Electrical Grounding for the house system

A forum for discussing boat or trailer repairs or modifications that you have made or are considering.

Moderators: Catigale, Paul S, Heath_Mod, beene, Hamin' X, kmclemore, tangentair

Post Reply
Interim
Engineer
Posts: 197
Joined: Thu May 08, 2014 9:31 am
Sailboat: MacGregor 26S
Location: Great Plains

Electrical Grounding for the house system

Post by Interim » Fri May 20, 2016 8:13 am

I'm rewiring my house system. When I look at wiring diagrams in the archive, it looks like I should run a negative wire to the item (like running lights) from the switch panel, but also run a negative line directly back to the battery.

Is this right? 3 wires? I thought that pos and neg off the panel would work.

Thanks for any guidance.

--john

User avatar
Tomfoolery
Admiral
Posts: 5208
Joined: Tue Jul 05, 2011 9:42 am
Sailboat: MacGregor 26X
Location: Rochester, NY '99X BF50 'Tomfoolery'

Re: Electrical Grounding for the house system

Post by Tomfoolery » Fri May 20, 2016 8:33 am

I don't know where that came from, but that's not required. It only takes two conductors to make a circuit, and you can, and normally will, bring the negative conductors back to the switch panel. Land them on a bus bar for neatness (or bolt them all together like MacGregor did :P ), and run a single line back to the battery bank. Size that conductor for the total current of all devices connected to that bus/bolt. Same with the positive conductor - size it for all the loads it carries, combined. Voltage drop may govern, and remember that voltage drop is calculated over the total out-and-back run. But if it's only lights, voltage drop isn't all that important, unless it dims your nav lights, of course. :wink:

That's all assuming you're talking about the 12V system, not 120V shore power.

Interim
Engineer
Posts: 197
Joined: Thu May 08, 2014 9:31 am
Sailboat: MacGregor 26S
Location: Great Plains

Re: Electrical Grounding for the house system

Post by Interim » Fri May 20, 2016 9:13 am

Yes, 12v system. Sorry.

And thanks. Simpler is better for me. Gosh, I might even get to do some night sailing this year!

--john

User avatar
Wind Chime
Captain
Posts: 868
Joined: Sun Jan 21, 2007 7:30 pm
Sailboat: MacGregor 26X
Location: Vancouver, B.C. Canada. 2000-26X, Suzuki-50hp, 8' Walker-Bay tender (with sailkit)
Contact:

Re: Electrical Grounding for the house system

Post by Wind Chime » Fri May 20, 2016 1:05 pm

We have the factory wire (+/-) running from battery(s) to our panel(s) and the negative is still factory single ground pole behind panel.

Thinking ahead; if we are increasing the load to the panels and require additional voltage we may need to max out the voltage to the panels by increasing the AWG size from battery to panel.

Stupid question: we could just run a larger AWG (+/-) wire, or could we also leave the existing wire and run an additional wire with the same AWG size as existing one? Does 2 smaller AWG wires = one larger AWG wire??

User avatar
seahouse
Admiral
Posts: 2182
Joined: Mon Dec 08, 2008 12:17 am
Sailboat: MacGregor 26M
Location: Niagara at Lake Erie, Ontario. 2011 MacM, 60 hp E-Tec
Contact:

Re: Electrical Grounding for the house system

Post by seahouse » Fri May 20, 2016 2:21 pm

Wind Chime wrote:We have the factory wire (+/-) running from battery(s) to our panel(s) and the negative is still factory single ground pole behind panel.

Thinking ahead; if we are increasing the load to the panels and require additional voltage we may need to max out the voltage to the panels by increasing the AWG size from battery to panel.

Stupid question: we could just run a larger AWG (+/-) wire, or could we also leave the existing wire and run an additional wire with the same AWG size as existing one? Does 2 smaller AWG wires = one larger AWG wire??
I have wondered the same thing Wind Chime, because it would seem that two wires (assuming equal conductor cross-sectional area), better able to dissipate heat, would actually have an increased carrying capacity over one single large one. (Extra terminal connections not considered).

User avatar
Tomfoolery
Admiral
Posts: 5208
Joined: Tue Jul 05, 2011 9:42 am
Sailboat: MacGregor 26X
Location: Rochester, NY '99X BF50 'Tomfoolery'

Re: Electrical Grounding for the house system

Post by Tomfoolery » Fri May 20, 2016 3:21 pm

While you could parallel conductors, which will halve the current and voltage drop for that run (assuming same material and gauge for each), there are potential hazards. Like losing continuity in one conductor, which results in doubling (assuming two conductors) the current and therefore the voltage drop while increasing the heating in the remaining conductor. The National Electric Code prohibits doing that for conductors smaller then 1/0, except in certain circumstances, but those circumstances require each conductor to be able to carry the entire load (Art 310). Granted it's 60Hz power and control wiring, and we're not talking about wiring a building, but there are valid reasons not to do it that still apply to DC wiring.

With such a low voltage as 12V, voltage drop is likely a bigger concern than heating in the wiring, which the insulation is designed for anyway. A 1V drop (out and back) is 8.33%, which is a lot. 1V on 120V is only .83%, which is a little. The current is the same for the same drop. So 12V systems are very sensitive to voltage drop, making the wiring relatively big, which renders the heating relatively small, at least compared to what the same conductor can carry with higher voltages, at least for runs longer than a pigtail.

User avatar
frede
Engineer
Posts: 114
Joined: Thu Mar 24, 2011 8:16 am
Sailboat: MacGregor 26M
Location: Washington Sailing Marina, Alexandria VA - 2005 26M 50hp Honda - Kool Breeze
Contact:

Re: Electrical Grounding for the house system

Post by frede » Fri May 20, 2016 3:22 pm

Wind Chime wrote:We have the factory wire (+/-) running from battery(s) to our panel(s) and the negative is still factory single ground pole behind panel.

Thinking ahead; if we are increasing the load to the panels and require additional voltage we may need to max out the voltage to the panels by increasing the AWG size from battery to panel.

Stupid question: we could just run a larger AWG (+/-) wire, or could we also leave the existing wire and run an additional wire with the same AWG size as existing one? Does 2 smaller AWG wires = one larger AWG wire??
If it is the factory wire (lamp cord) replace it!

Post Reply