wiring questions

[Acadia]

Hello,

I am doing some re-wiring in my boat and got a couple questions for the electrical experts...
The alternator is grounded to the engine. The positive lead has a short hop to the starter, and then on to the battery switch. I thought I read somewhere that even though you have an alternator that uses the mounting as it’s connection to ground, you should still run an individual wire from the negative terminal of the alternator to the system ground. Any truth to that and why? (alternator is a Balmar 100amp)
Question #2; When setting up batteries in parallel, is there a minimum wire size that you should use other than what you can find on the wire size charts. The batteries are 4 group 27, the alternator is 100 amps and the shore charger is 40. The 2 highest current draws would be the starter and an electric windlass (the windlass has an additional battery at the bow)

Thanks

[David M]

For the first question...your ground must ultimately have a clean run all the way back to your negative terminal on the battery(s). There is nothing wrong with running the alternators ground to the ground on the starter motor...so long as all the connections are clean, tight and of adequate gauge. Gauge your starter motor ground for the starter motors maximum output, so you are still good if your alternator mount starts to oxidize or shake loose...(at least electrically you are still good).

Starter motor grounds sometimes shake loose, so that may be Balmars concern. Balmars other concern is that some engines may not have a ground at the starter motor and may have the engine ground elsewhere or a poor engine ground. I think Ballmar just wants to make sure their alternator is going to work for you by giving you default directions of running the ground all the way back to the start battery or negative bus terminal. It would not hurt to run your alternator ground all the way back to the battery or the negative bus terminal for your batteries if you are concerned.

For your second question, gauge your wire for the greatest load or combinations of loads. For most boats, it will be your starter motor that has the greatest draw. For other loads guage them for their maximum draw. For devices that charge the battery, gauge those for their maximum charge.

Be sure to put heatshrink on all your crimp connectors to keep the oxidation from occurring inside the crimp-wire interface.

Does that answer it?

[Pblais]

The 2 highest current draws would be the starter and an electric windlass (the windlass has an additional battery at the bow)

That still leaves the sources for charging as well discharge. Electricity is not unidirectional. At high charging loads the wires can easily go red hot just as quickly in either direction. It's still amps related to distance and the distance includes positive lead distance plus the ground distance to compute wire size. I doubt the 100 amp alternator will ever deliver 100 amps but at peak output it's probably more than the windlass. Some room for error is not a bad thing. The issue with DC current and wire size is a basic burn the boat to the water line issue. 80% of all boat fires are electrical ignition.

[David M]

Did you have a question about the wire run? There are tables for the run as well as the gauge for a given current.

ABYC Wiring Standards

A nice calculator: Voltage Drop Calculator, Amperage Tables, Wire and Cable Technical information

[hellosailor]

I suspect the reason most folks don't run "extra" ground leads from the engine/starter/alternator is first, they are a waste of money in most cases, and second, "more stuff" just means more stuff that can find odd ways to break and create trouble.

The starter and alternator should both be making very good grounds simply by being bolted to the engine block, and in theory the engine block is firmly and closely grounded right to the batteries--making any extra ground wires an invitation for gremlins. (Just one man's opinion.)

[Whimsical]

Don't like the idea of a seperate cable from the alternator earth directly to the batts. If the main cable, or it's connections, should fail for any reason the the starter would draw it's power throught the alternator negative, toastie time.

Mike

[Acadia]

Thanks for all your imput..

I can't remember where I saw that thing about runing another wire from the alt negative terminal, but I think the argument had to do with corosion? So I guess I will just replace the wire that originates from the block to the batt.

[Rick]

There are two reasons for adding a discrete negative cable from the alternator case to the starter negative attachment on the engine block:

1. Copper is 50 times more conductive than steel and will provide a "preferential" current path from the alternator back to the start battery with a lower attendant loss.

2. Large charging currents provided from the alternator that ultimately charge a house battery will make small voltage differences within the engine over a sufficiently long time to cause ionic transfer within the internals. You will not be able to inspect any internal degradation like where the metals occupy both sides of a gasket between the front case and the engine block, for example.

Mike (Whimsical) makes the point that this discussion necessarily is predicated upon the idea that the alternator negative (in the case of a non-isolated alternator negative) current pass first to the engine block negative distribution point and THEN to the external battery(s) thereby eliminating the possiblilty of a bad connection (at that point) causing starter current to pass through a wire placed directly between the alternator negative and an external battery. Use a terminal stacking heirarchy wereas the starter battery cable terminal is the FIRST one in contact with the engine block with the others stacked on top of that one.

Engine manufacturers design their electrics to provide engine operation currents alone and not to power external devices that we might add. This is why stock wiring is not necessarily sufficient for external house battery charging currents when it comes to alternator positive and negative wires.

[Acadia]

Interesting, the reasons you gave above is very close to the arguments that I remember reading regarding a negative wire from the alternator to the grounding point. I also downloaded a Balmar user manual and they also recommend having a wire from the alternator to the grounding point on the engine block. Well I ended up running this wire. I did the best I could to get a clean contact to the engine block but I felt that by running that extra wire (alt/block) that it would provide a better connection for the circuit.

Thanks for all the input