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  1. #11
    Rolling Along Gyro Gearloose's Avatar
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    Quote Originally Posted by Luv2Ski View Post
    Do you know how the 12v feed to the breakaway switch is wired on your trailer?
    Steve, Great writeup...

    On my 303RLS the 2 Blue wire pair in the junction box do run back to the brakes, one side is to ground and the other side is to the Break Away switch and the 7 way plug for the brake controller . Somewhere in the trailer the Blue wires connect to 16Ga. Black wires that go to the brake magnets.

    I've never seen a fuse for a breakaway switch. I did check my RVIA manual and they suggested 50 amp auto-reset breakers for 10Ga. wire. Something to think about, yes there is a 10Ga. wire that comes from the battery for the breakaway switch, but the breakaway switch and wire that lead down to axles for brakes are 16ga. (at least on mine). So if you use a fuse for the breakaway switch, should it be sized for the 16Ga. wire so it doesn't make smoke? Anyway just thinking in text (out loud)....

    (Not to hijack the thread)
    I didn't like the resistance I measured to my brakes so I rewired the brakes and ran magnet coil ground directly to frame and doubled the size of wires going back to my brakes (basically paralleled the blue and black wires going back to brakes and ran the brake hot side through that. Parallel the wires through the axle also). Cut Resistance in half, and cut resistance to the right brakes. Did this project because I was getting a pull to the left under heavy braking and was thinking that the wire through the axles was to small and causing an imbalance of current to right brakes. The pull to the left seems to be gone now.

    pat
    Pat&Marlene Gyrogearloose - 2010 Itasca Meridian 34y - 6.7 w/Allison 6spd - Jeep Wrangler Rubicon - previous Reflection 303RLS

  2. #12
    Seasoned Camper Luv2Ski's Avatar
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    This question was asked in another thread here and I'm posting my thoughts here to keep the technical discussion on this thread. Keep in mind, these are MY thoughts only and I'm not accepting responsibility for designing anyone's system but my own. I'm just documenting my thoughts as I do this and reflect back on it.

    Quote Originally Posted by orbiker View Post
    What is the load of the circuit?
    4 brakes
    6 brakes
    Lippert documentation that came with my RV lists the magnets at 3.2 ohms each. Assuming they are all wired in parallel, 4 would be an 0.8 ohm load. 6 would be 0.533 ohms. The formula for parallel resistance is:

    R = 1/( 1/R1 + 1/R2 + 1/R3 ... )

    So for a battery voltage of 13.5 volts nominal, with 4 electromagnets in parallel you're looking at drawing 13.5/0.8 = 16.9 amps. For 6 electromagnets it's 13.5/0.533 = 25.3 amps. Keep in mind, this is theory. The "right way" is to take actual current measurements with your RV configured with all the usual stuff running that would be running as you go down the road. It's easy to measure current at the battery post with and without the breakaway switch pulled. The advantage of measuring at the battery post is you also capture the baseline current your RV would draw for its own needs while underway. In a breakaway event, your house battery must supply that baseline current PLUS the brake current for 15 minutes to meet the D.O.T. spec that RamGunner quotes here.

    Quote Originally Posted by RamGunner View Post
    I have heard others say that the connection to the battery must be "uninterrupted", but in a search this morning I was unable to find that wording. DOT FMSCA 393.43 seems to have the most complete wording, but doesn't state anything about the power source other than that it must provide braking for at least 15 minutes...

    (d)Breakaway braking requirements for trailers. Every trailer required to be equipped with brakes shall have brakes which apply automatically and immediately upon breakaway from the towing vehicle. With the exception of trailers having three or more axles, all brakes with which the trailer is required to be equipped must be applied upon breakaway from the towing vehicle. The brakes must remain in the applied position for at least 15 minutes.

    I see references to systems that have separate, dedicated batteries for the breakaway system as well as systems that use the main trailer batteries, but nothing relating to fusing the circuit.
    To measure the current, I recommend begging, borrowing or buying one of these. Be careful as many of the cheaper units do not measure DC current.

    Once you know the measured current the batteries need to provide (which includes the baseline RV current), you have to look at the ratings of the house batteries required to supply that current for 15 minutes. Don't look at cold cranking amps. Deep cycle battery amp hours are normally labeled for a 20 hour draw at a "useable voltage". Regular car batteries are normally labeled at a 10 hour draw also at a at a "useable voltage". For example, a deep cycle battery rated at 85 amp hours at a 20 hour rate means it will provide 85/20=4.25 amps for 20 hours at 10.5v or greater. A regular car battery rated at 85 amp hours at a 10 hour rate will provide 85/10=8.5 amps for 10 hours at 10.5 volts or greater. Typically this is a 100% discharge situation so the battery will be severely damaged if you actually do this.

    Due to a nasty phenomenon called the Peukert effect, the rated amp-hours does not apply at all levels of current draw. For instance, if you drew twice as much current, you might expect to get a useable voltage for 50% the time. In reality, it would be more like 40% of the time. Another way of saying this is that the higher the amperage you draw, the fewer amp hours you are going to get from a charge. A reputable manufacturer will go beyond the standardized amp hour battery specs described above. They'll publish charts that show voltage over time at various current draws. Or they'll provide a table of useable voltage time or amp-hour capacity at different currents. This data will likely apply to a new battery so derate accordingly if yours are old.

    If there is no additional data online, we have to get into some math to do an estimate of how long your house battery can hold the brakes while supplying a baseline load. You'll need a scientific calculator.

    -- start of math ---
    Let's compute the time your house battery can provide the needed (measured) current and see if it's greater than the required 15 minutes. In this example we'll assume a three axle trailer that needs 25.3 amps for brake current. The baseline amperage the RV draws when under tow was measured at 10A. Add them up and we get 35.3 amps. The formula for how long the house battery should provide the needed current can be estimated as:

    thours = 20(C/20i)k (for deep cycle batteries spec'ed @ 20 hr draw)
    thours = 10(C/10i)k (for car batteries spec'ed @ 10 hr draw)

    where:
    thours is the time in hours that the desired current can be provided from the house battery
    C is the labelled house battery capacity in amp-hours
    i is the current desired - i.e. the sum of brake current and RV baseline current
    k is the Peukert exponent. Use the following values for k
    1.5 for flooded batteries
    1.15 for AGM batteries
    1.25 for gel batteries
    1.05 for Lithium batteries

    So if we have an 85 amp-hours rating at 20 hour draw on a new flooded deep cycle lead acid house battery:

    thours = 20(85/(20*35.3))1.5 = .836 hours = 50.16 minutes

    This significantly exceeds 15 minutes so the house battery meets the requirement.
    -- end of math ---

    Finally, to size a self-resetting thermal circuit breaker, the current you need for the bakes (without the baseline) AND the wiring gauge of the circuit determine its size. The CB should be dedicated to just the breakaway circuit and not shared with other loads. You should not run more than 80% of the circuit breaker's rated amperage when the breakaway switch is pulled. Pat did a good job of upgrading the ampacity of the brake runs described in the post above.

    If you choose too high of a circuit breaker amperage, you run the risk of a short smoking the wiring and something catching fire. However the breaker would cycle indefinitely which lowers the "duty cycle" of current through the short until it eventually fails. My personal choice was to go with 40A Type 1. That might be a little on the high side given the small 16 ga. conductors on the breakaway switch and in the jacketed pair wiring going from the J-box above the pinbox to the brakes (thanks to Pat for sizing these out). I have to balance that concern against having confidence that the brakes will engage in a breakaway event without tripping the breaker. In hindsight, I might have been better off with a 30A breaker if breakaway testing showed that it never tripped. Given my faulty breakaway switch though, that testing didn't happen before I converted to the Titan EOH disc brakes.

  3. #13
    Site Sponsor gbkims's Avatar
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    Great information & research.
    Also good tip on CB at Autozone, thanks.

    X2 on Clampon AC/DC RMS Ammeter, had my Tenma for over 10yrs now.

    Gene
    - Gene

    Kim & Gene
    2015 Reflection 317RST
    2017 Ram 3500 CC LB 4x2 6.7 CTD AISIN 3.73 DRW Auto Level Rear Air, BD3, Prodigy P3, Aux Tank

  4. #14
    Setting Up Camp
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    Steve,

    Great write up.

    You mention a blue wire several times. Is that blue wire the one on the top left of the circuit breaker bank in your first picture? If it is not the same wire, do you perchance know what that blue wire is? On my 350m, that blue wire goes to the backup camera but I could not figure out what else, if anything, it tied to. As I am sure you are aware, none of those wires are affected by the battery disconnect switch. I have put a simple toggle switch in series with that wire so that the backup cam does not suck current from the batteries when I am boondocking or park the trailer for a couple of weeks. Was wondering what else might be disconnected if I forget to turn the backup cam back on before leaving my camping spot. I hope that it won't be the trailer brakes and/or breakaway switch.

    Thanks for the great write up and thanks for any further info you may have on the blue wire.
    Loran
    Loran and Debe
    2015 Momentum 350M (Carl)
    2015 F-350 SD Crew Cab, Platinum, (Whitey) DRW, Long Bed, B&W Companion 5th wheel hitch, Airlift LoadLifter 5000 Ultimate air bags

  5. #15
    Seasoned Camper Luv2Ski's Avatar
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    That blue wire in the pic is something else. It connects to the +12v from the tow vehicle umbilical. The blue brake wire is in the jacketed pair ROMEX type cable coming from the J-Box above the pin box. It goes to the brake magnets and is not in the picture.
    Steve and Cheryl

    2017 Momentum 328M w/Dual Pane Windows and 3rd A/C. Aftermarket mods: Titan EOH Disc Brakes, MORryde IS suspension and Reese 5th Airborne Sidewinder pin box
    2014 Ram 3500 Longhorn Megacab 4x4 DRW with 6.7 HD Cummins Turbo Diesel, AISIN trans, 3.73 axles and a Reese 20K puck mount hitch
    Call sign: AAØSB, Class: Extra



  6. #16
    Setting Up Camp
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    Steve,

    Thanks for all of your sleuthing and info on the potential safety hazards and knowledge of the GDRV wiring anomalies. Invaluable information.

    Loran
    Loran and Debe
    2015 Momentum 350M (Carl)
    2015 F-350 SD Crew Cab, Platinum, (Whitey) DRW, Long Bed, B&W Companion 5th wheel hitch, Airlift LoadLifter 5000 Ultimate air bags

  7. #17
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    as long as the wire is free air (not in a conduit or raceway ) the ampacity of the wire is increased. By code #14 is rated for 30A

  8. #18
    Seasoned Camper Luv2Ski's Avatar
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    Quote Originally Posted by tcioch View Post
    as long as the wire is free air (not in a conduit or raceway ) the ampacity of the wire is increased. By code #14 is rated for 30A
    The 16 ga. brake wire pair is romex type flat sheathing. For the span from the umbilical J-Box to the front battery compartment, it's bundled together with many other wires, held together by spiraled electrical tape and the bundle placed inside wire loom. This wire span is built similar to an automotive wire harness. I don't know if that makes the brake wires free air or not.

    Setting aside the conductor ampacity concerns, there is the matter of the ratio of the 16 ga. brake wire's pathway resistance to the very low resistance of the paralleled brake electromagnets. For 3 axle systems, the brake electromagnet load is about 533 milliohms. A reference I found has the DC resistance of 16 ga. stranded wire at 4.016 ohms per 1000 ft. I estimate the round trip of the breakaway current from battery terminal to battery terminal is 80 ft. The wiring resistance of the mostly 16 ga. wire would come to 4.016*80/1000 = 321 milliohms. Assuming that the battery voltage is 13.5v, the voltage available across the electromagnets is 13.5 x 533/(533+321) = 8.4 volts (about 62%). Power delivered to the electromagnets works out to about 133 watts. Pretty bad.

    Non-emergency braking current from the tow vehicle follows a very similar path of nearly the same length which means the electromagnets's percentage split of the variable voltage being input into the trailer's braking system is pretty much the same at 62%. This would explain why Pat saw a significant improvement in his non-emergency braking when he paralleled the positive brake wire and used the trailer frame as his neutral. This lowered the overall resistance of the brake loop thereby increasing current. It also allocated a greater percentage of the voltage to the electromagnets (close to 77% by my calculations). My power calculations estimate that before the mods, 133 watts were delivered to the 6 electromagnets. After the mods, it was closer to 260 watts.
    Last edited by Luv2Ski; 05-24-2019 at 01:24 PM.
    Steve and Cheryl

    2017 Momentum 328M w/Dual Pane Windows and 3rd A/C. Aftermarket mods: Titan EOH Disc Brakes, MORryde IS suspension and Reese 5th Airborne Sidewinder pin box
    2014 Ram 3500 Longhorn Megacab 4x4 DRW with 6.7 HD Cummins Turbo Diesel, AISIN trans, 3.73 axles and a Reese 20K puck mount hitch
    Call sign: AAØSB, Class: Extra



  9. #19
    Site Sponsor Cate&Rob's Avatar
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    Somehow . . . I missed this discussion, going all the way back to 2017! I will modify the title slightly to make the discussion in this thread easier to find. This is the same situation that I identified during my EoH disc brake installation. I resolved this in the same way . . . by putting a 40A circuit breaker in the emergency apply feed to the hydraulic actuator (which I measured as drawing 28A max) https://www.mygrandrv.com/fo...-System-Wiring

    Rob
    Cate & Rob
    2015 Reflection 303RLS

  10. #20
    Site Sponsor gbkims's Avatar
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    The duplex jacket wire I think is automotive brake cable.
    http://truckpartsetc.com/sales/PDFs/..._WireCable.pdf
    Duplex Brake Cable
    • Duplex brake cable is made with two polyvinyl chloride (PVC) insulated general purpose primary wires laid parallel and covered with a high quality 30 mil polyvinyl chloride jacket.
    • Internal wire is rated for 12V DC and meets S.A.E. J1128 specifications.
    • The jacket provides a compact cable and protects the insulated conductors against weather and acid effects.
    • Color code: black and white with gray outer jacket.
    • Sizes 12 and 10 gauge are recommended for electric brake wire.
    - Gene

    Kim & Gene
    2015 Reflection 317RST
    2017 Ram 3500 CC LB 4x2 6.7 CTD AISIN 3.73 DRW Auto Level Rear Air, BD3, Prodigy P3, Aux Tank

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