What size fuses

[john-206]

Just bought first boat. Can someone tell me in simple English the answer to the following regarding lighting circuits:<br />If I have 1 20w bulb on a circuit what size fuse do I need to protect it?<br /><br />If I put 3 20w bulbs on the same circuit does the size of the fuse need to be trebled?<br />Thanks<br />John

 

[Dunaruna]

Re: What size fuses

A 20 watt bulb draws about 2amps. Add a bit for cable length and terminal lose. Fit a 3amp fuse. If you run 3 x 20w bulbs, the circuit will draw 6 amps, your 3 amp fuse will blow. Fit a 10amp fuse.<br /><br />Aldo

 

[cbcrawford]

Re: What size fuses

Use your simple power formula to figure out draw, I was taught to use West Virginia, or W=VA. Use your basic algrbra to rearrange the formula to figure out what function, i.e. Watts in your case is 20, Volts in your case (assuming all is normal) are 12. Plug those in and solve for Amps. You can figure out any power draw using this formula, and just like Dun said, add just a hair to the final Amps because any circuit that is cold will briefly draw a little more cuurent (current=amps).<br /><br />Hope this helps!<br />CC

 

[john-206]

Re: What size fuses

And thats plain English???<br />Help it didn't

 

[Silvertip]

Re: What size fuses

The short answer is 2, 3, or 5 amp fuse. For three bulbs, 7.5 amp. But don't you want to know why? Here it is in plain english:<br />P (power in watts) equals volts x amps. Remember this is science and it has been proven correct. Same as SHO indicated. Your boat uses a 12V (actually 12.6V) electical system so the "V" (volts in the formula is 12.6). You said you have one 20 bulb so that's "P" in the formula. As you learned in your mathematics class, you need two out of three values to work a formula with three parts. <br />P=ExI becomes 20=12.6xI Since fuses are rated in amperes (amps) you are solving this formula for "I". You simply rearrange the formula so it now reads I=20/12.6. The answer is a circuit with one 20W bulb (or any other 20W load) would draw 1.58 Amps. Therefore a 2 or 3 Amp fuse would be required but you could use a 5A if the others are not available. To answer the second part of your question (3 x 20W bulbs) use the same formula. 3x20 = 60W total. I=60/12.6 which works out to 4.76A. a 5A fuse would be marginal so a 7.5 or 10 would be best. It's always good to get the "why" along with the "what" when you ask a question. This formula as others have indicated can be used in any direct current circuit and it also works very roughly for AC circuits. The only other consideration for your wiring is that the wire gauge (current carrying capacity, must be equal to or greater than the fuse rating or the wire becomes the fuse, it gets hot, melts the insulation, and once the smoke gets out you have all sorts of problems. (That last part was making a little fun!)

 

[john-206]

Re: What size fuses

Thank you!!!

 

[18rabbit]

Re: What size fuses

John – hopefully you were not sleeping during algebra. Electricity is a theory; a theory in the sense that we don’t really know if it exists. Don’t ask, long story. A basic equation of the theory goes like this:<br /><br />To find amps: Amps = Watts divided by Volts, or A=W/V.<br /><br />To find watts: Watts = Amps times Volts, or W=AV<br /><br />To find volts: Volts = Watts divided by Amps, or V=W/A <br /><br />That would be the same thing that SHO and UpInSmoke posted, just in plainer-plain English.<br />

 

[john-206]

Re: What size fuses

Thanks guys and in answer to the Algebra, science, formulas etc, some of us are to old to even remember if we learned it or not in school.<br />Good luck you young uns!!

 

[Silvertip]

Re: What size fuses

Hey John -- your communicating with a soon to be 66-year old. I remember learning it, but I don't remember when.

 

[jlinder]

Re: What size fuses

What is missing is the important part. Fuses are to protect you from burning up the wires or anything else in the circuit. You know what size fuse you need, now you need to make sure the circuit can support that.<br /><br />For a 10 amp fuse you can get by with 16 ga. or bigger wire and you won't have a fire hazard.<br /><br />But when you pick a wire size you also need to take into account the voltage drop. Wire has resistance, and when you draw current the voltage supplied to the far end is less than what starts out at the battery.<br /><br />These calculations are a bit more complex than the formulas you have already been given. Voltage drop is calculated based on the wire guage and length of the circuit.<br /><br />The good news is that running a couple of light bulbs is not a big draw, nor is critical. I would just go up some in wire size and you should be good. Probably about 12 ga. or 14 ga.

 

[john-206]

Re: What size fuses

Between you all you have helped me tremendously. Thanks to everyone who helped, arnt these forums great for us beginners. Anyone know anymore good ones???<br />John

 

[Boatin Bob]

Re: What size fuses

John...you don't need any others iboats is the best forum out there!

 

[jtexas]

Re: What size fuses

Those are the numbers for a parallel circuit, right?<br /><br />Assuming the lamps are designed for 20watts at 12volts, current is 20 ÷ 12 = 1.7 amps, so resistance is 12v ÷ 1.7i = 7ohms. If you put them in series, the voltage available to each will only be 4v, so each will draw 4v ÷ 7o = .6 amps. They'll only be about 35% as bright. The total current in the circuit will also be .6 amps (12v ÷ 21o) and a 2-amp fuse should be sufficient but much higher would probably be ok depending on wire size.<br /><br />I'm a rank amateur when it comes to electrics, so somebody correct me if I'm wrong. thanks!

 

[jlinder]

Re: What size fuses

jtexas,<br /><br />Only problem isthat with the loweer voltage you don't have 35% of the light, you have much less.<br /><br />The wattage in eac bulb is not 20 watts, but 2.6 watts (4 volts x .6 amps). About 1/8 the power<br /><br />On top of that the output of the bulbs is shifted to be more just heat, very little light. Just a little orange glow.<br /><br />On the other hand the bulbs will last forever.

 

[Silvertip]

Re: What size fuses

JT - boat lighting (or any other accessory) is never wired in series (except if you add the old Christmas tree lights)for a boat parade. Remember when one went out they all went out?

 

[--GQ--]

Re: What size fuses

 

[artherm]

Re: What size fuses

I have worked with several licensed electricians. I agree with Jack L. When laying out a circuit you first determine your wattage/amperage load. You then size your wire size to that load. You then properly size the fuse/circuit breaker to the wire size. This is right out of the NEC code book, and will keep you out of trouble.