Bilge pump questions

[Comodave]

The 14 gauge wire will carry 10 amps with a 10% voltage drop. It would take a 10 gauge wire to carry the 10 amps with a 3% voltage drop. The 3% drop calculations are for “critical” circuits. But I typically use the 3% drop calculation for pretty much everything since the cost difference in the wire is small and everything works better with good voltage.

 

[O C Diver]

If you do a little research you will find most of the fuses (glass and ATC) being used in boating and automotive applications are more designed for higher amperage failures like a positive touching a negative in a DC circuit. Failures from amperage at 110 to 120% of load rating can be measured in hours to days before the fuse fails. This is not a precision amperage disconnect, but modest circuit protection costing pennies to make.

Ted

 

[Discoverer]

10A fuse on a bilge pump that only draws 2-3A running is quite oversized. As I said the main issue isn't just the pump-it's the wiring. If there's a fault, the wire could overheat and burn before the fuse blows.

Ideally, the fuse should be just above the pump's max operating current but low enough to protect the wiring. Over-fusing is a real risk, especially in a critical system like bilge pumps.

 

[Delta Riverat]

You really want 2 fuses.

One close to the source to protect the wire, say 10A for 14g and then the one the load is rated for close to the load if the load is light.

As to the bilge pump, put in the largest flow you can get that fits your existing output hose. Which is 3/4" I suspect due to the size existing now. Yes, a locked rotor condition needs to be protected and calls for a properly sized fuse.

 

[rslifkin]

You really want 2 fuses.

One close to the source to protect the wire, say 10A for 14g and then the one the load is rated for close to the load if the load is light.

There is no reason to use 2 fuses. Just use the one smaller one as it will protect both the wire and the load.

 

[boomsurf]

Two pumps. High water pump Johnson 4000gph with a USS Ultra pump switch wired to a different battery than the primary pump. Primary more important to get down low rather than high gph. Fuse for wire.

 

[Delta Riverat]

There is no reason to use 2 fuses. Just use the one smaller one as it will protect both the wire and the load.

Yeah, if all you got on a whole circuit is one little pump.

 

[rslifkin]

Yeah, if all you got on a whole circuit is one little pump.

For a bilge pump, that should always be the case. Shared circuits are not appropriate for that purpose in my opinion.

 

[Fleming]

Maybe I'm wrong, but if the pump motor seizes, it's going to generate a lot more amps and should trip the 10A fuse.

You ARE wrong. Test it. Prove me wrong. Stick a screwdriver or rag in the impeller to block it and turn the pump on.
If you were an ABYC member, which I'm guessing you aren't, you could go on the website and watch the video about locked rotor current of bilge pumps that they made a few years ago. They did the experiment of locking the rotor while monitoring the current in the circuit. You can see the startup current, running current, and locked rotor current. They put in an oversized fuse like yours, and showed the locked rotor current stabilizing and then gradually dropping as the pump started to smoke, eventually catching fire (experiment done outside).
Here's their facebook post after they made the video where they are explaining all the stuff I've already explained but you don't believe. Log into Facebook

 

[Bmarler]

For a bilge pump, that should always be the case. Shared circuits are not appropriate for that purpose in my opinion.

Your opinion.
My boat is full displacement with a common bilge, no compartment separation. So I essentially have one big pump for it, plus a tiny nuisance pump.
Supplied power from a single, fused, non-switched cable at the battery bank. This goes to a terminal strip with two pump control panels fused for each pump.
I’m perfectly comfortable with this setup of two pumps on one supply.

 

[rslifkin]

Your opinion.
My boat is full displacement with a common bilge, no compartment separation. So I essentially have one big pump for it, plus a tiny nuisance pump.
Supplied power from a single, fused, non-switched cable at the battery bank. This goes to a terminal strip with two pump control panels fused for each pump.
I’m perfectly comfortable with this setup of two pumps on one supply.

I wouldn't consider that multiple pumps on a single circuit. That's just a dedicated power bus for the bilge pumps with individual pump circuits fed from that bus. Multiple pumps on one circuit wold be having 2 automatic-only pumps fed from a single breaker in the main DC panel and a single wiring run that splits to the 2 pumps down in the bilge or something like that.

 

[Discoverer]

You really want 2 fuses.

One close to the source to protect the wire, say 10A for 14g and then the one the load is rated for close to the load if the load is light.

No, you don’t need two fuses. Adding a second fuse doesn’t provide extra safety, quite opposite - it adds another potential failure point.

 

[jhance]

This is the wiring diagram from the manufacturer showing the bilge pumps and a single 10A fuse for the wiring and 500 gph pump. They must have made a calculation that they needed a 10A fuse instead of going with a smaller fuse per bilge pump manufacturer recommendation. Why?

 

[Delta Riverat]

They are protecting the wire to the pump.

 

[jhance]

They are protecting the wire to the pump.

I get that, but they must have considered the manufacturer's spec, "2.5A fuse," for the pump. Did they just disregard or feel it was not necessary?

 

[Delta Riverat]

Some pumps (motors) are "thermally protected" which means that even with a locked rotor condition they will not exceed a dangerous temperature. Or they were just "building to spec" and protecting the wire which a 10A fuse will do on a 14g wire. If you want, you can even test your pump on locked rotor. I have to advise that this can be a destructive test.

 

[NoRain]

You ARE wrong. Test it. Prove me wrong. Stick a screwdriver or rag in the impeller to block it and turn the pump on.
If you were an ABYC member, which I'm guessing you aren't, you could go on the website and watch the video about locked rotor current of bilge pumps that they made a few years ago. They did the experiment of locking the rotor while monitoring the current in the circuit. You can see the startup current, running current, and locked rotor current. They put in an oversized fuse like yours, and showed the locked rotor current stabilizing and then gradually dropping as the pump started to smoke, eventually catching fire (experiment done outside).
Here's their facebook post after they made the video where they are explaining all the stuff I've already explained but you don't believe. Log into Facebook

That’s fascinating, but quite frankly I find it very counterintuitive. Must be a function of it being a DC motor? Because I don’t think that’s the way AC motors behave. If I start jamming a too-thick piece of wood into my tablesaw until it stalls it will blow the breaker.

But facts is facts, and seeing is believing when it comes from a credible source, so I’ll take their word for it. Lesson learned, thanks.

 

[Fleming]

That’s fascinating, but quite frankly I find it very counterintuitive. Must be a function of it being a DC motor? Because I don’t think that’s the way AC motors behave. If I start jamming a too-thick piece of wood into my tablesaw until it stalls it will blow the breaker.

But facts is facts, and seeing is believing when it comes from a credible source, so I’ll take their word for it. Lesson learned, thanks.

From AI "When comparing AC and DC locked rotor current, the key difference is that in AC motors, the locked rotor current initially peaks very high due to the inrush current phenomenon, then drops slightly as the motor starts to rotate, while in a DC motor, the locked rotor current is essentially the same as the initial inrush current, remaining relatively constant until the motor starts to turn."

Basically, locked rotor current is the starting current, which is higher for AC motors because the rotating magnetic field has to 'build' to drag the rotor up to speed while in a DC motor the field is steady and unidirectional. I'm sure there are some smarter electrical engineers on here who can explain it better.

 

[SteveK]

Is it possible the 2.5a bilge pump is not original? The 10a fuse may well have served an original pump that needed a 10a fuse.
I believe a fuse should be 150% of the rated max draw. So a 6-7a draw protected by 10a makes sense.

 

[jhance]

Is it possible the 2.5a bilge pump is not original? The 10a fuse may well have served an original pump that needed a 10a fuse.
I believe a fuse should be 150% of the rated max draw. So a 6-7a draw protected by 10a makes sense.

I am pretty sure the bilge pumps are original and say to use a 2.5A fuse.

 

[jhance]

I just realized my 2003 bilge pumps say "ignition protected." Maybe this is why there is a 10A fuse on the circuit (instead of the 2.5A as recommended as seen on the top of the pump) as the manufacturer was not concerned about the pump locking and catching fire?

 

[SteveK]

ignition protected means it does not throw sparks during operation

 

[jhance]

ignition protected means it does not throw sparks during operation

Ah... different than thermally-protected then.