Debating the wire size needs...LED

[Rowglide]

OK so here is the layout.
I have a pair of Signal Mate LED port /starboard nav lights. They are to be fed with a pair of wires (+,-) to the bowsprit. The actual distance is 38' so X2 for pos and neg... so 76 feet.
The Amperage is minuscule! The current draw is 0.100A at 12VDC per lamp so 0.200A.
Using standard "wire size chart".(I used the one from Blue Sea Systems) There is a 5 Amp minimum load shown on the charts. As Navigation lights, 3% voltage drop is max allowed.
at 76' that becomes #8AWG wires...? Am I missing something here? It seems to me that the relatively small amperages of LED lights need to be addressed.
Is there a formula that would allow me to calculate a more suitable wire size that would also be acceptable under ABYC standards?
Jim

[Kelkara]

Originally Posted by Rowglide

Is there a formula that would allow me to calculate a more suitable wire size that would also be acceptable under ABYC standards?

The formula you're looking for is simply Ohm's law: V=IR.

V = 3% of 12V = 0.36V
I = 0.2A
R = V/I = 1.8 ohms.

This is the maximum allowable resistance of the wire in your circuit for a 3% voltage drop at 0.2A

Then it's a question of selecting a wire that has less than this resistance for a run of 76ft.

which according to this calculator: Wire Resistance Calculator & Table would be AWG23 wire ... which is too small for other reasons ... so basically you can get away with whatever boat appropriate wire that you have 76ft available.

discalaimer: I'm not an electrician.

[DeepFrz]

Blue Sea has an app that can help you with this. The size is 18 but Blue Sea recommend 16 gauge minimum unless inside a conduit or wire bundle.

The web app didn't seem to work for me but the Android app worked okay.

[Rowglide]

Awesome,
Thanks
I hadn't seen that app anywhere, but now that I have it my life will be much easier. I am always willing to err on the side of safety, so 16AWG will be the wire of choice. Beats looking at 5A charts, that was just getting scary!
Jim

[Lake-Effect]

Originally Posted by Rowglide

OK so here is the layout.
I have a pair of Signal Mate LED port /starboard nav lights. They are to be fed with a pair of wires (+,-) to the bowsprit. The actual distance is 38' so X2 for pos and neg... so 76 feet.
The Amperage is minuscule! The current draw is 0.100A at 12VDC per lamp so 0.200A.
Using standard "wire size chart".(I used the one from Blue Sea Systems) There is a 5 Amp minimum load shown on the charts. As Navigation lights, 3% voltage drop is max allowed.
Is there a formula that would allow me to calculate a more suitable wire size that would also be acceptable under ABYC standards?
Jim

I've heard that the ABYC will at some point update their chart to add lower current values, since LEDs are becoming so common.

Anyway, all these charts start with knowing the resistance per length of different wire gauges.

According to this chart, here are a couple of sample values:

• 16 AWG stranded: 4.03 ohms per 1000 ft => 0.00403 ohm per ft
• 18 AWG stranded: 6.54 ohms per 1000 ft => 0.00654 ohm per ft

Back to your requirement: 200mA, 76 ft, less than 3% drop.

First thing I'm going to do is round up:

• current to 0.5A, cos maybe you'll change the fixture down the road
• length to 80 ft

3% of 12v is 0.36v. Ohms law says that if there's 0.5A of current, then you can find the resistance to produce that drop as:


E/I = 0.36/0.5 = 0.72 ohm


So the wire run must have less than 0.72 ohms to drop less than 3% of 12v.


80 ft of 18 AWG would drop 80 x 0.00654 = 0.5232 ohms...


So you could get away with 18 AWG. But there are other qualities of the wire to consider: how easy it is to pull and to crimp connectors on, how it will stand up to bumps and knocks, etc.


Myself I would use 16 AWG for this.

[ok so I type slower than everyone else ...]

[Rowglide]

16 it is! I agree, and besides with fat clumsy fingers like mine, handling slightly heavier wire will be easier.

[Dave Lochner]

Originally Posted by Rowglide

16 it is! I agree, and besides with fat clumsy fingers like mine, handling slightly heavier wire will be easier.

Not to be insensitive to my northern neighbor, but this is precisely the reason that 16 ga is the minimum. Smaller diameter wires are just too fragile for marine use unless they are in a cable, like a microphone cable, ethernet, or N2K.

[S/V Alchemy]

Originally Posted by Rowglide

Awesome,
Thanks
I hadn't seen that app anywhere, but now that I have it my life will be much easier. I am always willing to err on the side of safety, so 16AWG will be the wire of choice. Beats looking at 5A charts, that was just getting scary!
Jim

Consider running 12 ga. to a breakered DC subpanel in the cabinetry in the saloon. Run all the saloon lights and the bow lights from here. Shorter total runs and the whole circuit is protected by whatever amperage breaker is appropriate. You just have to learn to switch on your nav lights from the middle of the boat.

I have always wondered why, on aft cockpit boats, the panels are all 3/4trs of the way aft. The logical place for the breakers is in the saloon, close to the mast, with equal runs fore and aft. The distance from the batteries can be compensated for by upsizing the cabling, but it's not nearly the problem it was with the phasing out of incandescents.

[Redline452]

In general (and as they say, "all generalizations are dangerous, including this one"), LED lights need so little current that you will be sizing wire for mechanical strength, and the ability to install terminations (lugs, connectors etc) rather than actual current-carrying capacity. #16 is good, 318 about as small as you'd want to handle, though you could go even smaller in a multi-conductor cable.

[Teknishn]

Originally Posted by Redline452

In general (and as they say, "all generalizations are dangerous, including this one"), LED lights need so little current that you will be sizing wire for mechanical strength, and the ability to install terminations (lugs, connectors etc) rather than actual current-carrying capacity. #16 is good, 318 about as small as you'd want to handle, though you could go even smaller in a multi-conductor cable.

Nailed it! Mechanical strength, crimps and terminations, and multi-conductor cabling. Oh, and use proper crimping tools with marine grade wire and terminations.

[foufou]

I find that crimping wire smaller than 16 gauge is very iffy even when doubling it. Go with 16 or larger

[Redline452]

Originally Posted by Redline452

In general (and as they say, "all generalizations are dangerous, including this one"), LED lights need so little current that you will be sizing wire for mechanical strength, and the ability to install terminations (lugs, connectors etc) rather than actual current-carrying capacity. #16 is good, 318 about as small as you'd want to handle, though you could go even smaller in a multi-conductor cable.

Aarrgh - the typo fairy strikes again. That's #18, not 318! .
The latter would be very very thin wire indeed, if it was a gauge.

[Cadence]

Originally Posted by Rowglide

OK so here is the layout.
I have a pair of Signal Mate LED port /starboard nav lights. They are to be fed with a pair of wires (+,-) to the bowsprit. The actual distance is 38' so X2 for pos and neg... so 76 feet.
The Amperage is minuscule! The current draw is 0.100A at 12VDC per lamp so 0.200A.
Using standard "wire size chart".(I used the one from Blue Sea Systems) There is a 5 Amp minimum load shown on the charts. As Navigation lights, 3% voltage drop is max allowed.
at 76' that becomes #8AWG wires...? Am I missing something here? It seems to me that the relatively small amperages of LED lights need to be addressed.
Is there a formula that would allow me to calculate a more suitable wire size that would also be acceptable under ABYC standards?
Jim

Jim,

I would not worry about wire size electrically. I would worry about the size needed for physical fatigue.

[SteveSails]

The 3% voltage drop rule is not critical with good quality LEDs like Signal Mate, they will work from 9V to 36V with constant brightness. The circuitry in the fixture maintains constant power so these LEDs actually draw more current at low voltage than higher voltages, just the opposite of an incandescent bulb. A #16 wire is adequate but what size breaker drives that wire? Reduce it to a 5Amp breaker or add a fuse to LED circuit.

[evm1024]

Also consider that the wire size should be large enough so that the breaker trips (at the source end) long before the small wire goes up in flames should there be a short somewhere along its length.

16 Gauge is the minimum size spec'ed by ABYC for that reason as well as the mechanical reasons noted.

As an example here are the resistances of 100' of various wire gauges taken from some random source (YMMV) along with the currents that would flow through that 100' loop at 12.8 volts.

18 gauge 0.639 ohms 20 amps
16 gauge 0.402 ohms 32 amps
14 gauge 0.253 ohms 50 amps
12 gauge 0.158 ohms 81 amps

So with a 100' loop of 18 gauge wire Shorted at one end and fed from a 10 amp breaker at the other we could expect the current through the loop to be limited to 20 amps by the wire resistance alone.

Looking at the A-series breaker Trip Delay Curve I see that it could take up to 20 seconds for a 10 amp breaker to trip when passing 20 amps. 20 seconds is plenty of time to start a fire....

With 16 gauge wire (and 32 amps) that same breaker would trip within 4 or 5 seconds and with 14 gauge wire and 50 amps that trip time would be 1 or 2 seconds. Of course those are worst case times with it more likely to trip in a shorter time.

When I first started to put in LED lighting I thought to use small gauge wire and use smaller breakers till this was pointed out to me. Now I tend to feed with 14 gauge for everything that doesn't need more current (I bought a 500' spool or was it 1000') and set the breaker size as needed. For small loads like LED in lockers I put a small inline fuse where the 14 gauge feed switches over to pigtails for the LEDs.