Minimum wire size

If I am not mistaken minimum wire size aboard should be awg18.
Why to have this limitation and not something more flexible relying on common sense?
I want to install a led tube above my galley countertop. The light draw 0.2A and if I follow the rule I will need to use wire that can support up to 20amps, what's the reasoning behind this?

L

Not sure exactly what you are saying. 18 gauge will not support 20 amps. If the LED light is 12 volts, it is a case of measuring the total length of the circuit and using a chart that will show what size wire need to be used based on circuit lenght and amp draw and desired voltage drop. Personally I really don’t use anything smaller tham 16 gauge. The places I can use a smaller conductor are few so I don’t stock smaller wire. It isn’t a problem to use larger wire than necessary.

Lou, check out the Blue Sea ampacity chart, it will tell you what you require. Free (although Blue Sea gets a good buck for its products).

The minimum for electric power (not data) is usually #16, not #18. The reason is not current carrying capacity, it's physical strength.

That said, as the boat's owner, you can use whatever you want to. There are no boat wiring police. Just be sure you have overcurrent protection based on the wire size installed at the source of power.

What I mean is what is the need for such size when you draw less than 1 amp? I know that going bigger is not a problem but when you try to do an aesthetic install and to hide the wire, I would prefer to use 22 awg that would be enough on the 3 feet length I need and easier to hide.
I do not understand the reasoning behind a minimal size v.s. simply using a wire that is big enough to support the load?

Lou_tribal said:

What I mean is what is the need for such size when you draw less than 1 amp? I know that going bigger is not a problem but when you try to do an aesthetic install and to hide the wire, I would prefer to use 22 awg that would be enough on the 3 feet length I need and easier to hide.
I do not understand the reasoning behind a minimal size v.s. simply using a wire that is big enough to support the load?

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Maybe you missed my post above. Or maybe you ignored it.

aboatman said:

Maybe you missed my post above. Or maybe you ignored it.

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I was typing when you posted
If I am right the e-11 state the minimum recommendation.

L

Your LED and its 18 gauge wire (more on this below) are protected by a 15 amp breaker more than likely. The wire needs to be big enough or the breaker/fuse small enough so that the breaker/fuse trips if the wire shorts to ground due to chafe or whatever. If the breaker is too big or the wire is too small, the breaker will not trip and the shorted wire could cause a fire.

18 gauge is nominally good for 20 amps so it will trip a 15 amp breaker if it shorts. I say nominally because the current carrying capacity also called ampacity is also dependent on its environment. A hot engine room environment results in lower safe current carrying capability.

If you want to use smaller than 18 gauge wire you can; just put a smaller fuse, say 5 amps where the small wire connects to the bigger supply wire or breaker.

Marinco and others have ampacity tables giving safe current carrying capability at different environment temps.

And finally ampacity is the minimum size wire to use in a circuit supplied by a certain fuse or breaker size. The other issue in sizing wire is the voltage drop. But your LED light probably draws less than an amp and that will result in minimal voltage drop.

And finally, finally- what are you tying this 18 gauge wire into on the supply side?

David

djmarchand said:

Your LED and its 18 gauge wire (more on this below) are protected by a 15 amp breaker more than likely. The wire needs to be big enough or the breaker/fuse small enough so that the breaker/fuse trips if the wire shorts to ground due to chafe or whatever. If the breaker is too big or the wire is too small, the breaker will not trip and the shorted wire could cause a fire.

18 gauge is nominally good for 20 amps so it will trip a 15 amp breaker if it shorts. I say nominally because the current carrying capacity also called ampacity is also dependent on its environment. A hot engine room environment results in lower safe current carrying capability.

If you want to use smaller than 18 gauge wire you can; just put a smaller fuse, say 5 amps where the small wire connects to the bigger supply wire or breaker.

Marinco and others have ampacity tables giving safe current carrying capability at different environment temps.

And finally ampacity is the minimum size wire to use in a circuit supplied by a certain fuse or breaker size. The other issue in sizing wire is the voltage drop. But your LED light probably draws less than an amp and that will result in minimal voltage drop.

And finally, finally- what are you tying this 18 gauge wire into on the supply side?

David

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To clarify, I want to install two led lights in a mahogany frame over my countertop. The wire will run from each light on 2 to 3 feet to the main light commands where it will connect to a 1 amp fuse and the switch ( each light is 2w so 0.16A in 12V ). I want to hide the wire in a nice mahogany rail so it does not show, so smaller is easier on the wire size. I know I can use the size I want but if I want to be compliant, if I am right, I need to use 18awg wire, which in my sense is overkill for the very small draw. The main light switches panel is powered by a 12awg (14 would be enough but 12 gives me more flexibility if one day I want become crazy about lightning lol) wire connected to a 10amp breaker on the distribution panel.
Ok I may be too anal on compliance but I want to do it right

L

Xsbank said:

Lou, check out the Blue Sea ampacity chart, it will tell you what you require. Free (although Blue Sea gets a good buck for its products).

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Thank you, their poster wit wire and fuse size is very useful.

L

16 gauge is the minimum like ab said

High Wire said:

16 gauge is the minimum like ab said

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What reinforces even more my point .[emoji2]
Or I am just becoming too old and grumpy lol

L

Never too old but definitely grumpy.

Stick whatever wire you want in there, you are the only one who will know.

Xsbank said:

Never too old but definitely grumpy.

Stick whatever wire you want in there, you are the only one who will know.

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Thank you very much I always like when you comfort me lol

L

Well, in this case I agree. With a 1 amp fuse protecting the wire, you can use just about anything you want to use, even 22 gauge.

David

Always glad to be a comfort Lou!

I think maybe the confusion is the distinction between distribution wiring size (which is 16 AWG minimum) and device wiring size (no minimum?).
Once you have landed the boat distribution wire to a terminal strip, etc, then it turns into a device wire. Lots of device wiring is less than 16. But, that is why it is usually protected by these flying fuse assemblies. VHF, chartplotter, etc.
So, if you have that lower Amp fuse close to the #16, you should be fine. If you are wiring a lamp, for instance that does not come with a flying fuse, then one should be added. I've made this a bit oversimplified, breakers can be used, and fuses don't need to be "flying", etc.

diver dave said:

I think maybe the confusion is the distinction between distribution wiring size (which is 16 AWG minimum) and device wiring size (no minimum?).
Once you have landed the boat distribution wire to a terminal strip, etc, then it turns into a device wire. Lots of device wiring is less than 16. But, that is why it is usually protected by these flying fuse assemblies. VHF, chartplotter, etc.
So, if you have that lower Amp fuse close to the #16, you should be fine. If you are wiring a lamp, for instance that does not come with a flying fuse, then one should be added. I've made this a bit oversimplified, breakers can be used, and fuses don't need to be "flying", etc.

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Ok I may have misunderstood what I read. I thought the min wire size only exception was electronic devices (radio etc) and internal appliance wiring.

Thank you for the clarification!

L

I should use the terminology used by ABYC:

k. Pigtails - External conductors that originate within an electrical component or appliance installed
by their manufacturer.

and:

h. Pigtails - Pigtails less than 7 inches in length are exempt from overcurrent protection requirements.

SO, if over 7", then that's where the flying fuse comes in.

Yep, if you run skinny leads, protect it with a low amp breaker like 1amp or 3amp. That meets the intent of the rules while maybe not the literal reading. If you are feeding the circuit from a higher amp breaker, then put an in line 1amp fuse in the line to your LED's.

When working on electrical systems , I never need to bother with "minimum wire sizes".

The bulk purchase of #10 allows the on hand wiring to be used almost everywhere , and the cost savings of bulk buying pays for the oversize wire.

Electric items do not suffer from almost no voltage drop.

FF said:

When working on electrical systems , I never need to bother with "minimum wire sizes".

The bulk purchase of #10 allows the on hand wiring to be used almost everywhere , and the cost savings of bulk buying pays for the oversize wire.

Electric items do not suffer from almost no voltage drop.

Click to expand...

In my case this is not a question of price but really size. 10awg would be massive to wire my little led lights.

L

As others have said, the required wire size is a function of the current, length of the wiring run, voltage and the acceptable voltage drop over the length of the wires. The formula for calculating the maximum length of a given size wire is:

L = (5 x x% x V)/(I x R)

where:
L is the maximum one way length in feet of the circuit
% is the acceptable voltage drop in the wires (normally 3% for lighting)
V is the voltage of the circuit (I use 12.5 for 12 volt systems)
I is the current to be carried
R is the resistance of 1,000 feet of wire.

Resistance of common wire sizes per 1,000 feet are

20 gauge 10.15
18 gauge 6.385
16 gauge 4.016
14 gauge 2.525
12 gauge 1.588
10 gauge 0.9989

The formula will tell you the maximum length of wire that you can use before the voltage drop goes over the desired percentage. Note that this formula is based on the one way length of the circuit in that the round trip length is built into it. The formula is based on electrical principles only and NOT ABYC suggestions.

Finally, the circuit breaker for the circuit should be based on the rated ampacity of the wire and the circuit should be fused based on the current requirements of the components the circuit supplies. So a circuit made from 16 gauge wire needs a 13 amp or less circuit breaker - typically a 10 amp breaker.

"In my case this is not a question of price but really size. 10awg would be massive to wire my little led lights."

True, but there is no downside .

I believe the 3 petcent drop recommended is for running lights, not any lighting cricuit.

As long as it is near 10 percent and not too close to ampacity limits, and you follow the engine room and bundled wire recommendations.... you should be fine, especially if still above the circuit protection amperage.

2 of my engineering friends have repeatedly told me the often suggested wire size for low amperage, 12V DC circuits is way larger than needed.

So I use the 3 percent rule for critical and wet area wiring, 10 percent for items that say they will run fine on 3 to 18 volts (or whatever the spread is), and somewhere above ampacity or above the 18g for signal wires or tiny draws like a few LED lamps.