AWG for AC earth ground?

[BAD ORCA]

Not 100% sure correct method for determining the AWG needed for the AC earth ground to the DC negative bus.

My guess would be the wire size needs to accommodate 30 Amps since thats what shore power receptacle trips at, maybe a bit more?

Not sure if round trip length matters for the AC side as much as the DC side, but its about 30' round trip from my panel to the neg. bus.

The current wire which is probably 40 years old looks to be 14 awg, maybe 12 single wire. I'm going from memory. Its currently conected to the back of the engine. I'm trying to consolidate all my grounds so using this opportunity to replace the old wire.

Online calculator using 30amps / 120 volts @ 30ft is suggesting 6awg.

Currently looking for info on Blue Sea's website, but any education/ recommendations would be appreciated.

[Hartleyg]

I would strongly urge you to reconsider attaching your vessels bonding system (which should be connected to the engine and all through-hull metals) to the AC shorepower ground. You DON'T want your vessel to become a part of the of grounding connection for your marina!
Connect your AC ground to the ground system of your AC circuits (through a galvanic isolator, of course) using the same size wire as the AC hot & neutral conductors.

Hartley
S/V Atsa

[HeywoodJ]

While I generally agree with Hartley, if you are wiring to ABYC standards then the AC ground has to be connected the engine (if included in the DC negative system) or to the DC negative bus (if not a fully bonded installation).

Voltage drop in the PE wire is not a concern, 10AWG will suffice. As long as it can handle the 30A and has a low resistance (so that flow is not impeded) voltage drop in handling a fault is actually a good thing.

[BAD ORCA]

Thanks for replies.

I eventually will have an isolation transformer aboard but im not ready to cross that bridge yet. I have way too much on my plate right now and its an area i need a lot more understanding in.

For now it all works perfectly so i want to keep everything the way it is until im ready to do a major wiring upgrade maybe early next year. I really am concerned with the condition and deterioration of the AC wiring so for now just want to clean it up and replace whats there that bothers me. That AC ground is in very bad condition, and its been suggested to get all of my multiple engine grounds consolidated on a single bus.

I do have a good galvanic isolator aboard.

[hjohnson]

Originally Posted by Hartleyg

I would strongly urge you to reconsider attaching your vessels bonding system (which should be connected to the engine and all through-hull metals) to the AC shorepower ground. You DON'T want your vessel to become a part of the of grounding connection for your marina!
Connect your AC ground to the ground system of your AC circuits (through a galvanic isolator, of course) using the same size wire as the AC hot & neutral conductors.

Hartley
S/V Atsa

The main issue is DC voltages on the PE and galvanic corrosion. These can be resolved using a Galvanic Isolator. We have one that will be installed on our boat. Relatively cheap insurance, and lets us wire to modern electrical standards (which do require bonding the PE to your negative bus and/or engine.

[Frankly]

If an inverter charger is in your future then the DC common to AC common connection should be sized no smaller than one size below the wire size of the feeding DC supply. Typically this is a lot larger than #10 found with a 30A shore power connection.


Frankly

[smac999]

Originally Posted by Frankly

If an inverter charger is in your future then the DC common to AC common connection should be sized no smaller than one size below the wire size of the feeding DC supply. Typically this is a lot larger than #10 found with a 30A shore power connection.


Frankly

That is inverter case to dc ground . Different wire.

[hjohnson]

Originally Posted by Frankly

If an inverter charger is in your future then the DC common to AC common connection should be sized no smaller than one size below the wire size of the feeding DC supply. Typically this is a lot larger than #10 found with a 30A shore power connection.


Frankly

The AC PE Bonding wire does not need to be any larger than your AC conductors.

[Frankly]

When Installing Charger, Inverter, or Inverter/Charger Follow A-20 and A-25

A-20 and A-25 contain specific installation directions that are not described in E-11.

2. Install the AC wiring to the charger or inverter including an AC grounding conductor of a size equal to the current carrying conductors unless the circuit exceeds 30A, in which case the grounding conductor may be one size smaller (E11.16.1.3.8.2). This is the typical grounding conductor that you would see with any AC appliance and returns with the other AC conductors to the power distribution panel.

3. Install a DC grounding conductor sized not less than one size smaller than the DC positive conductor and have a capacity such that the DC positive fuse has an amperage rating not greater than 135% of the current rating of this grounding wire. As a practical matter, this wire will be much larger than the AC grounding conductor. This requirement is the latest addition to the standards when it was discovered that faults in the DC side of an inverter or charger could provide sustained high currents that could start a fire from overheating the AC grounding conductor.



Discussion

The rules for grounding electrical systems have evolved over time. Boat builders, installers, and electricians continue to recognize hazards and increase safety measures. Battery chargers were originally treated like any other small appliance, first without having any safety ground as was common through the 1950's, and then by adding a safety ground to reduce shock hazards during faults.

It was found that faults in the DC wiring or the DC side of chargers could generate fires because high current could flow back from the batteries, so a fuse was added between inverters or chargers and the battery system. As the capacity of chargers increased, and with the introduction of inverters, these DC fuses became quite large. It was then determined that a fire hazard exists when a DC fault in a charger or inverter can pass DC current into the AC safety ground wire. The AC safety ground was not sized for the high DC currents, so a high capacity DC grounding wire is now required by standards A-20 and A-25.

Now three critical grounding wires for these systems have been identified. This may seem excessive, but this combination of grounding conductors has been shown to give protection against a wide variety of faults.


I copied this from the Blue Sea WEB site. There is a picture to help with identifying the conductors but it did not copy.


Frankly

[team karst]

Makes sense. If the B+ conductor shorts to the invertor case, for instance, then the chassis grounding conductor needs to clear the fault by blowing the B+ fuse, manytimes well over 100A rating. A AWG10 will surely overheat before the fuse blows, maybe the fuse won't open and the grounding conductor burns down instead.

[boatpoker]

PM sent

[boatpoker]

Originally Posted by team karst

Makes sense. If the B+ conductor shorts to the invertor case, for instance, then the chassis grounding conductor needs to clear the fault by blowing the B+ fuse, manytimes well over 100A rating. A AWG10 will surely overheat before the fuse blows, maybe the fuse won't open and the grounding conductor burns down instead.

I have retired but was ABYC Certified in Electrical and Marine Corrosion.
I have a basic rule on my own boat ... use the biggest conductors that
fit.
One must remember that "standards" are minimums.

No one has ever lost by going above "standards".

[smac999]

Originally Posted by team karst

Makes sense. If the B+ conductor shorts to the invertor case, for instance, then the chassis grounding conductor needs to clear the fault by blowing the B+ fuse, manytimes well over 100A rating. A AWG10 will surely overheat before the fuse blows, maybe the fuse won't open and the grounding conductor burns down instead.

The inverter chassis ground goes from inverter chassis to dc ground. And needs to blow the inverter fuse yes. Battery Fault Current flows From battery pos to case to battery neg. Often 300a fuses for a 2000 wat inverter. No current flows thriugh the ac to dc ground bond.

The post is about the wire between ac ground and dc ground. It’s a different wire. With an inverter you would need both wires. The ac to dc bond wire will never exceed the highest ac breaker. (Shore or gen). And 10awg is correct on most boats. (30a).

If you short ac hot wire to inverter chassis fault current through the ac to Dc ground bond would be 30a

If 300a dc was Able to get into the ac side your Shore cord would also have to be 2/0…

[goboatingnow]

Originally Posted by hjohnson

The main issue is DC voltages on the PE and galvanic corrosion. These can be resolved using a Galvanic Isolator. We have one that will be installed on our boat. Relatively cheap insurance, and lets us wire to modern electrical standards (which do require bonding the PE to your negative bus and/or engine.

Only outdated ABYC voluntary compliance “ guidelines “

[Frankly]

I have a policy of not arguing on IF but because life safety is involved I will make an exception.

We have a shore power ground (green #10 for typical 30A SP, may or may not have a galvanic isolator in series with this conductor). This wire is connected to the metallic case of the I/C as well as other AC receptacle/ appliances throughout the yacht.

We have a 12V DC negative (say 2/0 for discussion purposes, black or yellow insulation) connecting the battery(s) negative and typically connected over to the engine block (make the starter go round and round).

ABYC also requires that we make an electrical connection between these two systems (can cause corrosion issues but I don't get to make the rules).

If I use a #10 wire to make this connection and I have an I/C with a 200 amp fuse in the positive feed and a short occurs between the inverter 12 volts positive and the case (say a FET shorts out to heat sink), then the current path for that fault current is from the positive battery connection through the 200 amp fused feed, through the I/C short to the case, through this #10 wire, and back to the battery negative.

We now have hundreds of amps flowing through a 200 amp fused connection in series with a #10 wire. Want to guess which one now becomes the fuse.


Frankly