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Old 08-08-2020, 23:58   #1
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Bilge pump fuse

The Rule 3700 we have says to use 14 gauge wire with a 25Amp fuse since the unit draws 19Amps nominally. In longer runs, they of course recommend 12 gauge wire.

But I’m confused since 14 AWG has 15-amp capacity and 12 AWG has 20-amp capacity so, no matter which you went with, wouldn’t the cable likely melt via overcurrent before popping the fuse?

Seems like it should use 10AWG which matches the 25Amp rating and would blow the fuse before killing the wire, no?
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Old 09-08-2020, 00:41   #2
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Re: Bilge pump fuse

Looking at Anchor’s website, it would appear 14 gauge is sufficient up to 29amps, so perhaps I’m just getting my info on cable capacities from the wrong source.
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Old 09-08-2020, 02:36   #3
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Re: Bilge pump fuse

I would run 10 for that.
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Old 09-08-2020, 04:31   #4
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Re: Bilge pump fuse

The correct term for the current-carrying ability of a conductor is "ampacity". If you do some googling, you will find ampacity tables available. The primary consideration is the type of insulation of the conductor. There are others, but they are dwarfed by making sure you are using a table for the insulation of YOUR wires.

Now- here is the part that will help you. I will assume you have a 12 Volt bilge pump. Select a conductor that will give you a 3% voltage drop. You can use the ABYC tables for that, or an internet voltage drop calculator. Now look up the ampacity of the conductor you selected. I will bet big money that the ampacity of the conductor will be in excess of the current draw of the bilge pump.

In almost every case, voltage drop will be the limiting factor. If you size your conductor for proper voltage drop, you will also meet the ampacity requirement. There are very rare instances that this will not be true, hence my instruction to check the ampacity after you select the conductor.

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Old 09-08-2020, 04:48   #5
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Re: Bilge pump fuse

The Rule install guide seems fine in my opinion.

A quick internet search shows the Rule 3700 max current draw at 15.5amps (max amps will occur when pump is at max head, ie not actually pumping out any water). Your running amps likely to be closer to 14amps.

The 25amp fuse is within the max current capability of 14awg, the fuse is there to protect wiring against fault conditions, not to protect the pump.

So it seems that the engineers at Rule may somehow have got it right. Having said that there is nothing wrong with installing 10awg to minimize volt drop and maximize output for a minimal additional investment.
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Old 09-08-2020, 04:57   #6
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Re: Bilge pump fuse

The 25 and 20 amp limits are for house wiring. Marine wire has higher temperature insulation and therefore higher current limits.
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Old 09-08-2020, 06:22   #7
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Re: Bilge pump fuse

unless that bilge pump is only 1 foot from the battery I bet the voltage drop in anything less 10 awg wire from 19 amps is unacceptable
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Old 09-08-2020, 06:36   #8
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Re: Bilge pump fuse

Ampacity requirements depend on amps and circuit length.



14, 12, and 10 AWG can be used for the pump and still meet ABYC ampacity requirements.

From the chart with a 20A load and 3% voltage drop
14 AWG = up to 6ft
12 AWG = up to 10ft
10 AWG = up to 15ft

Keep in mind those distances are round trip. That being said using a higher gauge wire never hurt anyone and 10 AWG is widely available, cheap, and negligible extra weight. If you are more comfortable with 10 AWG then wire it with 10 AWG. Personally I would but primarily because I always have some 10 AWG on hand and don't have any 14 AWG (better to have a wire with a gauge to large than too small).
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Old 09-08-2020, 07:27   #9
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Re: Bilge pump fuse

Does anyone “fuse” a bilge pump? Surely a circuit breaker instead?
When there is any doubt always go with bigger wire, it doesn’t hurt.
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Old 09-08-2020, 07:52   #10
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Re: Bilge pump fuse

For that bilge pump I would just use 10 gage. At least 12 on a short run. But that's just me I guess.
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Old 09-08-2020, 08:02   #11
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Re: Bilge pump fuse

Quote:
Originally Posted by a64pilot;3204520 Does anyone “fuse” a bilge pump? [B
Surely a circuit breaker instead?[/B]
When there is any doubt always go with bigger wire, it doesn’t hurt.
I think many do use a breaker, Some of mine were. But I will say that when a breaker fails, and they do, the only way you will have a bilge pump again is after you replace the breaker. A fuse is a simple device.
-Interestingly, those little bilge pump panels with the Auto-Off-Manual have a fuse not a breaker.
-Many boats have the BP wired direct to the battery with a fuse and switch.
-A series of 30 small specialty Army boats we made specified only fuses, no breakers.
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Old 09-08-2020, 08:54   #12
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Re: Bilge pump fuse

Quote:
Originally Posted by a64pilot View Post
Does anyone “fuse” a bilge pump? Surely a circuit breaker instead?...

In most cases, fusing is short for "over current protection."

That said, a fuse can't fail shut, a breaker can. If the location is damp, a fuse is normally the safer choice, and a bilge pump should generally wire dirrect to the battery and NOT through the main panel because they are not wired through the main battery switch. There will normally be a switch, but the wire may not go through the cabin and so there may be no dry location for a breaker.

So yes, a fuse is probably the better choice for a sump pump in many cases. I've had both. On the smaller boats, one was fuse, the other breaker. On the 6-figure boat, fuses because they are better for this.
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Old 09-08-2020, 09:07   #13
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Re: Bilge pump fuse

No, not all panels are fused, and fuses most certainly do wear out, whether it’s from corrosion of the fuse element or from The element vaporizing or whatever, but as they age they will blow or break from vibration.

Many decades ago aircraft were fused, the FAR’s still stipulate that spare fuses be carried, and I believe even how many per each rating.
On edit, looked it up, it’s 91.205 and three compete sets per each rating are required to be carried.
However since I’d guess soon after WWII aircraft have circuit breaker panels, and not fuses, I guess the aircraft manufacturers just don’t know that fuses that cost pennies are better than circuit breakers that cost 50 times as much?

Where the switch is located doesn’t mean how it’s wired, just where it’s located, why would anyone want to put the circuit protection device in a wet area, and require you to get into the bilge to inspect / change it?
Mine is panel located, but wired directly to the battery

I dislike fuses because where is the spare? Bypassing a circuit breaker is as easy as connecting both wires to the same terminal.

Yes circuit breakers can fail closed, and a fuse can’t, but for a bilge pump, I’ll take that chance, because I want it to get power.
During phase maintenance I used to pull all circuit breakers, it was rare, but once in a blue moon I’d find one stuck that you couldn’t pull on an old Huey.
Maintenance manual didn’t have us pulling CB’s it was something passed down.

If you want to verify operation it’s easy, just short the thing and see if it pops.

Only advantage fuses have over CB’s is the price, pennies compared to tens of dollars
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