Bilge Wire Size Question.

[BAD ORCA]

This one has me baffled. I am rewiring my bilge pump and moving the location of the on/off switch. I have a Rule 2000 UL listed pump.

Originally the old on/off/auto switch was about 15 feet away from the pump and batteries (30 ft round trip). The wire leads that come off the pump appear to be 14 Awg, and the wire used to wire the pump to the switch and back was also 14 awg bilge pump wire (grey outer insulation). My new on/off switch location will also be about 15 ft away from the pump. The pump draws about 9 amps when on and is fused for 15 amps.

According to Blue Seas wire size chart that requires 10 Awg wire not #14, but the ABYC wire ampacity chart says #14 wire can handle 35 amps but gives no indication of at what distance. I can certainly wire it with 10 Awg but the leads that come off the pump would still only be 14 awg. Lets assume my boat had a very bad leak and the pump stayed on for a considerable amount of time, it seems the #14 wire leads are way undersized and could get very hot or melt. Also, the lead wires cant be changed at the pump so i would be butt splicing 10 awg to 14 awg.

If you consider a much larger pump like a Rule 3700, things get much worse and the recommended wire gauge goes up to 6 Awg. The spade connectors max out at 10 awg where they connect to the switch so something seems off in my understanding or calculations.

Do the pump manufacturers size the leads off of the pumps in the anticipation that they wont be on for very long so can be a much smaller gauge wire, or am i fundamentally misunderstanding something?

I'd like to wire my pumps so that the wire size is appropriate for a pump that may be on for an extended amount of time in an extreme emergency without melting the wires, but this doesnt seem possible.

 

[Davidasailor26]

There are two factors at play here - ampacity and voltage drop.

The first limit to consider is maximum ampacity of the wire. That’s what’s shown in the ABYC table you mentioned. Ampacity limits are about not heating up the wire to the point of melting the insulation. For your 15 amp fuse you’d actually be ok with 18 AWG wire (provided the insulation is rated 90C or more, and it’s not in an engine compartment).

The second, and often more restricting limit, is minimizing voltage drop. Breaking voltage drop limits won’t heat the wire or cause a fire, but they will decrease the effectiveness of the devices on the wire. Voltage drop is cumulative over all the legs of a circuit, and for a device like a bilge pump you want it to be as small as possible. The Blue Seas chart you mentioned is based on limiting voltage drop. If you assume the 30 feet of the circuit is the only part causing any voltage drop (ie the cables from the batteries to the switch you mentioned are really heavy and not very loaded) then 10 AWG is adequate. I’d probably be looking for a way to reduce that distance, maybe by running the pump wire straight to the battery instead of the switch. But the 14 AWG that came with the pump assumes a very short circuit length and will probably have pretty significant drop, so I’d be making a splice to replace that with 10 AWG as close to the pump as possible while keeping the splice somewhere above the water level in the bilge.

 

[jssailem]

 

[dlochner]

@Davidasailor26's description is spot on if you size the wire for a 3% voltage drop, if you size the wire for a 10% drop then 14 ga is OK. The bilge pump should run with a 10% drop. However, with he lower voltage there will be a drop in capacity, i.e., the pump will run a little slower and the output will be lower, which begs the question, how much slower and does that make a difference?

The answer to that question lies in marketing hype and myth busting. Bilge pumps outputs are typically rated at zero head and at float voltages, about 13.5v. In real life the bilge pump is pumping water up before it flows out and this head will reduce flow. Unless the engine is running and the alternator is charging, the voltage will always be much lower again reducing flow. The belief that these little pumps are going to keep the boat from sinking is a belief without much foundation.

In a catastrophe the pump, no matter its rating will simply be unable to remove water faster than it enters. Couple this with the fact the pump will run slower as the battery voltage decreases from the pump running. The real purpose of these pumps is to remove nuisance water that comes from deck and port leaks, condensation, water entering through a keel stepped mast and so on. About the only time they will keep your boat from sinking is at the dock while connected to shore power, if a drain fails and allows rain water to enter the boat.

The point of this digression is to say, using 14 ga wire is perfectly acceptable in this application

 

[BAD ORCA]

There are two factors at play here - ampacity and voltage drop.

The first limit to consider is maximum ampacity of the wire. That’s what’s shown in the ABYC table you mentioned. Ampacity limits are about not heating up the wire to the point of melting the insulation. For your 15 amp fuse you’d actually be ok with 18 AWG wire (provided the insulation is rated 90C or more, and it’s not in an engine compartment).

The second, and often more restricting limit, is minimizing voltage drop. Breaking voltage drop limits won’t heat the wire or cause a fire, but they will decrease the effectiveness of the devices on the wire. Voltage drop is cumulative over all the legs of a circuit, and for a device like a bilge pump you want it to be as small as possible. The Blue Seas chart you mentioned is based on limiting voltage drop. If you assume the 30 feet of the circuit is the only part causing any voltage drop (ie the cables from the batteries to the switch you mentioned are really heavy and not very loaded) then 10 AWG is adequate. I’d probably be looking for a way to reduce that distance, maybe by running the pump wire straight to the battery instead of the switch. But the 14 AWG that came with the pump assumes a very short circuit length and will probably have pretty significant drop, so I’d be making a splice to replace that with 10 AWG as close to the pump as possible while keeping the splice somewhere above the water level in the bilge.

Thank you for elaborating, much appreciated. In my mind i was sort of arriving at the same conclusion. Use #10 for the p+ from batteries to switch and back to pump and cut the leads about a foot or less from the pump. Looks like im going to West Marine today to be extorted for #10 wire

 

[BAD ORCA]

@Davidasailor26's description is spot on if you size the wire for a 3% voltage drop, if you size the wire for a 10% drop then 14 ga is OK. The bilge pump should run with a 10% drop. However, with he lower voltage there will be a drop in capacity, i.e., the pump will run a little slower and the output will be lower, which begs the question, how much slower and does that make a difference?

The answer to that question lies in marketing hype and myth busting. Bilge pumps outputs are typically rated at zero head and at float voltages, about 13.5v. In real life the bilge pump is pumping water up before it flows out and this head will reduce flow. Unless the engine is running and the alternator is charging, the voltage will always be much lower again reducing flow. The belief that these little pumps are going to keep the boat from sinking is a belief without much foundation.

In a catastrophe the pump, no matter its rating will simply be unable to remove water faster than it enters. Couple this with the fact the pump will run slower as the battery voltage decreases from the pump running. The real purpose of these pumps is to remove nuisance water that comes from deck and port leaks, condensation, water entering through a keel stepped mast and so on. About the only time they will keep your boat from sinking is at the dock while connected to shore power, if a drain fails and allows rain water to enter the boat.

The point of this digression is to say, using 14 ga wire is perfectly acceptable in this application

Thank you! I know the pumps wont keep up with a large ingress of water forever. My thinking is that they could manage a moderate size leak for example, from a tear in my dripless, or another source long enough that i could manage a fix or get help. My small 2000 pump is connected to start battery which is charged 24/7 so effectively could supply power forever when plugged in for nuisance or small leaks. Im going to add one or two 3700's connected to my lithium house bank as well for when im away from the dock. Two of these could run at full bore for 20 hours at high voltage if my house bank is full. Hopefully i will never need them, but for $500 it gives me a second chance. I plan to do a lot of solo sailing once the boat is ready. Of course i will have a lift raft and epirb as a backup to the pumps if things get really bad

 

[dlochner]

Thank you! I know the pumps wont keep up with a large ingress of water forever. My thinking is that they could manage a moderate size leak for example, from a tear in my dripless, or another source long enough that i could manage a fix or get help. My small 2000 pump is connected to start battery which is charged 24/7 so effectively could supply power forever when plugged in for nuisance or small leaks. Im going to add one or two 3700's connected to my lithium house bank as well for when im away from the dock. Two of these could run at full bore for 20 hours at high voltage if my house bank is full. Hopefully i will never need them, but for $500 it gives me a second chance. I plan to do a lot of solo sailing once the boat is ready. Of course i will have a lift raft and epirb as a backup to the pumps if things get really bad

If you want to have any chance of dewatering the boat endanger of sinking, you'll need a large diaphragm bilge pump, preferably engine driven.

 

[JamesG161]

You can find this Blue Seas APP that does the trick for sizing everything for you Electrical needs.

Jim...

PS: it does work on my iMac iOS too!

 

[BAD ORCA]

If you want to have any chance of dewatering the boat endanger of sinking, you'll need a large diaphragm bilge pump, preferably engine driven.

Thank you! I will look into them. I think the best i will be able to do is make sure my alternator can keep up with the amperage of all the pumps running at once.

 

[LakeOntario270]

I would use 12 awg not 10 awg. 12 awg will be much easier to work with and easier to connect to the 14 awg wires on the pump. The voltage drop will be not as good as the 10 gauge but still pretty good Based on what I've read on this thread.

 

[BAD ORCA]

I would use 12 awg not 10 awg. 12 awg will be much easier to work with and easier to connect to the 14 awg wires on the pump. The voltage drop will be not as good as the 10 gauge but still pretty good Based on what I've read on this thread.

Too late

I wired them with 10 awg. Thank you though. Pump definitely seems like it has a little more power now with the larger wire size. The motor seems to run a little stronger.

 

[Jan11]

Relating to this discussion, and many others, we should be aware that the standards for electrical equipment are not the same as the standards for wiring your boat... or house or car. They allow smaller leads from the equipment than you can use in wiring your boat. The thinking is that the equipment's leads are short and don't effect the voltage drop significantly and are not in bundles - or shouldn't be.
No one mentioned that wires in a bundle are rated lower than in free air.