Calculating the correct wire AWG for any given device

[jetboater4life]

After following another thread discussing the installation of a blue seas dual usb port (which I can't find now ) I decided to calculate what I thought the wire thickness should be. I then sent this in an email to Blue Seas and posted my question and their response below. So the point of this thread is: Do you think I'm doing this correctly or not? If not then what are your calculations on it.

My note to Blue Seas:
Hi,
I'd like your recommendation on the proper AWG for wiring this unit.
http://www.bluesea.com/products/1016/Dual_USB_Charger_Socket/featured
My calculations say an 18AWG wire can be used for a 20foot round trip distance. Here is how I think the calculation is supposed to go. Please correct me if you disagree. You need to calculate how much current the wire needs to carry. I would do this like this. There are two USB outputs on one of those plugs and I'll assume each one is capable of outputting 2.1A at 5V (the USB spec'd voltage). Converting that to watts would be V*I=W, so 5V*4.2A=21W Converting back to 12V DC coming from the battery gives us 21W/12V=1.75A. Assuming a 20 foot distance using Oms law I*R=V and targeting a 2% voltage drop at the usb plug you would solve for your max tolerable wire resistance like this 12V*0.98=11.76V -> 11.76V/1.75A= 6.72 Ohms of max resistance for the wire. From this table
http://www.cirris.com/testing/resistance/wire.html
an 18AWG wire has only 0.006385 Ohms per foot. So if your run of wire is 20 feet round trip then that is 0.0006385 Ohms/foot * 20 feet = 0.1277 Ohms. This is well below the 6.8 Ohms you need to maintain the proper voltage at the the plug. You also need to check the max current for the wire and 18AWG supports a a max current of 2.3Amps. I would use 16gauge
Here are a couple of calculators
http://www.gadgetjq.com/wiring_size_guide.htm
http://www.bulkwire.com/wireresistance.asp
http://www.offroaders.com/tech/12-volt-wire-gauge-amps.htm

Their response:
Good afternoon,
Thanks you for your question. Blue Sea systems also offers a wire sizing calculator called “Circuit Wizard” on our website. http://circuitwizard.bluesea.com/
Based on the information you’ve provided, our calculator agrees with you based on Ampacity. Since the lineal distance is 20’, the return wire will be the same length, so that’s 40’ total. With n allowable 10% voltage drop, Circuit Wizard recommends 16AWG.
It may be worth noting that ABYC recommends 16 AWG wire as the minimum size for use aboard boats. There are exceptions of course, for limited power circuits, but maybe this is a good choice for such a long wire run anyway.
I hope this feedback is helpful. Please let us know if you have any additional questions.

Engineering Manager

 

[Glassman]

Thanks for that! I'll be sure to add it to my favorites.

 

[rickd01]

Lineal?
Ampacity?

Is that part of the .....new school concept™

 

[KXCam22]

Thats pretty standard for DC wiring voltage drop where you include the return path in the calc. They allowed 10% which is extremely high in most cases but in the case of USB you only need to produce 5V output so if the arriving voltage is a bit low it shouldn't matter. A more-typical voltage drop with DC is 3%. I just built a big solar system where I used 2% everywhere on the DC side. I would tend to use #14 since I doubt #16 wire is much cheaper, unless you already have a supply. USB chargers are very handy. I might need to add one. Which one are you using? Cam.

 

[davel501]

I am guessing the tech at the dealer that used speaker wire to hook up my halogen transom lights did not use that calculator.

 

[Julian]

Adding this thread to the FAQ....

 

[jetboater4life]

Thats pretty standard for DC wiring voltage drop where you include the return path in the calc. They allowed 10% which is extremely high in most cases but in the case of USB you only need to produce 5V output so if the arriving voltage is a bit low it shouldn't matter. A more-typical voltage drop with DC is 3%. I just built a big solar system where I used 2% everywhere on the DC side. I would tend to use #14 since I doubt #16 wire is much cheaper, unless you already have a supply. USB chargers are very handy. I might need to add one. Which one are you using? Cam.

@KXCam22 I don't have any in my boat right now. I have a couple that plug into a standard cigarette lighter which we use in the car quite a bit and could potentially use that in the boat, however I think I'm going to go with the Blue Sea's one that I linked above. I would prefer to have it in white, and will probably put one in on the driver side and one near my glove compartment. Now that I'm getting more comfortable with wiring this stuff up, I might even put a new circuit with a USB charger in my car. I blew the 20amp fuse on the car cigarette lighter circuit this weekend because of all the electronics (including an DC to AC converter) my kids were running.

As far as the voltage drop, I also read that 3% was pretty standard and was surprised when I saw 10% in the Blue Sea guy's calculation. With the Ohms per foot of these marine wires, I don't see it as much of an issue. The real issue is the amount of current they need to supply.

 

[davel501]

This is another great article: http://www.boatus.com/boattech/articles/choosing-cables-and-terminals.asp

Interesting bits include glueing and heat shrinking connection and wire color matters.

Table 4: Marine Wire Color Code
ColorItemApplication
Yellow or BlackGroundReturn, Negative Mains
Light BlueOil PressureOil Pressure Sender to Gauge
Dark BlueCabin & InstrumentFuse or Switch to Lights
BrownGenerator ArmatureGenerator Armature to Regulator
BrownAlternator Charge LightGenerator Terminal or Alternator Auxiliary Terminal to Regulator
BrownPumpsFuse or Switch to Pumps
GreenBonding SystemBonding Wires (if insulated)
GrayNavigation LightsFuse or Switch to Lights
GrayTachometerTachometer Sender to Gauge
OrangeAccessory FeedAmmeter to ALternator, Generator Output and Accessory Fuses or Switches
OrangeCommon FeedDistribution Panel to Accessory Switch
PinkFuel GaugeFuel Gauge Sender to Gauge
PurpleIgnitionIgnition Switch to Coil & Electrical Instrument
PurpleInstrument FeedDistribution Panel Electrical Instruments
RedMain Power FeedsPositive Mains (particularly unfused)
YellowGenerator FieldGenerator to Regulato Field Terminal
Brown w/YellowBilge BlowersFuse or Switch to Blower
Yellow w/RedStarting CircuitStarting Switch to Solenoid

 

[steined]

Harbor Freight sells the marine guled heat shrink in a kit. It's a nice kit to have! I also use heat shrink connectors to help keep out the moisture. I ended up purchasing that duplex ANCOR wiring at HERN marina here locally, it was actually cheaper there than I could find it on the internet! In fact I am going back for more tomorrow.

 

[jetboater4life]

I'm going to try the solder, crimp, and heat shrink connectors from DelCity I bought a kit that has all the sizes I should need.

 

[Andy S]

I'm working on determining what I need for a few led projects, boat and truck, and ran across something that might be helpful.

For those that like a visual aids to help calculate wire size based on current draw, I found a chart on the the Blue Sea website, https://www.bluesea.com/resources/1437. On the site they have a link to an app you can load on your phone to do the calculation, I haven't loaded it yet so I have no comment on its usefulness.

On the bottom of the wire size web page there is a link to Part 2 which is "Select a Fuse and Fuse Holder For Your DC Product Installation" another helpful page with a chart to help determine fuse size.

 

[KXCam22]

The best wire connection is with an un-insulated butsplice and heatshrink tubing overtop to seal it. This is better than soldering. I frequently cut the plastic cover off a typical auto butsplice to get at the un-insulated one inside. Easy to do when i run out of the un-insulated ones. Inside the are the same. Did an 1100A cable at work yesterday with 1" diam conductors and the method is the same. Cam.

Ps it is usually legal to color code any wire with tape or heatshrink at prominent places.