Switching ground instead of positive lead

jspriddy · Sep 8, 2011

I'm in the process of rewiring a small bass boat. Is there any reason NOT to break the ground wire with the switch, as opposed to breaking the positive wire? In my case, it would make the hot wire much shorter, and therefore reduce the amount of hot wire subject to wear, shorting out, etc. Some of mine would not even pass through any hull panels. I realize that I'd still have to fuse the hot side.

John

NetDoc · Sep 8, 2011

Re: Switching ground instead of positive lead

The only reason is consistency. I wrangled with the same question and finally just decided to run #8AWG red to a fuse panel in the front and be done with it. I also ran a black for a ground as well. You're going to have to run one wire or the other, so staying consistent will reduce hair pulling time when stuff acts up. Murphy was an optimist.

seabob4 · Sep 8, 2011

Re: Switching ground instead of positive lead

Per ABYC Section E-11, AC and DC systems...

11.7.1.2.1. A battery switch shall be installed
in the positive conductor(s) from each battery or
battery bank with a CCA rating greater than 800
amperes.

Not to say you HAVE to follow ABYC standards, but they are the standards the majority of builders follow when building boats...

jhebert · Sep 8, 2011

Re: Switching ground instead of positive lead

NetDoc said:
The only reason is consistency.

I completely disagree.

Running unswitched positive circuits around the boat to various loads and then controlling them with a switch in their negative return would be a disaster. You will promote galvanic corrosion in all the devices that are left floating to battery positive.

Silvertip · Sep 8, 2011

Re: Switching ground instead of positive lead

Switching ground is a rather common technique in certain applications, such as the temp sensorr on an outboard or the various senders on EFI/DFI engines (although those may be 5 volt circuits). The difference in what you are proposing is that in control systems, the hot side of the system is turned off when the key is off. In your application the hot side of negative switched circuits may NOT be turned off and that would present the corrosion issue. You would still need to run ground wires which when chaffed (just like hot wires) can do so against hot or signal wires also creating problems. So why not follow accepted practice, protect wiring accordingly and save yourself and the next owner some grief should troubleshooting become necessary.

jspriddy · Sep 8, 2011

Re: Switching ground instead of positive lead

Not to say you HAVE to follow ABYC standards, but they are the standards the majority of builders follow when building boats...

(concerning the battery switch)

I was going to run the hot leads from a bus bar with an integral battery switch or a separate switch and bus bar, in that order. I didn't give enough detail.

Thanks, John

jspriddy · Sep 8, 2011

Re: Switching ground instead of positive lead

jhebert said:
I completely disagree.

Running unswitched positive circuits around the boat to various loads and then controlling them with a switch in their negative return would be a disaster. You will promote galvanic corrosion in all the devices that are left floating to battery positive.

I did not know about the corrosion issue. Thanks to you and Silvertip, however. My thoughts were that the less hot wire you had running around, the less chance of shorts. I did intend to disconnect from the battery when not in use.

My bilge pump and aerator are less than 3 feet from the battery, and to run them both to the control panel switches and back will be 30-40 feet, and through several hull panels, so you can see my line of thinking.

Also, thanks to Netdoc. It would create unnecessary confusion, especially to a subsequent owner.

On the subject of grounds, an aluminum boat should be grounded, just like an automobile frame, right?

Thanks again,

John

Silvertip · Sep 9, 2011

Re: Switching ground instead of positive lead

jspriddy said:
I did not know about the corrosion issue. Thanks to you and Silvertip, however. My thoughts were that the less hot wire you had running around, the less chance of shorts. I did intend to disconnect from the battery when not in use.

My bilge pump and aerator are less than 3 feet from the battery, and to run them both to the control panel switches and back will be 30-40 feet, and through several hull panels, so you can see my line of thinking.

Also, thanks to Netdoc. It would create unnecessary confusion, especially to a subsequent owner.

On the subject of grounds, an aluminum boat should be grounded, just like an automobile frame, right?

Thanks again,

John

On the subject of your bilge pump. You hesitate to run a hot wire to the switch on the helm and then to the pump but you would still need to run a ground wire from the pump to the helm switch and back again unless you want to put the switch next to the pump. So where is the savings? None - nada!!!

NO! The hull of an aluminum boat is not the same as a car. Again -- corrosion. The only ground on a boat is the negative terminal on the battery. While there may be multiple ground busses located throughout the boat, the fewer the better.

jspriddy · Sep 9, 2011

Re: Switching ground instead of positive lead

Silvertip:

My concern was not the wire or the cost of it, but to reduce the amount, or run, of hot wire. For the bilge pump alone, hot wire would be reduced by at least 75%. To my way of thinking, if you reduce the run of hot wire by 75%, then you reduce the chance of shorts (read this potential fires) by 75%, all other things being equal.

I was not intending to use the boat as a ground for accessories or whatever, only asking if it should be grounded for any reason. I'm not an electrical engineer or I probably wouldn't be here asking questions. There's no reason for a bunch of exclamation marks.

John

j_martin · Sep 9, 2011

Re: Switching ground instead of positive lead

jspriddy said:
Silvertip:

My concern was not the wire or the cost of it, but to reduce the amount, or run, of hot wire. For the bilge pump alone, hot wire would be reduced by at least 75%. To my way of thinking, if you reduce the run of hot wire by 75%, then you reduce the chance of shorts (read this potential fires) by 75%, all other things being equal.

I was not intending to use the boat as a ground for accessories or whatever, only asking if it should be grounded for any reason. I'm not an electrical engineer or I probably wouldn't be here asking questions. There's no reason for a bunch of exclamation marks.

John

The exclamation point was probably generated by a sinking feeling. (experience)

The fuse or breaker is before the protected wire. Makes no difference how long the wire is, if it's properly fused there is no fire hazard.

Normally you'd have a heavy (8 - 10G) wire fused by 30 amp breaker to a distribution panel. That panel will have fuses for each individual circuit. The one for a bilge pump would typically be 3 - 5 amps. 14g or 16g wire from there to switch and to pump would be well protected.

The rule is the fuse protects the wire. Fuse has to be small enough to blow before the wire heats up. There's tables to look it up, and it even depends on if the wire is in the open, or in a bundle.

hope it helps.
John

jspriddy · Sep 10, 2011

Re: Switching ground instead of positive lead

Thank you, John. That does help. This is a simple boat but the difference between boat and automobile wiring makes it whole 'nother ballgame.

...heavy (8 - 10G) wire fused by 30 amp breaker to a distribution panel. That panel will have fuses for each individual circuit. The one for a bilge pump would typically be 3 - 5 amps...

If I'd used my idea of switching the ground, I'd wind up with a bunch of inline fuses, and I hadn't considered that. I already have distribution panel (if I can find someone small enough to get under the helm and take it out) so I kinda know where I'm going now. Thanks.

I definitely don't want to get that sinking feeling. Sorta takes on a double meaning when you talk about boats, doesn't it?

Have a great weekend,

another John

raymondpickens · Sep 10, 2011

Re: Switching ground instead of positive lead

I had a toyota truck and I like to tinker with things, in thus finding out that toyota uses switched grounds on some of there system, mainly lights. Total headache when wiring extra relays. I would stick with the K.I.S.S. Philosophy. Oh and dont ask how long I sat there with a test light scratching my head trying to figure out WTH was going on there hahahha. Spare the next owner this headache.

ssobol · Sep 10, 2011

Re: Switching ground instead of positive lead

Switching the ground will only make a difference when the load is off. You could have a longer run of energized wire that could wear and short to ground.

A typical circuit is something like this:

batt(+) --> switch --> fuse/cb --> load --> batt(-)

Switching the ground would look something like this:

batt(+) --> load --> switch --> fuse/cb --> batt(-)

In the first instance when the load is off, the circuit will only have power as far as the switch (first thing in line). In the second there will be power all the way from the batt through the load and back to the switch. The longer the run the more opportunity for the wire to be chafed and short to ground. In the first case you have a better chance of isolating a short by turning the switch off or having the CB trip. It all kinda depends on the layout of the boat wiring whether this makes a practical difference or not.

For circuits that are on, there is no difference.

jspriddy · Sep 10, 2011

Re: Switching ground instead of positive lead

raymondpickens said:
I had a toyota truck and I like to tinker with things, in thus finding out that toyota uses switched grounds on some of there system, mainly lights. Total headache when wiring extra relays. I would stick with the K.I.S.S. Philosophy. Oh and dont ask how long I sat there with a test light scratching my head trying to figure out WTH was going on there hahahha. Spare the next owner this headache.

Yeah, even some older American cars used controls on the ground, especially on A/C compressors. We knew that from experience and didn't think anything about it. No electronics then, just switches and sending units and circuits to trace and check.

All these things, the corrosion, the fact that a properly fused circuit poses no danger, and the simplicity have convinced me. It was just a thought that struck me and I threw it up there. Looks like I hung one over the plate, and you guys hit it out of the park.

I've a new question to pose but I'm going to do a little more detective work on my boat and start a new thread if I can't find it in previous threads.

Thanks to everyone who's responded,

John

jspriddy · Sep 10, 2011

Re: Switching ground instead of positive lead

Thanks, SS. I've pretty much canned this idea. I appreciate your thoughts.

John

no704 · Sep 15, 2011

Re: Switching ground instead of positive lead

Re: Switching ground instead of positive lead
Switching the ground will only make a difference when the load is off. You could have a longer run of energized wire that could wear and short to ground.

A typical circuit is something like this:

batt(+) --> switch --> fuse/cb --> load --> batt(-)

Switching the ground would look something like this:

batt(+) --> load --> switch --> fuse/cb --> batt(-)

Nice explanation, but the electrons (the bits that actually flow) start at the - and go to the +. So your actually better protected with the fuse near the - terminal so that it blows before your load reaches overcurrent. Just actual theory, never could figure out why everyone alwas fuses the +?

Fed · Sep 15, 2011

Re: Switching ground instead of positive lead

Just actual theory, never could figure out why everyone alwas fuses the +?

Fuse the positive because the negative is grounded.

jspriddy · Sep 15, 2011

Re: Switching ground instead of positive lead

Nice explanation, but the electrons (the bits that actually flow) start at the - and go to the +. So your actually better protected with the fuse near the - terminal so that it blows before your load reaches overcurrent. Just actual theory, never could figure out why everyone alwas fuses the +?

Fed said:
Fuse the positive because the negative is grounded.

I guess it's late and this will probably hit me in the morning, but would a fuse blow (or circuit breaker trip) in an overloaded circuit if it was on the negative side of the load, or short, as the case may be?

John

NetDoc · Sep 15, 2011

Re: Switching ground instead of positive lead

jspriddy said:
but would a fuse blow (or circuit breaker trip) in an overloaded circuit if it was on the negative side of the load, or short, as the case may be?

The "short" answer is yes. When a fuse exceeds its rated load it will pop no matter what side the device might be on.

dewey0726 · Sep 15, 2011

Re: Switching ground instead of positive lead

If you notice almost everthing you buy has the hot switched or fused. It is for a reason. If you have a short in the circuit you want to kill the hot wire. Ground won't hurt anything. Also if someone else is working on something, they will probably assume the hot is switched or fused and work on it with the switch off. If you have your ground switched or fused they will "get the juice". Being a 12 volt system, it probably won't kill anybody, but may really wake someone up. But you could do some sparking and burn something up. Anyway, It's my opinion you need to switch the hot whenever possible. And about the fuse. It will blow no matter where you put it in the circuit and no matter which way current flows through it *** long as it has more current going through it than it is rated for.

Switching ground instead of positive lead

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