wire size

[closeanuf]

What size wire do I need to use for a Maxair?

 

[closeanuf]

I don't have one yet.  I'm putting ceiling in and want to run wires now before I do .  Less than 10 ft run.

 

[Almost There]

10 gauge will work fine. You  might get away with a little thinner wire but if you're pre-wiring a van then putting in 10 all the way through will give you the economy of using the same gauge all the way through the van.

I wired in 3 LED lights, the roof vent fan and 8 - 12 V outlets, max run about 15 ft. It was all wired back to a Blue Sea 12 outlet fuse block. I ended up using almost all of 300' of each of red and black 10 gauge wire.

 

[Trebor English]

For just the fan its under 3 amps.  This post shows the amps I measured using my MaxxAir fan.  

https://vanlivingforum.com/showthread.php?tid=23120 
 
AWG size 22 is big enough for just the fan.  I use extension cord size, #16, for everything small like LEDs and fans.  It is the size people use in houses plugged into outlets that have 20 amp breakers.  Zip cord has ribs along one conductor, smooth along the other.  I use the ribbed conductor for +12, smooth for -12.  I use white zip cord and red and black markers to mark the + and - in an easy to see way.  I just mark a spot at each end not the entire length of the wire.

 

[tx2sturgis]

14 gauge is fine, more than enough. You can go as small as about 18 gauge with negligible voltage loss. 

If you can find the 14 gauge wire locally or online in jacketed cable or 'trailer wire' you will have double insurance against shorts to the metal ribs and body framing edges.

It will usually be grey, with white and black, color coded wires inside.

If you are on a budget, you could even use zip-cord or lamp cord, but it won't be color coded, it will usually have a ribbed side and a smooth side, to help you observe polarity.

 

[B and C]

Industry standards are the black wire is the hot wire and the white is the ground wire.
On the zip wire, the ridge or ribbed wire is the hot wire and the smooth one is the ground wire.
The wire with the tracer color (usually white) is hot and the one without is ground.

This will save someone some head scratching if they have to work on it later.

 

[highdesertranger]

"Industry standards are the black wire is the hot wire and the white is the ground wire."

except in the automotive industry, then red is hot and black is negative/ground.

highdesertranger

 

[B and C]

^^^True. Forgot to add that one. They were talking black and white and zip cord wire.

Confusing stuff isn't it, especially for a noob.

 

[tx2sturgis]

As long as the OP observes correct polarity end to end, they could be any color. 

I believe the MaxAir and the Fantastic fans are shipped with white and black input wires, black being positive.

Remember, RVs are wired internally to residential 120v standards, so black is supply, or hot, and white is neutral on 120 volts, but white is battery ground on 12 volts.

So as I said, black and white wires will work fine.

If standard red and black wires are used, then the red (hot) wire from your fuse panel will attach to the BLACK fan wire, and the black ground wire will attach to the WHITE fan wire. 

Confused?

Yep...join the crowd.

 

[closeanuf]

So 14 will work.  Thanks

 

[Weight]

Just for clarity. In direct current systems, red is used for positive, unless in a sub circuit, then it could be any color or strips. Negative is usually black or yellow. Yellow is not as often used in USA automotive.

 

[Zoomyn]

Remember every fuse, splice, crimp connector and terminal lug all also takes its cut out of battery charge state...   I'm a big believer in heavy copper, all those little savings add up over time, try not to pay big money but use the best quality and size wire possible.  Fine, it might not be vital boondocking on NM BLM land with lots of solar input but get in a 4-day cloud funk in Oregon w/o solar and it matters.

Maybe look at wire economy this way - At 3 amps using 10AWG for the 10-foot run will give you six free minutes run time after an hour versus using 22AWG but only a minute 'free' power is gained per hour by using 16AWG versus 10AWG...  YET: Adding more load changes those percentages so sometime in the future a bump in the fan circuit to 12 amp draw --- 22AWG is turning into a really dim 50 watt light-bulb style heater element (BAD);  the 16AWG will run the load 2-1/2 minutes less than the 10AWG for the same work request on the battery.  14AWG runs for 1-3/4 less than 10AWG. 

Play with this calculator to get the feel of it - https://www.powerstream.com/Wire_Size.htm and scroll down..

 

[tx2sturgis]

The question was simple...so simple answers are all that is really needed.

Sure, we can talk about minute power losses, negligible as they may be. This could apply to ANY wire length. At some point you have to balance performance vs practicality, just like the electric power companies do when they string high tension power lines across the country. 

The fact is that these roof fans are generally run at lower settings, pulling one amp or less for a few hours a day, and normally not 24/7.

For a 20 foot run or less, 14 gauge is good enough. Sure, 10 gauge will work....heck... 4/0 battery cable will work too...

But what is a reasonable, simple, answer?

14 gauge will work.

 

[tripper]

There is an excellent DC wire size spreadsheet located here: Correct DC Wire Size
They also have a more in depth app you can get at the same site for both Android and Apple phones.

 

[tx2sturgis]

I plugged in some numbers and the calculator came up with 18 gauge wire...which is even smaller than 14...so either one would work fine.

 

[JD GUMBEE]

I use extension cord size, #16, for everything small like LEDs and fans.  It is the size people use in houses plugged into outlets that have 20 amp breakers.

Trebor, I get ruffled when people "nit-pick" others posts. Still, I thought about a newbie self-teaching and reading this thread. I felt the need to explain one thing for the sake of those newbies. (Sorry in advance.)

While #16 of the proper style is perfectly fine for small loads, for a newbie, it can be very misleading to compare 120V AC and 12V DC. The wire needed is completely different.

Low voltage DC needs many more conductors to carry the load than house voltage. The size you need increases the longer the wire gets.

12/24 volt DC systems, as a general rule, need lots of strands and as many of them as possible.

Think welding cable or amplifier power cable.

 

[Zoomyn]

 ..if current is constant at 3a how does using a heavier wire allow you to draw the 3a 10% longer?"  Posted by gsfish

The target theme was conserving energy begins when choosing each part of the larger machine. I apologize, it was too easy to compare & contrast 10AWG vs. 22AWG.

Okay - 'how?' - we count prevented wire losses saved as possible longer fan run times.

Our fans (I got one too) run at 3 amps X 12 volt = 36 watts.  If we lose or drop 3 volts in the wiring, the fan still tries for 36 watts by increasing current flow (amps) to make its 36 watt power rating (9 volts X 4 amps) so that stays the same.  And the 3 volts lost heating skinny wires is 3 volts at 4 amps equaling a 12 watt EXTRA load so our thrifty little fan is now using 48 watts instead of 36 - and the extra current is not good for it, makes excess heat trying to do its designed work.

In the above made-up skinny wire example it'd be 24% longer run time with 10AWG, a 14 minute bonus per hour, by investing in heavy copper.

In reality:

10AWG : 10 feet : 12 volts : 3 amps consumes 36.2 watts (of battery charge) per time unit,
14AWG : 10 feet : 12 volts : 3 amps consumes 36.5 watts
16AWG : 10 feet : 12 volts : 3 amps consumes 36.8 watts
22AWG : 10 feet : 12 volts : 3 amps consumes 39.0 watts

Which brings us back to 10AWG being able to run one hour and five-ish minutes on what one hour 22AWG battery draw was...

BONUS ROUND: raising 12V current needs from 3 amps to say 12 would increase losses to 4x the 3A figures...
14AWG : 10 feet : 12 volts : 12 amps consumes 151.5w; that is an increase to 5.2% loss versus 1.3% loss for the 3 amp. Just the way it works, low voltage DC needs more copper...

Sure it looks like tiny numbers on paper (negligible?) but sketch them out across a couple of long hot summers it's a whole herd of 'free' hours of fan usage - or dragging battery charge low slightly less often - or running engine slightly less often to get batteries ready for tomorrows req'd work - blah blah blah..  

My career was working on 750VDC subway cars with mains fused at 1000 amps, easy million watts when that main fuse popped... I like BIG WIRE. Everywhere. :dodgy:

And oh yeah there is a billion English speaking people in this world who might want to read about wire size so I don't mind typing this up...

 

[tx2sturgis]

 If we lose or drop 3 volts in the wiring, the fan still tries for 36 watts by increasing current flow (amps) to make its 36 watt power rating (9 volts X 4 amps) so that stays the same. 

This is not how they work, so some assertions in your thesis are incorrect.

There are SOME electric motors and electronic components that DO accept more current as the input voltage drops, but these vent fans don't do that. Only IF they were RPM governed (which they are not) would they pull more current if supply voltage was reduced. 

If the supply voltage of let's say 13 volts, and about 3 amps, WERE to drop by 3 volts, which it won't do on 10 feet of 18 or 14 gauge, but let's say it did, then 10 volts times 3 amps is 30 watts. But in reality, the current supplied (at the fan motor) also drops, so the result is even less than 30 watts power consumption. In this scenario, with a voltage drop along the supply wires, there WOULD be some power consumed in heating the wiring, so there is the loss you speak of.

In fact the simple 3 speed Fantastic Fan has nichrome wire, arranged in coils (as a voltage-dropping power resistor network) to gain the 2 lower speeds.

IF simple resistance, and a dropping input supply voltage, to the motor ramped UP the motor wattage, then there would be MORE current pulled on the 2 lower speeds. This is not what happens. Lowering the supply voltage to the motor also lowers the power consumed. 

Of course those 2 'resistors' DO heat up a bit, consuming energy. And that is the reason I bypassed those, and installed a variable PWM motor speed controller.

The push-button multi-speed electronic vent fans do operate differently. The ones I know of use a digitally controlled PWM circuit, although some may employ a steady, but variable, supply voltage.

 

[John61CT]

Blue Sea app "Circuit Wizard" is excellent for judging wire size.

​UL1426 spec tinned boat cable is the best for low-volt house wiring, if the cost is an issue just settle on three gauges and buy in bulk.

Yes many will say overkill, but if you plan to keep your rig for many years IMO worth doing it right and only once.