Fuse amp selection

[EJ3]

I recently installed a fuse box for my bildge pump, nav lights and live well pump. I know what size fuse to get for the bildge pump since it was on the box. But the cheap nav lights, from Walmart, don't say what size fuse to use. Does anyone have advice on what size is a good size fuse for these?

Is there a good rule of thumb for fuse selection and its intended applicaiton or mabey even a reference chart of some sort?

 

[gonefishie]

Re: Fuse amp selection

there should be at least a current rating printed on the box of the light. The number of lights you got on the circuit determine the size of the fuse. For example, the light draw 1A each and you got 3 on the boat = 3A fuse. The rule of thumb is to never use fuse larger then the current rating of the circuit. Not that you can't do it, but it ain't a smart thing to do.

 

[MrBigStuff]

Re: Fuse amp selection

Fusing should be chosen based on the wiring it feeds, not the load. The load will only draw what it needs. If the fusing protects the wiring, it won't matter if it shorts out as long as the wire can support that amount of current without being damaged. If you start selecting fuses based on loads, you run the risk of over loading a circuit. Always safer to look at it from the other perspective.

 

[gonefishie]

Re: Fuse amp selection

I agreed but not in electronic application. What you're saying is true in an electrical application, i.e wiring up a building. You're only trying to protect the wiring as most thing plug into the outlet have it own fuse to protect itself. This is a boat application, you can't overload a circuit when there aren't any extra outlet to plug in other stuff.

 

[Silvertip]

Re: Fuse amp selection

You are both right and both wrong. That must leave me in the middle. Lets look at some typical fresh water boats.

Trolling Motor: Usually it's the only thing on the troller battery and a 40 - 50 breaker obviously protects the wiring.
Accessories on Troller Battery: Bow Mount locator has its own fuse so it protects the locator. If a short occurs ahead of the fuse you have a fire which is generally the case when you combine circuit and device protection. Since devices added in a boat are generally hard wired into a feeder, you have the scenario where the length of small gauge wire between the feeder and the in-line fuse becoming a fuse itself. It will melt if shorted.

Console: Gauges have their own fuse so it protects the wiring.
Radio: Inline fuse protects the radio, but it's generally fed from another circuit. Do I hear circuit overload? Again, a short ahead of the fuse can melt the radio leads before blowing the fuse for the circuit it's fed from.

And on we go. That's why fusing is important. If every circuit was dedicated, then you can have device protection so long as the "ampacity" of the smallest wire and fuse is not exceeded.

 

[wire2]

Re: Fuse amp selection

Common sense should prevail when exact ratings are unknown. Motors and bright lights draw the most current on a boat, electronics and indicators much less.

If you know the wattage of a load, divide by 12 to get amps.

A fuse that held for a long time, then blows, usually indicates a problem, such as a failed device or wiring short.

 

[gonefishie]

Re: Fuse amp selection

Well, being in the middle, you're not right either. Unless you're a special case, I've never seen anyone hook up their bow mount locator to the trolling motor battery. Why in the world would you want to hook up anything to the trolling motor battery other then the trolling motor itself. I take that back, I guess you can if you don't use the trolling motor very often and idle around with your bow mounted locator on. If a short happens between the in-line fuse and the "feeder", I'm assumming you mean the fuse panel, the in-line fuse WILL blown before the wire melt. If you have a fuse panel then every circuit on the boat is dedicated, assuming the wiring is done right. If all three lights connect to the same terminal on the fuse panel that doesn't have anything else connect to it then it is dedicated. You feed each device through the fuse panel, the current goes through a fuse before it reach the wire that goes to the individual device. Think of it as your house wiring. The in-line fuse that goes between the battery and the fuse panel is like your MAIN BREAKER that goes in between the service drop and your breakers box. Your fuse panel is the breaker box with breaker/fuse to protect the different circuit. For example, if your toaster is shorted, the kitchen circuit breaker will trip and no wire behind wall melt. If something shorted the wire before the circuit breaker, the MAIN BREAKER will trip and no wire behind wall melt.
Anyway, EJ3 is probably confused as hell right about now.

 

[MrBigStuff]

Re: Fuse amp selection

Silvertip, I see your point by your description. When I run feeders, they are fused within inches of the source if not inside a distribution panel. There is no opportunity for chafing or other damage to result in a short in this limited length. Sometimes, this results in more than one fuse on that circuit. The OEM in-line fuse is fed by MY fused feeder line.

If the person is foolish enough to run a long feeder before the in-line fuse, you could have a problem.

The analogy to house wiring doesn't hold up to my experience. I see people farmer rigging electronics all the time. It doesn't matter that there isn't a plug. They have discovered crimp connectors and think they're the solution to all wiring add-ons.

The danger I was trying to highlight is the situation where someone calculates their "new" load based on the stuff they added to the circuit and plops in a fuse based soley on that. Load based calculations alone can be dangerous.

 

[Chris1956]

Re: Fuse amp selection

Fuse size is dictated by wire size. 14 AWG wire should be fused at 15Amps, 12 AWG wire should be fused at 20Amps....

You cannot protect an electronic device from malfunctioning, with a fuse. if the device shorts out and draws more current, even if you had an inline fuse with the correct current rating, the device has already shorted out, and is broke.

Inline fuses (supplied with radios and FF etc), are used to protect the wiring upstream of the device. A lot of boat wiring is unfused, and therefore requires some protection, even if it is downstream of the power source.

I suppose these downstream fuses might provide some minimal protection if the device had a power surge, but fuses are usually not sensitive enought to perform that function.

 

[gonefishie]

Re: Fuse amp selection

Fuse size is dictated by wire size. 14 AWG wire should be fused at 15Amps, 12 AWG wire should be fused at 20Amps....

You cannot protect an electronic device from malfunctioning, with a fuse. if the device shorts out and draws more current, even if you had an inline fuse with the correct current rating, the device has already shorted out, and is broke.

Inline fuses (supplied with radios and FF etc), are used to protect the wiring upstream of the device. A lot of boat wiring is unfused, and therefore requires some protection, even if it is downstream of the power source.

I suppose these downstream fuses might provide some minimal protection if the device had a power surge, but fuses are usually not sensitive enought to perform that function.

It's true that a fuse can't stop a device from malfunctioning but it sure would stop it from being burn up. That is the main purpose! The original question was very simple and the answer was kept as simple as possible without risking anything. I'm sure the guy didn't needs a lecture.

 

[Silvertip]

Re: Fuse amp selection

Ah ha -- I love discussion. First, I have two locators. One at the helm, and one at the bow. The bow locator has the transducer on the motor and since the troller battery is all of about 30 inches away it makes very good sense to use it rather than run a line eight feet back to the console fuse panel. The locator has an inline fuse about six inches away from its attachment at the trolling motor socket and the locator itself is about 14 inches away from the fuse so how much more convenient and safe can you get. Besides, the amount of current it draws from the troller is miniscule. Think of this as exactly like the fuse that's in the plug of your Christmas tree lights. Guess why the light manufacturers had to add those? Yup, short the lights and up in smoke goes the house because the wires were too small to carry the current and breaker didn't know the circuit was on fire. And no -- the trolling motor does not interfere with the operation of the locator and the locator is turned off when the troller is pulled up. I refer to the feeder as the same as a branch circuit in house wiring -- and yes, I do know how that works. So the branch circuit (the circuit feeding the troller socket is protected with a 40A breaker at the battery. If the troller draws more than 40A (as if it were stalled rotor), the breaker opens. No issue. If there is a "dead" short in that branch/feeder circuit, again the breaker opens. If something goes sour in the locator, it's inline fuse blows and if there is a dead short in its wiring again the fuse blows. Now here's the rub -- so far we've talked about shorts. Electrical issues can involve high current draw that is not a short. Take your toaster example (which is a good example -- or any heating appliance like heaters, irons, or coffee pots). Do you realize that most of the kitchen fires are started by appliances that are left plugged in and unattended. They begin to draw too much current, wires get hot, the breaker in the service panel keeps delivering juice because the branch circuit current hasn't reached 15A or 20A depending on breaker size so fire erupts. People constantly leave electrical applicances on and think nothing of it when they leave to go shopping. Why trust a 10 cent switch on a coffee pot. Or a timer or heat sensor on a dryer. They fail at frightening rates. My point here is you cannot expect a fuse or breaker in a service panel (on your boat or in your house) to open if current draw in an appliance cord or the light circuit on a boat exceeds the ampacity of the wire feeding it. Quite simply, you have a fire if that happens prior to the point where the device fuse is located. Think about it. The radio in your boat has an in-line fuse located about a foot from the radio. There is about another foot to 18 inches of wire between that point and the fuse panel. If a short occurs between those two points you will have a melt down unless that circuit is fused exactly like the inline (or less). The entire issue here is moot if as you say -- each and every device in the boat is fed by a dedicated circuit. That is simply not the case in every boat I've ever owned. Circuits are often shared and there is nothing wrong with that as long as the wire size is the same to every device that's sharing that circuit -- OR -- that the entire circuit is fused for the smallest wire size in the circuit. And lastly, house wiring (which is AC) is indeed breakered at 15A for 14ga wire and 20A for 12ga. Those rules don't apply at 12V DC. Here's the ampacity charts that prove it. Check out the 14 and 12ga amp ratings.

 

[wire2]

Re: Fuse amp selection

Fuse size is dictated by wire size. 14 AWG wire should be fused at 15Amps, 12 AWG wire should be fused at 20Amps....

Well, in some cases. In a house, office or factory branch circuit, the NEC regs are calculated to maintain a 2% or less voltage drop. #14 wire is .002525Ω per ft so you can run up to 79 feet, (39?' of 2 wire). That's at 12 amps, based on the code rule of 80% limit of the overcurrent device.

Theoretically, you *can* use #18 ga for a 20 amp load
If you're 1? ft from a breaker, #18 ga has .006385Ω/ft, 3 ft total has .019155Ω for a drop of .38 volts. That's only 2.77%, quite acceptable for low voltage DC circuits.

Lift your car hood, you'll see a single #18 feeding both headlights, 60 watts each, 10 amps total. Does it get a bit warm? Yes. Does it matter? No.

There's a pair of tables at the bottom of this site, (found by Wildmaninal), http://www.submarineboat.com/sub/wiring.html
the last one gives acceptable wire length for various currents.

 

[wire2]

Re: Fuse amp selection

Silvertip, (et al) just so you know, I started typing before you posted, went for supper, finished it and posted, then saw yours.
But no matter, we seem to be in full agreement.

 

[QuadManiac]

Re: Fuse amp selection

You are all correct, to some degree. The safest and proper way to protect ALL systems is to use a fuse based upon wire size at the source of the wire run to protect that wire from overheating if it chafes to ground or the run becomes overloaded by additional unplanned hardware additions or if an upstream fuse or breaker malfunctions. THEN fuse each piece of electronics equipment with a fuse rated just marginally above the device's surge current, close to the destination (commonly an inline fuse included with the device) to protect the device from becoming a fire hazard if it fails. Electronics equip typically already are provided with a fuse onboard or inline. Resistive loads, such as bulbs, typically are not fire hazards if they fail, and so do not usually require a separate fuse at the destination.

Wire run is protected, equipment is protected.

 

[gonefishie]

Re: Fuse amp selection

Ah hah! So you're a special case. What kind of boat do have now? you have any problem getting on plane with the batt. that close to the bow? In my boat, the only shared circuits are the one for the console gauges, storage box light and the navigation lights. Everything else is on a seperate terminal and fuse on the fuse panel. How was ya'll Turkey day by the way?

 

[Silvertip]

Re: Fuse amp selection

Gonefishie -- As a matter of fact I do not have a problem getting on plane with the troller battery in the bow. The boat is powered at its maximum, it has Smart-tabs and stays on plane down to about 12 MPH. And all of that is with a full live well, two aboard and a full load of gear.