Wire sizes and Fusing

[Optimistic Paranoid]

Wire Sizes and Fusing: 

There is a maximum amount of current that each size wire can carry safely before suffering either long term, or in some cases RAPID deterioration.

The technical term for this is its AMPACITY, which is a made-up term from the two words Ampere Capacity.

In addition to wire size, it is also partially dependent on the temperature rating of the insulation on the wire.  Wire with insulation rated for 90 degrees Celsius has a higher ampacity rating than the same size wire with insulation rated for 60 degrees Celsius does.  Bundling a bunch of wires together calls for derating the ampacity, because the wires in the center of the bundle can’t shed heat like they could if they were run singly.

Anyway here’s a simple table of fuse sizes matched to wire gauge size that’s close enough for government work, as they say.

For 14 gauge wire, it’s 15 amps
For 12 gauge wire it’s 20 amps
For 10 gauge wire it’s 30 amps
For 8 gauge wire it’s 50 amps
For 6 gauge wire it’s 80 amps
For 4 gauge wire it’s 125 amps
For 2 gauge wire it’s 200 amps
For 1 gauge wire, it’s 250 amps
For 1/0 gauge wire it’s 325 amps
For 2/0 gauge wire it’s 400 amps

You can ALWAYS put a smaller fuse on a wire safely.  For instance, If I were installing a fuse block for my house circuits, and it was rated for a maximum of 100 amps, I would run 4 gauge wire(125 amps) from the battery to the fuse block, and fuse it close to the battery with a 100 amp fuse.

 

[rvpopeye]

Good post , I'd only add that the longer the wire , the less amps it will pass without heating up .

 

[GotSmart]

Also the type of wire also makes a difference.  Aluminum wire is inferior to copper wire, and has a tendency to compress at the point where it is clamped.  When I had my house, I had to tighten the wires going to the AC system from the fuse box twice a year to keep it operating properly.  

Spend the extra money and buy copper.

 

[Optimistic Paranoid]

Good post , I'd only add that the longer the wire , the less amps it will pass without heating up .

Yeah, voltage drop calculations probably deserve a separate post.  Thing is, I hardly ever worry about them, I just over-engineer the wiring myself.

Regards
John

 

[rvpopeye]

I was thinking about that after I posted.
In a small (under 25') vehicle , those sizes should be just fine.

I bought a big spool of 10GA and just use it for everything, double and triple for heavier loads......

 

[blars]

The temperature the insulation is rated at also matters -- I think that table is for the most common 90C rated wire, 75C wire needs smaller fuses. If packed tightly, it needs to be derated. The voltage drop is usually more concern on 12v wiring, and that is lenght sensitive.

 

[Optimistic Paranoid]

The temperature the insulation is rated at also matters -- I think that table is for the most common 90C rated wire, 75C wire needs smaller fuses.  If packed tightly, it needs to be derated.  The voltage drop is usually more concern on 12v wiring, and that is lenght sensitive.

blars, if you feel these are way off, please feel free to post different figures, and your reasons for recommending them.  I don't mind.  I long ago (reluctantly) renounced all claims of infallibility, though it IS making it harder to pick up chicks.

Seriously, I cribbed these figures years ago from an audio site, and these are apparently - for better or worse - what they use when doing installations.  I've never had a problem with them, but that could just be dumb luck.

Regards
John

 

[highdesertranger]

somewhere I have a table for wire size vs length for amp rating for 12v DC. however I can't find it at the moment. I really need to clean up my favorites. highdesertranger

 

[akrvbob]

So what wire sizes do you use for typical vandwelling installations?

 

[Spaceman Spiff]

I used this calculator for sizing my 12 volt system wires.  Gives correct wire (to ABYC standards) for both ampacity and voltage drop.

http://circuitwizard.bluesea.com/#

For fuses, MaineSail had recommended the largest fuse for wire size (in 12V systems) to minimize voltage drop across the fuse.

http://www.pbase.com/mainecruising/fuse_voltage_drop
http://www.bdfuses.com/fusesnwires.php

-- Spiff

 

[Optimistic Paranoid]

So what wire sizes do you use for typical vandwelling installations?

Being a lazy bastard, my theory is to just buy 12 Gauge Automotive Zip Cord and use it everywhere.  Using zip cord is easier than running separate power and ground wires.  The red and black makes it simple to keep positive and negative straight.  I can color code my runs with short pieces of colored heat shrink on each end.  It's easier to keep a lot of one size around than to try to stock different sizes of wires.  And I pretty much don't have to worry about voltage drop on any realistic run length.  It is, of course, massive overkill on things like LED lights, which would probably be happy with 18 gauge.  But as Master Redneck Engineer, I LIKE massive overkill, as long as it doesn't add a lot of unnecessary weight.

Regards
John

 

[Almost There]

Being a lazy bastard, my theory is to just buy 12 Gauge Automotive Zip Cord and use it everywhere.  Using zip cord is easier than running separate power and ground wires.  The red and black makes it simple to keep positive and negative straight.  I can color code my runs with short pieces of colored heat shrink on each end.  It's easier to keep a lot of one size around than to try to stock different sizes of wires.  And I pretty much don't have to worry about voltage drop on any realistic run length.  It is, of course, massive overkill on things like LED lights, which would probably be happy with 18 gauge.  But as Master Redneck Engineer, I LIKE massive overkill, as long as it doesn't add a lot of unnecessary weight.

Regards
John

It's also an economical bastard thing - buying 3 or 4 different sizes of wires gets expensive. Other than lumber and maybe screws, I'll have enough left overs to do another van by the time I'm finished...

Thanks John, you just answered one of my questions - 12 gauge zip cord it is for all the 12 v in the van.

That is, if it's the right wire for the Whynter fridge wiring - is it?

 

[Optimistic Paranoid]

That is, if it's the right wire for the Whynter fridge wiring - is it?

It will safely carry 20 amps.  I don't know what the Whynter needs off hand, but it's hard to believe it would be more than 20 amps.

Regards
John

 

[FALCON]

Are you saying you use AWG12 for ALL the wires? Even the wires going from Array to CC, to Battery, to inverter?

 

[SternWake]

Please make sure the 12 awg zip cord is actually copper, and not aluminum or CCA, copper clad aluminum.

The AWG sizes for fridges, well throw out ampacity charts and wire them fat and short as convenient.

Danfoss/Secop wants their compressor controllers wires directly to battery terminals, fised at battery terminals, and the wire thickness they recommend is MUCH MUCh thicker than what ampacity chart say is adequate for the amperage their compressors can pull.

When Danfoss compressors suffer too much voltage drop, the compressor will not kick on, but the condenser fan will. Since the fan is usually as loud as the compressor in most stock danfoss/secop compressors, the owner believes the fridge is operating properly, when it is not.

I assume the Whytner's compressor is as susceptible to voltage drop as actual Danfoss Secop compressors. The Sawafuji compressors do not have as high a starting surge and are more tolerant of thinner copper to them.

So read this PDF from Danfoss, and make sure the fridge is well wired with minimal connections, and if reliability is important, get that Effing Ciggy plug and ita 18 awg cord out of the circuit.

Notice that for 12 awg is only acceptable for 8 feet of wire total, 4 feet to and 4 feet from:
http://www.ra.danfoss.com/Technical...nit_101n0600_12-24vdc_08-2011_dehc100m602.pdf

99% of reported issues with 12v compressor fridges is either the ciggy plug itself, or the poor stock wiring leading to the Ciggy plug.

Wire it thick and get that Ciggy plug out of any install which is to be more permanent. I installed a 45 amp anderson powerpole on the original powercord of my Friends ARB fridge. He can still use it on a ciggy plug, but it performs better when the powerpole is plugged into 10AWG that goes only 12 inches or so to the fuse block.

Throw out ampacity charts where battery charging or running a fridge is concerned. Huge performance/reliability increases are possible with 'oversized' wiring.
Voltage drop is the Nemesis of 12vDC systems, and thicker copper is the prescription for a cure.

Inverters need thick and short copper to prevent that pesky low voltage alarm from sounding. I use 8awg for 16 inches one way on a 400 watt inverter.

 

[Optimistic Paranoid]

Are you saying you use AWG12 for ALL the wires? Even the wires going from Array to CC, to Battery, to inverter?

No, those things need much fatter wires.  I'm saying just use 12 gauge for stuff like lights, fans, roof vents, etc.  You could get by with lighter wires on much of that stuff, especially LED lights.  But I think it's simpler to just have one size wire and not have to concern myself with voltage drop calculations.  Especially for utility circuits where you aren't sure what you might wind up plugging in to it down the road.

Regards
John

 

[Tulsatraveler]

[quote='GotSmart   Aluminum and copper have different coefficients of expansion and joints between them will - over time - work loose. T hat goes for connections of aluminum wire to switches and plugs.  My condo was wired with aluminum 30 years ago and I had trouble with flickering lights and things plugged into wall plugs not working right.  i finally went through the house and put a short pig-tail ( 4 to 6 inches) of copper wire between the aluminum wire and the switch or plug.  At the connection between the aluminum wire and the copper pigtail i used a "]
Also the type of wire also makes a difference.  Aluminum wire is inferior to copper wire, and has a tendency to compress at the point where it is clamped.  When I had my house, I had to tighten the wires going to the AC system from the fuse box twice a year to keep it operating properly.  

Spend the extra money and buy copper.
[/quote]

 

[DannyB1954]

The amp rating for wire has more to do with heat than efficiency. larger wire will give less voltage drop. I like to go one wire size over. 12 gauge for a 15 amp fused circuit is good for me.

 

[ccbreder]

''"Notice that for 12 awg is only acceptable for 8 feet of wire total, 4 feet to and 4 feet from:
http://www.ra.danfoss.com/TechnicalInfo/...00m602.pdf ""
The figure 2 chart seems to be incorrect, notice it has two lines for 12 awg. The first line must be 14 awg. The asteric (*) indicates the foot measurements are from the battery to the compressor. Not round trip.

 

[dolomiti]

I'm not sure if this is the best thread for this, but I am hooking 2 12volt 160ah batteries in parallel and used a switch to starter battery cable I found at auto zone. It is 4 GA and 19" long. Is this substantial enough? My concern is that when I hooked a battery charger to the setup after putting them in parallel (charger was on one battery's + and the other battery's -) I heard gurgling sounds from the batteries. Is this normal? Maybe I didn't have the cables tightened enough? I just don't want to ruin my 2 new AGM batteries/have them blow up and ruin other stuff.