Confused by Chassis Grounding

[BrettB]

I have an inverter and two solar MPPT that I need to still hook up chassis grounds on. I keep reading and continue to confuse myself.

My Victron 3k inverter is hooked up with 4/0 wire. Does this mean I need to hook the chassis ground up with 3/0 wire(ie one size less)?

Then based on https://www.marineelectronicsjournal...show=VIEW&a=34 DC negative, engine negative and then grounding plate are ultimately hooked all together, so can I just hook the chassis ground to a negative battery terminal?

Lastly, does this mean the whole length back to the grounding plate needs to be the same size or perhaps larger(depending on length) than my largest grounding wire? So 3/0 or 4/0 to the grounding plate?

[smac999]

Yes 3/0 chassis ground. Back to the battery bank. Doesn't need to be bigger elsewhere. It's for a dc short internally. The amps are going back to the battery. Which will blow the inverter fuse.

[GordMay]

Yes, ABYC "A-25" requires chassis ground to be same size as DC Positive wire.

[CharlieJ]

Gord: A-25 morphed into A-31 about five years ago.
smac999: There are some OCPD concerns if an installer uses one size smaller than the B+.

Quoted from the current A-31:

GROUNDING CONNECTIONS
31.6.5.1The DC grounding conductor (see Figure 1, 2, 3and 4) shall,
31.6.5.1.1 be connected from the metallic case or chassis, to the engine negative terminal or its bus,and
31.6.5.1.2 shall be of an ampacity equal to that of the DC positive conductor.
EXCEPTION: The DC grounding conductor may be one size smaller than the minimum size conductor required for the DC current carrying conductors (see ABYC E-11, AC and DC Electrical Systems on Boats, “Allowable Amperage of Conductors For Systems Under 50 Volts”) providing the overcurrent protection device in the DC positive conductor is rated no greater than 135% of the ampacity of the DC grounding conductor and the conductor is no smaller than 16 AWG

[GordMay]

Charlie:
Thanks for the correction (update), and the applicable, and definative quotation.
I'm so out of date, I'm getting dangerous!

[Ahmet Erkan]

Originally Posted by GordMay

Charlie:
Thanks for the correction (update), and the applicable, and definative quotation.
I'm so out of date, I'm getting dangerous!

No Worries Gord, it is not dangerous if you are erring on the safe side. I am not a mechanical engineer but I have never had a problem with a bracket I designed which may have been a little stronger than what was needed. By the way, ABYC is a recommended standard, not the law. Law is the laws of physics and nobody can argue with that law and win (or live long).

Two principles that are relevant in this case :
1. "Ampacity" of a cable is determined by the resistance of the conductor and the temperature rating of the insulation.
2. Purpose of a fuse at the source is to protect the downstream power cables from overheating or blowing open during a short circuit. (ie: the fuse blows open instead of the conductor)
In order to minimize a voltage sag at the load, you may use a cable with an ampacity higher than what is needed and protect that wire at a current less than its nominal rating. (with proper design margins)
If you have installed a fuse or a CB at the current you need which is less than the ampacity of the power conductors, then it is perfectly OK to use a ground wire at the ampacity your fuse (or CB) is capable of protecting.


Why write all this instead of just say follow the ABYC recommendation?
I like to "blindly comply" only with the rules that I do not thoroughly understand. Actually ABYC chose the prudent path by recommending a bigger cable at first, and then revising their recommendation to a smaller (cheaper) cable only after analysis, recommendations and concurrence by many experts in the industry. ABYC is open to comments and recommendations from the industry to improve their standards, I am sure of it.
Cheers and all the best.