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Old 16-10-2016, 13:08   #1
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Latching relay

Hi, I want to install a switch on the panel at my boat's helm to control a relay to switch on/off the 12v fresh water pump. To cut down on current draw I want to use a latching relay. I understand I will need to use a momentary switch. The question is what amp size relay to use. The pump is rated for a 10 amp fuse. Also any suggestions for a reliable relay brand?
Thanks, Rains808
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Old 16-10-2016, 14:10   #2
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Re: Latching relay

Wouldn't it be easier to just switch the power to the pump directly? A simple toggle or rocker switch is all you would need.
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Old 16-10-2016, 14:29   #3
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Re: Latching relay

Thanks rwidman, wouldn't I have to run a heavy gauge wire back to the pump rather than a light one to handle the power requirements of the pump? If so what gauge should I use for the pump?
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Old 16-10-2016, 15:17   #4
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Re: Latching relay

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Thanks rwidman, wouldn't I have to run a heavy gauge wire back to the pump rather than a light one to handle the power requirements of the pump? If so what gauge should I use for the pump?
#12 unless it's a really long run. There are tables in the West Marine catalog to calculate wire size vs. voltage drop.

Without seeing your actual boat, I can't say for sure but you probably don't have to run a pair of wires from the vicinity of the pump to the helm. You can interrupt the circuit anywhere it's convenient.
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Old 16-10-2016, 19:41   #5
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Re: Latching relay

Thanks for the ideas. She's winterized now, so time to considered all this in the next months.
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Old 16-10-2016, 21:01   #6
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Re: Latching relay

Adding a relay is adding another point of failure. Not to mention a handful of extra connectors. For the cost savings involved(after paying for the relay and the connectors, and a spare relay to be safe) it hardly seems worth the added risk of failure of such a critical piece of equipment.
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Old 16-10-2016, 21:46   #7
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Re: Latching relay

Not to mention, that the current to latch the relay will be a constant drain on the batteries. It may be small but it must be taken into account.
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Old 16-10-2016, 22:37   #8
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Re: Latching relay

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Originally Posted by pdbms View Post
Not to mention, that the current to latch the relay will be a constant drain on the batteries. It may be small but it must be taken into account.

I thought the whole point of a special latching relay was they took a momentary current to open or close the relay but otherwise weren't drawing anything.
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Old 17-10-2016, 05:00   #9
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Re: Latching relay

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I thought the whole point of a special latching relay was they took a momentary current to open or close the relay but otherwise weren't drawing anything.
You thought correctly.
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Old 17-10-2016, 05:14   #10
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Re: Latching relay

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You thought correctly.
^^^ THIS ^^^
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Old 17-10-2016, 05:16   #11
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Re: Latching relay

Curiosity kills me as to why you want a FW pump switch on the "helm" ???

EDIT - ALSO ... with a latching/momentary, you'll never be sure whether the pump is on or off at the helm... Unless of course you install an indicator... Which is really starting to complicate a simple system...
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Old 17-10-2016, 14:56   #12
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Re: Latching relay

Thanks Adelie for clearing up the latching relay issue.
Here's the whole story: when we bought the boat 4 years ago it did not have a fresh water pump on/off switch, but the wiring was there from the past. The switch had obviously been bypassed. A mechanic installed a relay controlled by an existing control panel switch at the helm (nav lights, wiper, etc. located there also). All worked well so we could switch off the pump when we were away from the boat until the coil on the simple relay (automobile type) burnt out this year. So, not having a spare I bypassed the relay. My idea now is that at the start of next cruising season to install a latching relay so the relay would not need to be energized constantly. In terms of the wiring the batteries are located in a hatch in the stern. The pump is in a hatch just forward of that. So the wiring would have to run from the stern to the helm (about midship) and back to the pump. I want the pump on its own wiring circuit to hopefully avoid the lights flickering as the pump cycles when it's on. No indicator for the pump needed since the kitchen sink is near the helm so it's easy to tell if the pump's on or not. To me a relay makes sense rather than running heavy gauge cable from the batts to the switch to the pump.
So...right now I'm shopping for a proper relay to install next year.
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Old 18-10-2016, 07:07   #13
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Re: Latching relay

All righty...

This is starting to make a little more sense now given your layout! I would still say a proper breaker panel is the way to go, but your control wire is already there... It probably BTW is large enough for a 5A pump, but your latching idea is fine...

Latching relays are obviously a bit harder to find when simple SPST 12v relays are practically laying in the street for the taking... $5 at an auto store, a 5 pack with harnesses from Amazon for $14... They take something like 20ma to energize, so literally nothing

Trying to stay on the low $$ side for a Latching Relay, you'll need a short piece of DIN rail
https://www.amazon.com/20-21-9-012-0...latching+relay

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$45 with socket
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Old 18-10-2016, 07:08   #14
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Re: Latching relay

A handy wire size chart for you to bookmark or print...

http://assets.bluesea.com/files/reso...on_chartlg.jpg
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