. So what is the purpose of grounding for a non-dangerous (from electrocution) 12 volt circuit?

[@BoatHowTo]

see our pinned comment above

[@jdvnautic]

Hello Nigel and Jan In the Victron 1.6KVA System schematic pdf download, the grounding conductor of the Multiplus case is connected to a main negative busbar and the main negative busbar is connected to the chassis with a grouding conductor. Can you connect it like this without any problems? Kind Regards.

[@BoatHowTo]

see our pinned comment above

[@joesmith1142]

like the other posters, i would really like to know as well where we have to terminate this ground. on my new to me, yet to be installed inverter, that means a 4/0 ground wire. im assuming on a FRP hull, a ground this big would have to go all the way back to the DC negative bus at the engine? that would be the largest earth ground on my boat by a fair margin.

[@BoatHowTo]

see our pinned comment above