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Old 30-01-2014, 18:42   #1
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float switch far away from pump... should I use the neg side for the float switch?

Hello all,

I am replacing the wiring of the aft shower float switch. The pump is more than 20 feet from the shower and also about 20 feet from the batteries.
If I use the positive side (+), the power from the batteries travel to the panel, then around the salon and back to the master cabin and up to a locker where a fuse and a switch is and then down and aft to the shower sump where the float switch is, and then back to where the pump is. The total length is closer to 60 feet. If I use the negative side, I could pick up the ground from about 3 feet from the float, then go... directly to the pump..? or should I fuse this wire..?
The pump positive wire comes from the panel so it has a breaker and I can turn it off if need to...
Using the negative side avoids any stray current in case the float switch corrodes or breaks ... but the sump is not connected to anything else... the water there goes out the boat via a hose connected to the pump...
I guess my question is, should I avoid the long positive wire and use the shorter negative to the float?
Any comments are welcome and appreciated.
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Old 30-01-2014, 20:24   #2
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Re: float switch far away from pump... should I use the neg side for the float switch

If you switch the negative side then everything up to the switch is at a +12 volt potential. So any stray paths will have current flow and can cause electrolytic corrosion.

In a perfect world it wouldn't matter where the switch was.
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Old 30-01-2014, 20:28   #3
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Re: float switch far away from pump... should I use the neg side for the float switch

I agree with John. Float switch the hot side, always.
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Old 30-01-2014, 21:20   #4
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Re: float switch far away from pump... should I use the neg side for the float switch

Gee there will always be +12V potential at one side of the switch. With the other leg tied to ground and switch on the switch goes minus. Probably just as likely getting stray current on the minus side, depending on source polarity.

Really don't see where it makes that much difference. Most stray current is going in the prop shaft or ground plate anyway. Depends on what's in the bilge really.

Besides current flows minus to positive, least wise that's how I heard it explained by the EE types.
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Old 30-01-2014, 21:31   #5
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Re: float switch far away from pump... should I use the neg side for the float switch

Don't ever switch the ground side. It's dangerous, confusing to whoever works on the boat in the future, and violates all conventions and standards.

In addition, your pump won't work if you wire the float switch into the ground. To operate the pump, you'll have to turn on the manual switch and have the water turn on the float switch. The float switch and the manual switch are wired in parallel, and therefore have to switch the same side of the circuit.
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Old 30-01-2014, 21:44   #6
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Re: float switch far away from pump... should I use the neg side for the float switch

BAD IDEA ! as previously described.
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Old 30-01-2014, 21:48   #7
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Re: float switch far away from pump... should I use the neg side for the float switch

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Originally Posted by Sun and Moon View Post
In addition, your pump won't work if you wire the float switch into the ground.
Actually it would work just fine. Well The DC - side. We're not talking AC ground.

Though spot on about a manual switch and the float needing to be on the same side of the pump. But I think he's talking about a shower sump not a bilge switch

To the OP as long as one side is fused your good, btw.
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Old 30-01-2014, 22:48   #8
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Re: float switch far away from pump... should I use the neg side for the float switch

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Originally Posted by sailorchic34 View Post
Gee there will always be +12V potential at one side of the switch. With the other leg tied to ground and switch on the switch goes minus. Probably just as likely getting stray current on the minus side, depending on source polarity.

Really don't see where it makes that much difference. Most stray current is going in the prop shaft or ground plate anyway. Depends on what's in the bilge really.

Besides current flows minus to positive, least wise that's how I heard it explained by the EE types.
Through hulls, bonded parts, most everything is going to be at 0 volts.

If +12 stops at the float switch that is one place for failure and causing corrosion. One potential point for failure. Everything else on the pump is connected to ground and so at the same potential as anything else in the bottom of the boat.

If the switch is on the ground wire, the second connection on the float switch is at 12 volts, both connections at the pump are at 12 volts, anything in the pump is 12 volts, the negative wire to the switch is at 12 volts, the connection to the switch is at 12 volts. Depending on how you want to count it about 6 points for failure to cause electrolytic corrosion.

Don't care which way you like to talk about current flow in this case. It's where the potential difference is removed from wet salty deep bilge places.
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Old 30-01-2014, 23:17   #9
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Re: float switch far away from pump... should I use the neg side for the float switch

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Through hulls, bonded parts, most everything is going to be at 0 volts.

If +12 stops at the float switch that is one place for failure and causing corrosion. One potential point for failure. Everything else on the pump is connected to ground and so at the same potential as anything else in the bottom of the boat.

If the switch is on the ground wire, the second connection on the float switch is at 12 volts, both connections at the pump are at 12 volts, anything in the pump is 12 volts, the negative wire to the switch is at 12 volts, the connection to the switch is at 12 volts. Depending on how you want to count it about 6 points for failure to cause electrolytic corrosion.

Don't care which way you like to talk about current flow in this case. It's where the potential difference is removed from wet salty deep bilge places.
Actually, I agree with you completely. But the OP is talking about a float switch in a shower sump, not in a wet salty deep bilge, and I assuming a diagram pump out of the shower sump, because it would be a non issue with the pump in the shower sump. So you have an isolated sump, not connected to the bilge with I'm guessing mostly fresh grey water.

As I see it, as the float has one side hot and the other normally not, but in this case tied to DC ground, with the switch isolated from the bilge in a current isolated enclosure (shower sump), I just don't see a major problem. You have + on one side of the switch anyway, in either case. With the pump not in the bilge its not really a problem. If the pump was a typical submersible pump then yes there's a problem. But the OP says the pump is elsewhere.

Maybe I'm extra blonde tonight. But with a shower sump switch, which is what I think the OP is talking about, electrolysis is probably not a problem. A deep bilge is another story.
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Old 31-01-2014, 07:07   #10
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Re: float switch far away from pump... should I use the neg side for the float switch

IMHOP... Largest concern is voltage drop rather than possible stray currents. Voltage drop is a function of wire size times total length, so if switching the low side of the sump pump results in a shorter total wire run, better solution.

Layout A:
Battery Voltage 13.85
Pump draw 2A
High side switch
Total wire length 60'
Required wire size 16 gauge (voltage drop 0.49012)

Layout B:
Battery Voltage 13.85
Pump draw 2A
Low side switch
Total wire length 40'
Required wire size 18 gauge (voltage drop 0.53182)

Above calculations assumes "Layout B" low side connection is direct to batteries or low side bus with adequate wire size for total circuit loads connected.
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Old 02-02-2014, 07:03   #11
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Re: float switch far away from pump... should I use the neg side for the float switch

Perhaps you guys haven't noticed but the automotive world is changing this very thing. Many newer car and trucks ( like Ford for a few years now) have switched minus...switched minus turn signals and lights... Why because they can use smaller and lower current rated switches. So the big "normal" thing is changing. This is requiring special devices for trailer lights even. think about it double grounds and no " hot " wont work.
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Old 02-02-2014, 08:01   #12
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Re: float switch far away from pump... should I use the neg side for the float switch

Quote:
Originally Posted by sailorchic34 View Post
Actually, I agree with you completely. But the OP is talking about a float switch in a shower sump, not in a wet salty deep bilge, and I assuming a diagram pump out of the shower sump, because it would be a non issue with the pump in the shower sump. So you have an isolated sump, not connected to the bilge with I'm guessing mostly fresh grey water.

As I see it, as the float has one side hot and the other normally not, but in this case tied to DC ground, with the switch isolated from the bilge in a current isolated enclosure (shower sump), I just don't see a major problem. You have + on one side of the switch anyway, in either case. With the pump not in the bilge its not really a problem. If the pump was a typical submersible pump then yes there's a problem. But the OP says the pump is elsewhere.

Maybe I'm extra blonde tonight. But with a shower sump switch, which is what I think the OP is talking about, electrolysis is probably not a problem. A deep bilge is another story.
I guess it would be a rare failure mode to fill the sump with salt water, leave it there for a long time, and have a leak forming a stray current path.

Quote:
Originally Posted by justlearnin View Post
Perhaps you guys haven't noticed but the automotive world is changing this very thing. Many newer car and trucks ( like Ford for a few years now) have switched minus...switched minus turn signals and lights... Why because they can use smaller and lower current rated switches. So the big "normal" thing is changing. This is requiring special devices for trailer lights even. think about it double grounds and no " hot " wont work.
First, I have a turn signal light, it draws X amps of current. That means all the way from the positive battery post through the light and all the way to the negative battery post the exact same amount of X amps of current is flowing. I now insert a switch in a variety of different places in this circuit. Why does the switch see lower current in some places in this circuit?

I have a few problems with your relating cars to boats. Most boats do not have a metal chassis that is everywhere that can double as ground. Most cars do not have a watertight shell under them collecting conducting salt water and are sitting in a pool of salt water. Most cars if you rot out something in the electrical system it comes to a stop on the road. On your boat you can destroy a through hull starting to sink your boat, or prop. AAA is much farther away, if available at all, and a lot more expensive on the water.
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Old 02-02-2014, 10:49   #13
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Re: float switch far away from pump... should I use the neg side for the float switch

Every car to ever have a dome light is switched negative at the door jam switch. Not that it matters to a boat.

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Old 02-02-2014, 14:20   #14
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Re: float switch far away from pump... should I use the neg side for the float switch

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Don't ever switch the ground side. It's dangerous,
AC possibly if not properly marked..... 12-24 DC non issue. As stated previously... advantage of "low-side" switching is total wire length required.
Hence automotive changes... elimination of "common chassis ground on automotive systems due to implementation of IC integrated systems requires excessive wire runs with high-side switching. Not relevant to OP.

Relative to boat circuits, one should remember "ground" does not equal low-side return (or should not, if properly designed) . "Ground" is usually bonded to battery low side (bat -) along with lifelines, through hulls, AC ground (not neutral) etc. Battery low-side is a separate circuit from boat "ground". One serves to to prevent galvanic corrosion/stray currents, the other provides the low-side return to DC supply.

Equating automotive systems to marine is apples and oranges.
Irrelevant
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