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Old 02-21-2024, 10:53 AM   #1
Mini-Skoolie
 
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Wire gauge selection

Hi all. Couple questions on selecting the correct gauge wire.

For 12v outlets what is a good amp goal?

When running wires for switches does it need to match gauge of power wire or can it be smaller?

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Old 02-21-2024, 11:21 AM   #2
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First thing you need to know is what is the amperage of the potential loads on each 12v circuit.

Draw a simple map and plug in the numbers but generally speaking, a 16 ga wire at 12v can handle about 10 amps.
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Old 02-21-2024, 11:50 AM   #3
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Yeah but distance is a big factor too right? For example if I want to put a 12v outlet in bedroom at rear. Rough estimate of 60’ round trip. Using online calculator that gives me 6awg!! Haha.
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Old 02-21-2024, 12:37 PM   #4
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Google "12v ampacity chart" and you'll find lots of resources like this one: https://www.bluesea.com/support/arti...r_a_DC_Circuit
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Old 02-21-2024, 01:24 PM   #5
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Wire Guage

Another thing you want to mke sure the wire you use is 100% copper.. I got some fuse links that said 12AWG Copper well it's 50% or so Copper the other 50% was Tin Plating. So about half the current of real 100% Copper..

.
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Old 02-21-2024, 01:52 PM   #6
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Quote:
Originally Posted by Vapor Ghost View Post
Another thing you want to mke sure the wire you use is 100% copper.. I got some fuse links that said 12AWG Copper well it's 50% or so Copper the other 50% was Tin Plating. So about half the current of real 100% Copper..

.
Ok yeah thanks. I’m using all ancor marine grade wire. Expensive but best quality
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Old 02-21-2024, 07:38 PM   #7
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Originally Posted by wencington View Post
Yeah but distance is a big factor too right? For example if I want to put a 12v outlet in bedroom at rear. Rough estimate of 60’ round trip. Using online calculator that gives me 6awg!! Haha.
As Ewo1 mentioned, both issues (heat and voltage drop) are very dependent on amps.

For heat issues, a 12 gauge copper wire with insulation rated at 90 DegC can handle about 40 amps if in free air and 30 amps in conduit with 1-3 conductors. That heat limit is independent of wire length as the whole wire will get hot - close to 50 degC above ambient temp at 40 amps.

But voltage drop depends on both amps and length. For critical loads, 3% voltage drop is OK (12V dropping to 11.6) and for non-critical loads (lights) a 10% drop can be ok (12V to 10.8V). Back to the example above, a 12 gauge wire running 40A can run just 6 feet before reaching 3% drop and a 10% drop happens in about 20 feet. But 60 feet of 12 gauge wire will stay under 3% drop if the amperage is under about 4 amps. So it will depend what you are plugging into that 12V circuit on the far side. A water pump drawing 12A and limiting drop to 3% would need 6 ga wire (drop of ~2.3%). (If the pump will work fine with a 10% drop, 12 Gauge wire will be OK)

So for short wire runs, you can just focus on the wire's amp limits based on temp thresholds. For longer runs, voltage drop comes into play.
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Old 02-21-2024, 09:08 PM   #8
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Ambient Temps, Wiretypes

Terrific post, Jono. Always a pleasure to read your well articulated explanations. I'll add tables below & offer some considerations regarding the ambient temperatures & wire type.

--------------------

Following the most common advice, “buy only high-quality marine-grade wire", is ambiguous and can be unnecessarily expensive.

Other suggestions are more explicit, recommending tinned multi-strand “Boat Cable” labeled “UL 1426 Type III,” which indicates that a wire is finely stranded (Type III) and complies with Underwriters Laboratory (UL) standard UL 1426, the wire standard most commonly applied to boats.

When it comes to a vital systems component, paying more for a high-quality product is excellent advice. However, it is important to recognize that the term “marine-grade” is a marketing construct and there are a variety of wire & cable types which meet or exceed our needs on skoolies.

Cable (multiple conductors, or a group of wires, encased in sheathing) does not require conduit & can be run free air or incased in foam. Often, both concealed & exposed applications are acceptable.

Wire (single electrical conductors) ought to be inside of conduit. It's not engineered to be incased in foam nor installed free-air. Both exposed & concealed applications ought to be inside of conduit.

The air around the conductor dissipates the heat. The conduit provides the air and protects the wire from the physical world.

The wire type you selected falls under the 75C° column of the ampacity table. The conductor's ampacities are measured at 86°. See the note at the bottom of the table for temps other than 86°.


As wire temps increase, the wire's potential to carry ampacity is reduced. Our bus skins are already hot and wire, carrying current, produces yet more heat.

(To derate the wire's ampacity, locate the ambient temp from the table, then multiply by the factor above.)
For instance, a steel roof bus at 120° the factor is .75 for 75°C wiretype


Please take a few minutes to read through the Electrical thread in my signature below. Also this link naffainc.com/NEC_300-4_Protection_From_Physical_Damage.
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Old 02-21-2024, 10:54 PM   #9
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Conduit (minimally wire loom) is a really good idea. Be aware that more than three
conductors in a conduit will also have to be de-rated for ampacity because of heat. The charts and factors come from the NEC, but lots of available info on searching. Quality wiring = quality sleep.
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Old 02-21-2024, 10:56 PM   #10
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Thank you and Jono both so much for the detailed information! I’ll review tomorrow when I’m back at bus and respond



Quote:
Originally Posted by DeMac View Post
Terrific post, Jono. Always a pleasure to read your well articulated explanations. I'll add tables below & offer some considerations regarding the ambient temperatures & wire type.

--------------------

Following the most common advice, “buy only high-quality marine-grade wire", is ambiguous and can be unnecessarily expensive.

Other suggestions are more explicit, recommending tinned multi-strand “Boat Cable” labeled “UL 1426 Type III,” which indicates that a wire is finely stranded (Type III) and complies with Underwriters Laboratory (UL) standard UL 1426, the wire standard most commonly applied to boats.

When it comes to a vital systems component, paying more for a high-quality product is excellent advice. However, it is important to recognize that the term “marine-grade” is a marketing construct and there are a variety of wire & cable types which meet or exceed our needs on skoolies.

Cable (multiple conductors, or a group of wires, encased in sheathing) does not require conduit & can be run free air or incased in foam. Often, both concealed & exposed applications are acceptable.

Wire (single electrical conductors) ought to be inside of conduit. It's not engineered to be incased in foam nor installed free-air. Both exposed & concealed applications ought to be inside of conduit.

The air around the conductor dissipates the heat. The conduit provides the air and protects the wire from the physical world.

The wire type you selected falls under the 75C° column of the ampacity table. The conductor's ampacities are measured at 86°. See the note at the bottom of the table for temps other than 86°.


As wire temps increase, the wire's potential to carry ampacity is reduced. Our bus skins are already hot and wire, carrying current, produces yet more heat.

(To derate the wire's ampacity, locate the ambient temp from the table, then multiply by the factor above.)
For instance, a steel roof bus at 120° the factor is .75 for 75°C wiretype


Please take a few minutes to read through the Electrical thread in my signature below. Also this link naffainc.com/NEC_300-4_Protection_From_Physical_Damage.
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Old 02-22-2024, 02:44 PM   #11
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Wire Guage

I as a rule go about 25% over the wire guage needed.. That way you wont be wondering could I have done that better and safer.. No little naging thoughts should I have done that different..
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Old 02-22-2024, 02:51 PM   #12
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Quote:
Originally Posted by wencington View Post
Hi all. Couple questions on selecting the correct gauge wire.

For 12v outlets what is a good amp goal?

When running wires for switches does it need to match gauge of power wire or can it be smaller?
Well you got a whole bunch of good technical answers...so where is your diagram ?

What you gonna design???
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