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Old 26-03-2010, 17:30   #1
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AWG 6 Gauge for 30 Amp Iota Charger ?

I just bought some 6 AWG for my 30 amp Iota charger. The guy at West Marine looked at me like I was crazy when I said I was getting 6AWG for carrying 30 amps. Am I wrong here? Is that way overkill?

As an example he showed me the ProSport charger, they have 6 12AWG leads that go to - up to - 3 battery banks. He said "Yeah see this is a 20 amp charger and they only need these skinny little wires to carry that" I said, "yeah but each one of those leads only carries 10 amps to each bank." He then pointed to the box and said, "No see, 20 amps". Sometimes I am a little skeptical about how much those guys know. I have met some very knowledgeable ones and some that I am a little unsure of. The questionable people always seem to be the ones that talk the loudest.
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Old 26-03-2010, 17:37   #2
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Rest assured, it's not really overkill. You'll have lower voltage drop at the batteries, and that's always a good thing.

BTW, I use AWG6 routinely to install marine SSB transceivers....for runs up to about 20' each way. Marine SSB's typically draw 30A at voice peaks.

Oh, yeah....I've got two Iotas. The 45A model is used to charge the two T-105's which run all my radios at my home/shop. I use AWG6 cable between the charger and the battery...the one-way run is less than 2 feet! The 55A model is used on the boat to charge the two T-105's for my windlass. The run is about 4' each way and I use AWG4 cable for that installation.

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Old 26-03-2010, 17:44   #3
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Hi unbusted,

I tend to share your scepticism when it comes to salespeople's technical knowledge. Some are excellent, others... well, sometimes they just grab kids outside the high school.

Your wiring has to be sized according to both the maximum current and the round-trip distance to and from the device. Standard tables to look up the proper size are published by the ABYC and re-printed in most books on boat maintenance and repair. Most applications use the table for 3% voltage drop; non-critical things like cabin lighting or winches can use the tables for a 10% drop.

Using your 30 amp charger as an example: 10 gauge wire is good for a 10' round trip, 8 gauge will get you a 15 foot round trip, and the 6 gauge you selected will give a 20' round trip before the voltage drop in the wires exceeds 3%. Going to 4 gauge would give you a 40' round trip from charger, to battery, and back.
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Old 26-03-2010, 17:48   #4
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Hi unbusted,


Using your 30 amp charger as an example: 10 gauge wire is good for a 10' round trip, 8 gauge will get you a 15 foot round trip, and the 6 gauge you selected will give a 20' round trip before the voltage drop in the wires exceeds 3%. Going to 4 gauge would give you a 40' round trip from charger, to battery, and back.
I like being right. The trip is only about 11 ft but what the heck. It's only hurting my wallet right? Thanks guys.
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Old 26-03-2010, 17:51   #5
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Going a size up won't hurt the devices. And remember, wiring run lengths are cited in the tables as the total distance there and back- after cutting the distance in half (to get the one-way length) and then cutting it by another third (because for some reason you can never run cables in a straight line), it's surprising just how big a cable you need to span what looks like a pretty short hop.
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Old 26-03-2010, 17:53   #6
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it's surprising just how big a cable you need to span what looks like a pretty short hop.
Tell me about it I keep cutting it and cutting it and it still won't reach the battery terminal.
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Old 26-03-2010, 18:03   #7
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Matt's explanation and advice is right on the mark for most applications. However, for battery charging you need to keep in mind that a 3% drop is significant and, IMHO, far too much. Further, there are the usual problems of surface corrosion, bad or loose connections, etc. which seem to crop up more than they should.

Example: if your battery charger is putting out 14.4 volts and there is a 3% voltage drop in the cabling to the battery, that's 13.96 volts at the battery.

The difference between the open circuit voltage of a fully-charged flooded battery and one which is 50% depleted is.......just 3% (12.6 and 12.2 volts, respectively).

Where does the lost 3% go? It's converted to heat in the wiring and connections, and is lost. Not a big deal, but while you're at it why not make it as good as you reasonably can?

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Old 26-03-2010, 19:21   #8
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Here's the chart. Print it out and carry it like your organ donor card. Wire run is measured as both up and back to the unit being wired. Calculate Foot Amps like this: Total round trip distance X Amps = FAmps
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Old 26-03-2010, 19:43   #9
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The trip is 11 feet, so that's 22 feet because you measure it round distance.
With 6AWG cable carrying 30A you will have a voltage drop of about 0.26 volts.

That is equal to a 25% drop in the output from a "12 volt" battery. Or, your charger has to work 25% harder to charge the battery, or the battery will never quite be fully charged. However you want to look at it.

In the best case if your charger has a SEPARATE SENSING LEAD, then you can ignore the voltage loss in the cabling and just let the charger overheat as it works harder than it has to. In the worst case...Well, who cares if a battery is charged to 12.4 or 12.8 instead of 12.6 anyway? (Not!) Undercharged, overcharged, all good ways to sell new batteries before their time.
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Old 26-03-2010, 20:20   #10
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For what it's worth I've tended to over spec my wiring. The old wiring on this boat had corroded to the point where some melting had occured. On a friends boat he found some paper beginning to seer close to old wiring. The wiring was quite adequate when installed I'm sure but corrosion increases resistance and there doesn't seem to be a shortage of corrosion on sea going vessels.
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Old 26-03-2010, 20:38   #11
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The trip is 11 feet, so that's 22 feet because you measure it round distance.
No no it is 11 ft round trip 5 ft to one side and 6 to the other (two 6 volts mounted port and stb). But I am a little confused, the thicker the wire the greater the voltage drop? Christian, that chart means nothing to me. : ) It might as well be in Spanish. Can you explain it? What is meant by Ampacity? On other charts I have googled I have seen 6 AWG rated at 37 amps, see: http://www.powerstream.com/Wire_Size.htm

Thanks for your help guys.
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Old 26-03-2010, 20:52   #12
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Hola Unbusted! Como esta? Look, take the total wire run round trip. In your case we'll say thats 11 feet. Multiply by total amp output or draw. Total = FAmps, or Foot Amps. Enter the chart under the "12 Volt" heading for 3% volt drop. scroll down 'til you find the FAmps number closest to but higher than the total you came up with using the equation. In your case thats 330 FAmps... 11' X 30 amps = 330 FAmps. Closest higher FAmps on the chart is 348. Now scroll left til you come to the wire size directly across from 348...AWG 10 wire. TAH DAH! For the maximum voltage drop of three percent, you need to use size 10 AWG wire for an 11 foot run. Comprende mi amigo? PS, as you scroll left, make sure your amp requirements are under the ampacity numbers mentioned there. In your case it reads 51 amps if run in the engine room, 60 outside the engine room area. Via con queso, mi amigo!
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Old 26-03-2010, 21:17   #13
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Ampacity generally refers to the "amperage capacity" that a wire can safely carry for extended periods. That varies with things like ambient temperature, so a cable in an engine space would have less "ampacity" than one running in a cooler area.

If your wiring run is 11 feet round trip, not 22, then your voltage drop would be half as much also, i.e. 0.13V instead of 0.26V. Much better, if still not ideal.

Thicker cables have less resistance, and higher carrying capacity.

There are gobs of online charts and interactive calculators to figure these things, even in English.<G> When you are specifying wires for distribution runs to lights, radios, etc., the generally accepted standard on 12V systems is to use a wire size which can safely carry the load, and will not cause more than a 3% voltage drop. So if your battery is putting out 12.4V, you expect the lighting to see about 12.08 volts after the 3% drop in the wiring. Which is generally acceptable--but sometimes you may choose to do better. For instance, if you are pushing your battery capacity and often sailing at night, you might want less voltage drop, so your depleted batteries would still supply more voltage to make your nav lights BRIGHTER. Or, to provide more power to your radio. Or to the starter. Or from the charger to the batteries.<G>
Then there's the racer's point of view: That wire is damned heavy, use something smaller!

Decisions, decisions.
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Old 27-03-2010, 04:54   #14
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Jentine had it posted here....

Wire sizer software

This is the direct link to the most recent version.

Wiresizer 3.o

Great little utility for computing wire sizes.
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Old 27-03-2010, 05:47   #15
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Check out the blue sea systems site they have a wire calculator there sized for 3% and 10% drop.
My attitude is "the bigger the better".
But large wire is not all there is to it.
Proper connections are a MUST. This includes proper terminals, proper crimping and then heat shrink to seal it in. I also use Blue sea thermal paste in the connection as well to seal out moisture. And try not to use the engine room for wire runs unless you have to.
Also secure the cable runs to prevent motion and therefore early death of the wire from vibration.

http://beta.circuitwizard.bluesea.com/
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