How to Wire a Bilge Pump

Over and over, people want to know: “How to wire a bilge pump with float switch?”

Since we’re boat switch people, it’s most often the confusion arises from whether or not to use a 3-way switch to wire the bilge float switch through the switch at the helm.  We’ll explain how to wire a bilge pump below:

There are generally two configurations:

  1. an ON-OFF manual only bilge pump rocker switch, or:
  2. an ON-OFF-ON, manual and automatic bilge pump rocker switch

This is not to say that option 1 does not have a bilge float switch at all… it’s just that the power to the float is not passed through the bilge switch at the helm.

bilge pump rocker switch
manual and auto bilge pump rocker switch

In both cases, the manual switch and float switch, and manual feed are in parallel.  This means it’s an OR circuit… either the manual bilge switch OR the bilge’s float switch can turn on the pump.

So what’s the real difference if both bilge pump wiring strategies have a float switch?  Let’s look at a few schematics both with bilge pumps wired with a float switch:

Manual Only Bilge Switch

bilge switch wiring diagram with float switch

With a manual bilge switch – our recommended way – the float switch is always connected straight to the battery via a fuse.  It cannot be turned off by the helm bilge switch.

Manual / Auto Bilge Switch

manual auto bilge pump wiring diagram

A Manual / Auto bilge switch, is an A-B switch with a center off position.  There are two paths through it… one direct (bipassing the float), and one in series with the float switch (series means AND… both the auto switch AND the float switch must be closed for the pump to activate).

In Most Cases, use an ON-OFF Switch

On New Wire Marine custom marine switch panels, we recommend using an standard ON/OFF manual bilge switch.

We DO recommend every boat have an automatic (float switch) feed for the bilge pump, BUT recommend the “auto” float switch lead be run directly from the boat’s battery (fused of course).

Why an ON-OFF Bilge Switch with Float?

Two primary reasons:

  1. Human error proof, and:
  2. Simplified cabling w/ less voltage drop

#1 – Human Error Proof

This method of by-passing your battery switch and switch panel makes certain that your float switch will turn on your bilge pump if it ever begins to fill up with water.

If you ever leave your boat at the dock, this could keep your boat from sinking if you forget to turn your “auto” switch on, or even if you have your battery switch OFF.

The reason typically cited to NOT connect devices straight to the battery is that a trickle current (like from a GPS, or VHF) could drain it.  BUT, there is little to no risk of this when a device (like a bilge pump) is connected with a physical disconnect switch (like a float switch).

Furthermore, even if you trailer your boat how do you compare:
  1. The risk of draining a battery because you left the plug in and the bilge pump had to pump rain water out all night
  2. With the risk that you decide to unexpectedly leave your boat at a friend’s dock for the weekend and return to find it swamped?
We do recommend an automatic bilge pump, but there is little justification for putting a switch on the “automatic” feed, and there is potential risk reduced by NOT putting the auto feed on a switch.
how to wire a bilge pump with an ON-OFF rocker switch
bilge pump wiring allows indicator light to shine on rocker switch

#2 – Built in Bilge Running Indicator

There’s also a cool feature of wiring your bilge pump float switch like this:  In auto mode (either auto on the AUTO-OFF-MAN switch, or the “OFF on an ON-OFF switch) whenever the float floats you get a bilge running indicator on the switch.

Here’s how that works for the various switch types:

  • With an ON-OFF bilge switch, the light is normally OFF when the switch is down – but when the float floats the single light comes on.
  • With the AUTO-OFF-ON, printed switch, the auto light is on all the time (the manual light comes on when the float floats)
  • With an AUTO-OFF-ON, etched switch the single bottom light comes on in manual mode, OR when the float floats (but not when simply in ‘auto’ mode.

This is due to the splice in the bilge… the 12V coming from the now-closed float switch, runs back up to the switch… hitting terminal 3 (or 1 on an man/auto) on the bilge switch.  And BAAM!  The indicator light comes on.  Even though the switch is in the OFF position.

Makes for a super nifty built-in indicator light, so you know when your float switch is running your bilge pump… see diagram on the left

#3 – Backlit Bilge Rocker Switch Wiring Diagram

Of the three bilge pump switches the only one that’s not extremely simple is the backlit auto/manual bilge pump switch.  (learn more about how our awesome backlit switches work here) Even that one is still pretty straightforward though, here are some diagrams that show the single jumper required on the back of the switch.

In the right-hand diagram, you can see how the back feed from the float switch might come back up the manual line and land on Terminal 10 illuminating the bottom indicator light that the float switch has activated.

auto bilge rocker switch wiring diagram
bilge pump wiring diagram with manual auto switch

 #4 – What if an ON-OFF bilge switch is not Possible/Feasible?

Okay okay… sometimes it’s quite a pain to change from a manual/auto bilge switch to an ON-OFF due to already pulled cabling, or accessibility of the bilge, etc.  There could also be an operational reason to use an ON-OFF-AUTO switch like you want to be able to fill your bilge with water for cleaning.  Or you’ve just always had a manual/auto bilge switch and it’s hard to teach an old dog new tricks…

We’re definitely not saying it’s just flat-out wrong – or unsafe – to use an ON-OFF-AUTO bilge switch.  In fact, we sell those too.  It’s really up to you, you’ve read our rational on how to wire a bilge pump, let us know which way you want to go when you contact us about your new custom switch panel!