Fuse and wire size

[DT86]

I am in the process of adding some aux lighting to my tractor. I know just enough about this type stuff to be dangerous and usually learn by trial and error. (Aka "f'ing something up, scraping half a day work and starting over") I'm trying to do this right the first time.

I need help figuring out if the fused switch panel pictured below is up to the task and what size wire to run to it.









It has two 15a fuses, two switches on one and a switch / power plug on the other.

How many 27w LED lights will a single 15a fuse handle?

The battery is about 11' in a straight line from where the switch panel will be, so by the time I snake the wire through the tractor it will probably be about 15' of wire. What size wire should be used between battery and panel, and what size fuse should I put at the battery end?

Thanks in advance, and please don't laugh too hard at my ignorance. I'm a farmer, not an electrician.

 

[murphy1244]

What gage wire is on it now?

 

[DT86]

Each side has 14g wire where the fuse is.

 

[Fallon]

American Wire Gauge table and AWG Electrical Current Load Limits with skin depth frequencies and wire breaking strength

Fuses protect the wire & switches, not things connected to the wire. A fuse needs to ALWAYS be rated LOWER than the switch & wire. If you blow fuses, don't put in a bigger fuse. Then the wire melts instead of the fuse.

Amps = watts / volts

In an automotive or tractor the volts are considered 12v. Use the above formula if a device gives watts and not amps. Be careful on LEDs as they may be talking about watts equivalent to an old incadesent bulb (using that number will be safe & not over tax a circuit though)

 

[N2DFire]

Watts to amps (A) conversion calculator
DC watts to amps calculation

The current I in amps (A) is equal to the power P in watts (W), divided by the voltage V in volts (V):

I(A) = P(W) / V(V)

In theoretical terms 27W/12VDC = 2.25 Amps. In the real world we know a 12 Volt system is slightly more than 12 Volts at times. (13V = 2.08A, and 14V = 1.92A)

I like working with easy numbers so I'd say 2 Amps per light head = 7 Lamps per 15 Amp Fused circuit which is split between the 2 switches for Ckt 1 and shared between the Switch & Power Plug on Ckt 2 so for Ckt 2 you need to subtract the max amperage of what ever devices you plan to plug in from the 15A total or you could end up overloading the circuit and blow fuses.

As for fuse at the battery - if you're just pulling 1 wire from the battery to power the entire panel then you need a 30Amp fuse and a LARGE gauge wire (10 to 6 ga depending on the wire construction) - at that point I'd look at using a automotive self resetting circuit breaker.

Wire Gauges - Current Ratings

Also - in looking at the one wire end I could see and making a wild @$$ guess as to number of strands - I'm guessing your 14 ga wires are only good for about 10-11 amps (unless the mfg says otherwise)


I'm not an electrician or a farmer nor did I sleep at a Holiday Inn Express last night Just a few basic AC/DC fundamentals classes YEARS ago in college & the power of Google

ETA: If it were me and I were using this, I'd cut the feed wires on the panel side of the fuse holder (ditching the fuse holder), solder and heat shrink new wire onto it, feed them separately all the way back to the battery, and then add in blade type automotive fuse holders right off the battery. That way you have only 1 fuse per circuit instead of 2 (easier troubleshooting a blown fuse) and your fuse protection is as close to the power source as possible.

 

[skyhook]

At 14 volts DC (from your battery) each 27 watt light will use about 2 amps. (simplified ).
So you can safely put two lights on each switch if needed, since you have 3 switches,
you can put a total of 6.
The power outlet can safely handle 10 amps, don't forget.....you must ground that other lug on the power outlet, do not add a power source to this lug.
I would bring both "fused" wires directly to your battery.

 

[DT86]

Thanks guys. I have a little bit better understanding now.

The only question I have is about one recommendation of four lights per 15a fuse vs another of 7. Using the provided formula, and the fact that all seem to agree on 2a per light, it seems that 7 lights on each fuse is correct.

Should have mentioned the plan was to run four lights on one switch and two on another. The third would be empty for now.

 

[N2DFire]

Thanks guys. I have a little bit better understanding now.

The only question I have is about one recommendation of four lights per 15a fuse vs another of 7. Using the provided formula, and the fact that all seem to agree on 2a per light, it seems that 7 lights on each fuse is correct.

Should have mentioned the plan was to run four lights on one switch and two on another. The third would be empty for now.

Well the 7 lights figure is based on a couple assumptions:
1) That the switches are each rated for the max capacity of the circuit (15A) or higher
2) That the remaining wiring is also capable of handling the current capacity of the fuse

(You'd think these were pretty safe assumptions but it's not always so)

1) I had to REALLY blow up the picture to read the switch rating - it looks like 20A at 125VAC so they should be ok (in this application)
2) As I mentioned before - looking at the wire and making a guess on the strand count and using the table I linked - I don't think the wire is able to handle much more than 10 Amps. Additionally - you never really want to run anything at it's max abilities so for a 10A wire - 8A worth of steady current is pretty good. If the Mfg says the wires are good to 15A then 7 lights is ok, otherwise you're back down to 4 per ckt.

In your case with a 4/2 split, I would put the 4 lights on one switch in Ckt 1 (the one w/ 2 switches) and put the 2 lights on the switch on Ckt 2 (single switch & power plug). Again with my previous suggestion to use other fuse holders and larger wire. If in the future you really needed more capacity on the empty switch of Ckt 1 - it wouldn't be hard to separate it off and pull it's own fused feed from the battery as well.

 

[Tractor Seabee]

On DC circuits do not forget voltage drop. You cannot use the tables set up for house wiring. I do not have the info handy but it is all on the internet. Also your friendly auto supply house can keep you out of trouble. Trailers are especially bad as you have to calculate from the battery to the last device. As the distance goes up the voltage goes down and the amps on that wire/fuse go up. The wire is a big resister in the circuit if not large enough. The weak link may be the contacts in the switches. You did not mention their amp rating.

Ron

 

[N2DFire]

On DC circuits do not forget voltage drop. You cannot use the tables set up for house wiring.

Ron - you are 100% right. Both times I posted I was at home and all my good reference books were at work, so I did a quick Google search for "wire current capacity" - totally forgot to put "DC" in the search and paid no attention to the fact that was a AC table.

http://assets.bluesea.com/files/resources/newsletter/images/DC_wire_selection_chartlg.jpg

Based on this chart - 14ga should be fine for 15A (10% loss at 12-14 Volts is not much to worry about for such short runs) and somewhere in the back of my mind from my old electric RC racing days I knew this but kept talking myself out of it.

Thank You Ron for catching my mistake (twice)