HELP! What size fuse?

[Elumia]

I think I said that the size of the wire determines the max load (edit: yes, you should determine the load first, but sometimes you are handed a wire and you have to start there). The same thing you guys have been saying, but adding some real life numbers. (Edit: I guess I didn't know if Shootr had pulled any wire yet, so I wanted to make sure the point wasn't lost)

I just wanted to point out to people that it is not just a simple question of what size fuse or what size wire, but that both must be taken into consideration given the voltage and the load requirements. The biggest mistake someone could make is oversizing the circuit protection for the size wire they used, may as well not have any then.

I think we are all in agreement!

Mark

[GeorgeTelford]

George,

Good catch on voltage drop but if you'll review my post you'll see I suggested oversizing the wire by 25%. I didn't explain it but that's what its for. Since most people are designing a 4' or 5'x8' trailer voltage drop shouldn't be an issue. The original poster asked for a simple way to size fuses and using wire ampacity is the simplest.
Bruce

Hi Bruce

The simplist method was outlined in my very first post (remembering that the original poster asked about fuse sizing) Here is my original simple answer

for the simple circuits in a tear select a fuse of only marginally higher ampage than that circuit will use max

If someone asks about the wire gauge required, I can do that too, but if someone asked me what gauge wire they needed to run a 20 Amp device, my first question is always going to be "how far is the device away from the battery" and then you times that by 2 (Positive and negative wires)

If there and back is 1 Metre then 13 (AWG American Wire Gauge)
up to 2 metres 11 AWG
up to 3 Metres 9 AWG
up to 4 Metres 7 AWG
and so on and so forth

Because if you used the chart you posted for a tear, lets say there and back to the device was 4 Metres (13 Foot ish is easily possible within a tear)
then the cable suggested by your chart is 12 (13 is nearest on my chart) this is only 2.5 sq mm the reality is that it should be 7 AWG which is 10 sq mm

even if you allowed the 25% extra that you mentioned, that would make 3.12 sq mm, the wire would still be over 3 times smaller than needed.

[GeorgeTelford]

Hi elumia

If you are handed the wrong wire, hand it back.

Correct methodology is

I have a device that requires 15 Amps Max, it will be located 7 Feet away from the battery, therefore I need 9 AWG wire to avoid voltage drop and say a 20 Amp fuse.

[bdosborn]

Well, lets work some numbers.

Lets say we have a 15 amp load from all the devices we want to put on a circuit.

- First we size the wire: 15AX1.25=18.75A. So by the chart we would pick a #12 wire since its good for 20 Amps.

- Now lets assume that the maximum wire length is 6' since that's about the longest run I have in my trailer as well as Madjacks.

- The voltage drop would be 3% which is within the IEEE recommendations for power quality.

-Now we pick a 20A fuse to protect the wire from faults and we're done.

Lets assume we need a longer wire, say 10'. Now the voltage drop is 5% or 12 volts at the device if the battery is fully charged. This is still within the operating range of most 12v devices. We're wasting 5% of the circuit load heating the wire but it will work safely.

So if we limit our discussion to wire length typical to those found in a Teardop trailer, sizing wires and fuses is easily done using the method I posted.
Bruce

[Miriam C.]

My learning curve just flattened a bit I think. All of you have helped, however I try hard to not think in math terms after 9am. So If I need to run a 12v fan wire further than 6 ft should I just go to 12g wire and manipulate my wire to the shortest distance.

Do I got it.
Miriam

Thank you Bruce for answering so promptly :rofl:

[Elumia]

here's a wire size table and voltage drop calculator.

http://www.powerstream.com/Wire_Size.htm

Shootr, hope you're not more confused than you started. This gives you an idea of why it seems there are more circuits in your car's fusebox than in your house.

Mark

[GeorgeTelford]

Hi Bruce

I was right there with you, until you said 6 Feet, straight away you need to double that (positive and negative wires, there and back) 12 Feet and dont forget that stuff is rarely on the same plane (ie it could be in the roof when the battery is at floor level) anyway lets stick with 12 foot for now.

My chart works in metres so 12 foot is 3.658 Metres, now on my chart that works out at 7 AWG

This will be just a tad better than the 3% Voltage drop reccomended.

I just tried both on the online voltage drop calculator, mine came out at 2.65% voltage drop and the 12 AWG comes out at 6.53 %

I always aim to exceed the standard of 3 % why waste power creating heat in wires?

Hi Miriam

it depends on the ampage required by the fan

[bdosborn]

Okay, I just have to clean up this thread. I let it die because I was tired of arguing the point but there's some bad information posted here. George had posted some wire sizes that are grossly oversized. His mistake was doubling the length of the wire to account for the out and back lengths of wire. Most voltage drop calculators, including the one that is linked in the post, use the one way wire distance. The calculator automatically doubles the wire length to account for the out and back length of wire. So as most of you who have ever worked on a car have guessed, the wire sizes George posted are about twice as big as needed. Since copper has doubled in price over the last year I thought I would post a correction. Also, odd awg sizes of wire aren't available to most of us, only even sizes, so don't bother looking for odd sizes.
Bruce