Size of DC ground wire question

[rbk]

Originally Posted by gscriba

I can relate. Most of us have an area of expertise and when we have to ask what may be "simple" questions (for some) it is truly humbling.
I appreciate the help on this thread.

Just out of curiosity, which was it? Were you thinking a bonding system or an Rf counterpoise (comment about electrical noise) Or something totally different? You seem to have the DC side down but what are your plans...?

[gscriba]

Originally Posted by Frankly

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.


I lifted this from Blue Sea (they probably lifted it from ABYC). Kind of complicated to follow but suggests/ requires the conductor connecting the DC negative (which is almost always connected to the engine block) to the AC ground on inverter equipped yachts be sized same or slightly smaller as the wire size feeding the DC to the inverter (which is typically quite large).

Frankly

Not looking for bonding nor counterpoise, though I do have an Icom IC-710 that came with the boat (so much to learn there too, OMG).
In addition to what others have states, what Frankly stated above has helped a lot.

So the plan is to essentially create a "trunk" with 4/0 from stern to just aft of mid ship with 600 amp rated bus bars at both ends. All negs will lead to the just aft of mid ship bus bar, and from there through the shunt to the house Neg. All connections will be top notch clean and secure. Start AGMs will be separate. HVCO and LVCOs incorporated with REC-ABMS controlling. So many more details, but that's the "short" of it. Ha! I can hear the groans loud and clear. Mai Tai anyone?
Enjoyed this discussion. Busy with wife, kids, work, fun and boat. Implementing plan next week!

[GordMay]

ABYC E-11, Figure 18

[David Mathis]

To simplify wiring, avoid grounding issues often experienced, and avoid voltage drop from small ground wires, I installed a separate grounding block behind the instrument panel for electronics added. I run a #6 ground wire from the battery to the grounding block. Terminals are crimped then soldered. The terminals are well greased before making the connection. I've done this on boats I've owned for the past 40 years, presently a 43' Wellcraft, and have had no grounding problems with any electronic installations. Tracing instrumentation problems can often be frustrating and time consuming. The separate ground wire installation for gps, depth, vhf, etc. has resulted in an absence of problems due to faulty grounding.

[Dsanduril]

I like the trunk layout and it's how I generally wire the boat. One thing to consider, if the positive and negative trunks do not parallel each other very closely you will get pretty strong magnetic fields wherever they are separated. The higher the current (and the trunks carry all of the current on the boat) the stronger the fields. This can affect your compasses - autopilot fluxgates installed belowdecks can be particularly susceptible. And since electrical loads can vary a lot your compass can be fine one minute and way out the next.

If +/- are run parallel (preferably even twisted) they will largely cancel each other out although since they are not coax there will always be some field disturbances close to the wire.

[deeper]

DC "ground", to where? If its part of some circuit for some purpose, then size that circuit correctly, but grounds shouldn't be arbitrarily "grounded" to all and sundry - that's asking for a current loop or an inadvertent dissimilar metal cell, with serious consequences.

People talk of "grounding" on a boat, but often you are better off to focus first on quality isolation, and then bond things only when you are forced to.

[Frankly]

One of the issues with electrical terminology is the sloppy use of the term "ground". If I understand the OP, he was referring to the 12V return conductor from the engine to the negative terminal of the battery. His original question concerned the size of this conductor relative to other conductors in the 12V system. Lot of possibilities for that selection, don't think any have been overlooked other than a superconducting set up.


In my particular set up, I used a single 4/0 from the engine block to the battery shunt. It is the only 4/0 on my yacht. It also serves as the return from the 100 Amp alternator. My thinking was what a waste of expensive copper wire to just start the engine once a day when it could just as well do double duty. Nothing magic just my way of addressing the problem. Worked out fine and there is also a short jumper from the alternator case to the same engine location but it is a lot smaller. Working with 4/0 cable on a typical yacht situation is a real PITA.

Because when we start talking about "real grounds" the engine block is almost always involved you need to take a look at the bigger picture.


Frankly

Frankly