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  1. #1
    Bronze Member
    Join Date
    Dec 2001
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    92
    Location
    Grundy County, IL
    Tractor
    JD 755

    Default What size wire for trailer brakes?

    My 16' utility trailer has brakes on one axle. I might put brakes onto the other axle too but that's another story. It has a small gauge wire running the brakes now. It's wired half-baked in general, so I figured when I improve it I'll put the proper gauge wire for the brakes.

    So what size wire to feed two brakes, and what size to feed four? Thanks!

    Edit: I found this article which says 18 ga. for a 5-wire system, which is the type I have. So maybe mine is good to go.

    http://www.etrailer.com/faq-wiring.aspx

  2. #2
    Veteran Member Treemonkey1000's Avatar
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    Jan 2008
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    1,894
    Location
    Renton, Washington
    Tractor
    Kubota L3750

    Default Re: What size wire for trailer brakes?

    18 seems pretty small. 14 gauge or 12 gauge is what I would use myself. Easier to work with and if you nick it or it gets corrosion you still have some diameter left to carry the current.
    1st Peter 1:6-9

  3. #3
    Elite Member Skyco's Avatar
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    Jul 2007
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    2,718
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    SC/NC

    Default Re: What size wire for trailer brakes?

    4 brakes draw about 10 amps so 18g is too small IMO.

  4. #4
    Veteran Member kevinj's Avatar
    Join Date
    Jul 2003
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    1,340
    Location
    West MI
    Tractor
    BX-1860, L3940HSTC, B7500, Farmall M, Farmall 560D

    Default Re: What size wire for trailer brakes?

    #12 minimum. My local dealer has a 2 cable brake wire in 10, 12, 14 for different applications.

    http://i.b5z.net/i/u/1080235/f/Brake...agram_6-09.pdf
    Kevin

    BX1860, mower, snowblower - B7500HST, FEL, Bush Hog - Grand L3940 HSTC, FEL, snowblower, 6' Landpride brush hog, 6' King Kutter tiller, Corn Pro 18+5 Fifth Wheel, 1950 Farmall M, Farmall 560D

  5. #5
    Veteran Member K7LN's Avatar
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    Feb 2009
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    1,545
    Location
    Michigan
    Tractor
    JD455 & JD790

    Default Re: What size wire for trailer brakes?

    According to the AWG tables for chassis wiring, #18 is good for 16 amps in open air (unbundled). It drops to 2.3 amps for power transmission (transformers). The voltage drop isn't too bad for the short distance involved, but I'd be a whole lot happier with #14 for 2 brakes and #12 for 4 brakes.
    JD 790 w/70 FEL & 7 BH on turf tires
    JD 455 w/60" MMM & 54" front blade

  6. #6
    Super Star Member Diamondpilot's Avatar
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    Jan 2007
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    15,305
    Location
    Daleville, IN
    Tractor
    Jinma 254/284 Ford 861 Powermaster at work

    Default Re: What size wire for trailer brakes?

    We use #10 at my shop to rewire brakes. #18 is fine for the lights.

    Chris

  7. #7
    Platinum Member
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    Mar 2008
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    556
    Location
    Colorado
    Tractor
    TN55

    Default Re: What size wire for trailer brakes?

    Go to 6, or even better, 7 pole wiring (the defacto standard today). Add back up lights to the rear of the fenders, pointed out about 30 degrees. You will not be disappointed. Don't put too large a wattage lights or you'll blow the factory fuses.

    Use a gauge thicker ground than the thickest signal wire, as all of the signals add up, makes sure the worst case will work well. And wire all the lights and brake ground to a common bolt near the tongue. Simplifies troubleshooting, improves current flow/continuity.

    Brakes don't care which wire is hot, and which one is ground.

    Personally, I have gotten to where I use sealed lights (already have a ground wire on them), a terminal strip at the tongue in a weather tight box using water tite fittings (you can find these for trailers, but you can use the standard electrical stuff at Lowes Depot). 3M makes an insulation penetrating splice connector (similar to telephone wiring connectors) filled with silicon grease. Works great, no stripping required, connects two or three wires. Sometimes Walmart stocks them, otherwise look around for a box, as more than a couple, you don't want to buy them at the auto parts store.

  8. #8
    Veteran Member dstig1's Avatar
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    Apr 2010
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    W Wisc
    Tractor
    Kubota L5240 HSTC, (Kubota L3130 HST - sold)

    Default Re: What size wire for trailer brakes?

    Quote Originally Posted by Diamondpilot View Post
    We use #10 at my shop to rewire brakes. #18 is fine for the lights.

    Chris
    So electric brakes are pulling 30A? Really? Sounds high to me... But that is the NEC spec on 10 ga. Sure, you lower the voltage drop issues, but trailer wiring distances aren't that great. Another poster noted that 4 brakes draw 10A...By all NEC codes you would be fine with 16ga for that. Me? I just rewired my recently acquired trailer (16', 2 axles, both have electric brakes) with 14ga for the brakes. Part of the issue was looking at the leads going to the brakes. They were small -probably 16. I didn't want to try and use large gage wire to feed them, so instead I split the wiring and did one axle with one run to the J-box, and the other axle with a second run. And I know that this was overkill. But it made connections easier.

    I used 16 for the lights as I could find a nice roll of 4-bonded trailer wire in that range. Yeah 18 probably would have been fine for lights.

    BTW I found these crimp on butt connectors that are also heat shrink. Those work well. Crimp on , then heat gun to shrink and seal. That makes a lot fosense for trailer work and I found them everywhere in electrical depts. I also used the 3M 806 (?) connector with the silicone goop in them for running splices. Got them from etrailer.com for not too much - you can buy individually from them. Otherwise you go to Shop 3M: Purchase 3M products online. and you can only buy a whole box of a couple hundred connectors for a couple hundred bucks... Great if you build trailers for a living I guess.

    -Dave
    -Dave

    "Being a pessimist is great. You can't lose. Either you end up being right...or you are pleasantly surprised."

    L5240HST, QA, 824 Loader, 48" Forks, 48" Grapple, rear blade, box blade, landscape rake, Ancient Farmi Skidding winch
    Trailer - 10k/16' twin axle w/elec brakes
    2005 F250 5.4V8(3V) 3.73/4wd tow vehicle

  9. #9
    Super Star Member Diamondpilot's Avatar
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    Jan 2007
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    15,305
    Location
    Daleville, IN
    Tractor
    Jinma 254/284 Ford 861 Powermaster at work

    Default Re: What size wire for trailer brakes?

    I use 10 GA because I buy it by the roll and keep it on hand to cover all the things I need. As for the connections I solder and heat shrink all. I have used the butt connectors with heat shrink and it works good also.

    Chris

  10. #10
    Platinum Member ModMech's Avatar
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    Dec 2009
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    684
    Location
    Canton, TX
    Tractor
    Kubota B2410

    Default Re: What size wire for trailer brakes?

    NEC has no impact on trailer wiring, AC and DC don't behave exactly the same. It is a LOT easier to open an AC circuit than a DC one so you need to be more careful with DC ragarding ampacity of the wiring and connectors to avoid shorts.

    If you are dealing with automotive wiring, use SAE standards and methods as outlined in various SAE papers and publications.

    For automotive wiring at 10A resistive loads, 16 Ga is really the smallest wire you should use and 14Ga would be a better choice but not needed.

    Every trailer ground should be at least 10GA and common with all other grounds connected at a central point. However, this is rarely done and is one primary reason why so many trailers have lighting problems.
    JohnG
    We have Democracy, but were founded on Liberty.
    Democracy is two wolves and a lamb voting on what to have for lunch. Liberty is a well-armed lamb contesting the vote - Benjamin Franklin (1706-1790).

    I am for LIBERTY!

    I Support electric trailer brakes on EVERY axle over the towing vehicle's GVW, CDLs for RVers and safety inspections on ALL licensed vehicles!

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