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Old 12-01-2020, 02:47 PM   #1
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ABYC wire AWG calculator

Hi all,


When I was rewiring my boat, I made a wire gauge calculating spreadsheet to help with determining exactly how much of my paycheck was going to go into copper. I adapted it for wiring my Sprinter conversion, and figured maybe it might be of interest here. I'm an EE, so you can trust me. (this is a lie, don't trust EEs, trust EEs who specialize in vehicle wiring -- which I am not. Use at your own risk.)

https://docs.google.com/spreadsheets...it?usp=sharing




You give it the run length, whether it's one-wire or duplex, the maximum current, and whether it's critical or not. Critical means "I need this equipment to operate the vehicle safely" and is a bit subjective. In critical circuits the allowable voltage drop in the wire is 0.3V, vs 1.2V for non-critical circuits.

The spreadsheet calculates the allowable resistance based on that voltage drop, and uses the resistivity to calculate a required gauge. It also calculates the required gauge from the ABYC chassis wiring ampacity tables. Then you give it the actual AWG you plan on using, and it tells you how much voltage and how much power you will burn in the wire. It will highlight your selected AWG in red if you violate either the ABYC chassis wire current limit or the allowable voltage drop in the circuit. This lets you make more intelligent decisions about where you want to cut corners.

ABYC (American Boat and Yacht Council) standards are the default in marine wiring, and they translate well to RVs.

Most people just use the ABYC tables. This is generally fine, but they don't account for every situation. In some situations (long runs of bundled wires behind walls) they are justifiably conservative. In other situations (short, high-current bus wires) they end up giving laughably large gauge recommendations.

Don't use this blindly: make a holistic analysis of your wiring diagram, summing currents from the outside in, to make sure you aren't going to overload a circuit. And always ensure that every circuit is fused well below the current limit of your wire.


Hope it's helpful for you.
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Old 12-02-2020, 09:54 AM   #2
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I'd NEVER argue with an EE, been there done that some years ago almost daily. Throw in UL standards and you've got one helluva riot going on there.

I would add double consider any sort of disconnect connector OR a buss bar, switch etc into the maximum current capacity you'll experience. Those too have their max amps ratings. A good example of this is the heater motor blower switch inside the E-Series cabin. After a time running the blower at high speed the switch melts needing replacement, often times the chassis wiring connector as well. It seems that switch and connector are just not rated to handle the blower's amp draw over time. I've rarely had one last 5 years during winter/summer "normal" use.

We should also note the blade fuses used most commonly in vehicles can "wear out" over time as well--they're not seeing current overloading so much as just showing the poor quality most have become.

I'd also suggest most DIY's dealing with electical issues or designs get some sort of current registering meter like this: https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

I have several including one for the Maxi-Fuse up to 60 amp draw.

HTH
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Old 12-02-2020, 12:17 PM   #3
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All good advice. I've more or less abandoned fuses entirely, except for a big 200A MEGA or class T on the batteries themselves. Breakers cost more up front, but are so much more convenient -- you get a switch in the bargain, and you don't have to bust your fingers pulling ATC fuses after you lose the plastic puller dingus, or swap a 20A into a 5A circuit because you ran out of tan ones.



I did the boat with the usual Carling breakers (the ones Blue Sea sells), but for the van I've moved over to using MCBs like this:

The advantages are many -- they break faster (magnetic), more reliably (thermal), they slot onto a DIN rail so I don't have to cut a custom panel, and my favorite part, you can use what are called shunt trips to trigger them. A shunt trip is a relay that clips to the side of the breaker. When you energize the relay, it trips the attached breaker. This opens up all sorts of opportunities -- for instance, when the battery breaker trips, so do my MPPT and DC/DC breakers, ensuring they don't drive a disconnected bus. Any sort of fault condition that can be made into a signal can trip the associated breaker, making things safer and more predictable. Carling makes similar hardware, but you can't buy it from Blue Sea.



You can also buy bus bars which fit across a stack of MCBs, so I don't have to crimp and wire each switched terminal of the breakers onto a screw-down bus bar.


I know it runs against the grain of the accepted ATC fuse and Carling breaker solutions, but I think it's significantly better and safer.
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Old 12-02-2020, 01:59 PM   #4
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Quote:
Originally Posted by JWA View Post
A good example of this is the heater motor blower switch inside the E-Series cabin. After a time running the blower at high speed the switch melts needing replacement, often times the chassis wiring connector as well. It seems that switch and connector are just not rated to handle the blower's amp draw over time. I've rarely had one last 5 years during winter/summer "normal" use.
I didn't realize this is a thing! When I first had my dash off to address a different issue (broken 12V socket retaining clip), I was surprised to find my fan switch melted. I replaced it with a new OEM unit, but it sounds like I'll have to keep an eye on that.

I like the idea of the MCBs, although I have no plans to go back and replace my Blue Sea breakers (mine are the pushbutton type). I imagine it adds considerably to the cost though?
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Old 12-02-2020, 02:48 PM   #5
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Got mine mostly on eBay for around the price of Carling breakers. The shunt trips, bus bars, and associated hardware definitely adds to the cost, though.


Switches will often melt when the spade terminals corrode a bit and increase the connection's resistance. You'll see the same thing on old fuse boxes -- the connectors corrode a bit, add some heat, and suddenly your 10A fuse pops at 5A. Best practice is to use emery paper to clean both the switch and wire terminals until shiny, then reassemble with silicone grease or SS-30.
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Old 12-02-2020, 07:09 PM   #6
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No doubt on the corrosion, although it is extremely difficult to clean the harness side terminals. I usually resort to some electric cleaner and repeated plug/unplug to cut through any corrosion, but it's not ideal.

Judging by the thick layers of soft dirt found in every crevice when I bought my van, it lived a dirty/dusty former life. Aside from the blower switch, I've also had to pull apart the radio faceplate to clean all the button pads and the volume pot. So. Much. Cleaning.
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