# Fuse and wire gauge

So i am building a automatic stair lighting system, i will be using a ws2812b strip 60led per meter stip and using 3.5 meters of it. I have worked out the system under max load could consume 13.2 amps if all leds are at max brightness white (will be set less than that in the code but needs testing) and will follow adafruits wiring guide. I want to add an inline fuse at the 5v positive end before the rest of the circuit. Can i use a 12v fuse at 15amps ? would that be ok for a 5v system ? If i set the leds to half brightness so the circuit draws say 7 amps would it be better to use a 10 amp fuse? and could i use a car fuse or does it need to be a glass one ? I am also unsure about the guage wire to use, the power supply will be under the stairs so it could be 1 to 2 meters away from the strip. I am also thinking that voltage drop maybe an issue along the strip and i may have to run the wires to the top of the strip aswell. And can i use 20amp connector blocks to splice one wire into 2 or 3 to connect up the various sensors and take the 5v to the top of strip. Any advice or safety considerations would be most appreciated, also i will be using a 5v 40amp meanwell psu with only one 5v out

Can i use a 12v fuse at 15 amps ?

Yes, 15 amps is 15 amps.

For white, rather than FF,FF,FF, suggest you use 7F,7F,7F (1/2) or 3F,3F,3F (1/4).

16AWG has ~4 ohms per 300 meters. 3 meters would be .04 ohms @ 13.8 Amps gives .55V Suggest you make 3 power runs to the strip therefore the voltage drop would be .18V.

Why not use 5x20 mm slow-blow fuse in an inline holder?

So the voltage rating on the fuse matter does not matter? I am assuming that in a short circuit situation the power supply draws more amperage than what the circuit can handle and that is what blows the fuse, is that correct. I only have one 5v out on the psu how do i splice the wires to go to the sensors and multiple points on strip, many thanks for your answers

Amps are amps.

The voltage rating at these low DC levels (12v/5v) is not an issue.

At higher voltages, the voltage rating is important, has to do with the possibility of arcing.

Note: if you do proceed with the 3 power runs, each run can have a 5 amp fuse.

Full brightness for red, blue or green is one LED fully ON (20mA). Therefore you can think of it this way, equivalent brightness for white can be considered 1/3 full current (7mA) into each of the 3 LEDs (21mA total). And remember we are dealing with PWM going to the LEDs.

stebod: So the voltage rating on the fuse matter does not matter? I am assuming that in a short circuit situation the power supply draws more amperage than what the circuit can handle and that is what blows the fuse, is that correct. I only have one 5v out on the psu how do i splice the wires to go to the sensors and multiple points on strip, many thanks for your answers

Above 20V or at high power the voltage rating will start to matter - especially for DC. Breaking a high energy DC arc is difficult, its a fireball. You'll see all sorts of exotic fuses for higher voltages and currents and powers, including some with exploslve charges.

You'll almost never need to use an exotic fuse if you avoid high voltages and powers.

For wire guage always consult the AWG

(see the "Ampacity" @20 deg C column)(looks like you probably need 20 to 18 guage) (smallest to largest)

A 1mm^2 copper wire of length 1m is about 17 milliohms.

Resistance goes up with the length and down with the area. V = IR will do the rest, no need for any lookup tables once you know this.

A 1mm^2 copper wire of length 1m is about 17 milliohms.

Resistance goes up with the length and down with the area. V = IR will do the rest, no need for any lookup tables once you know this.

Quite frankly, I don't see how you can feel comfortable telling this to a newbie who would have no way of knowing (without the AWG) that 1 mm^2 is 17 guage (1.04 to be exact)(AWG) which is in between 16 and 18 guuage and which almost nobody sells. This is useless information for a newbie. In my opinion, for a newbie , it makes much more sense to look up the Ampacity and find the appropriate guage. For that matter, he doesn't need 16 guage. He should be fine with 18 guage. 1 mm^2 is of no use to a newbie. You can't even buy 17 guage (the closest to 1mm^2), at least not easily. Where is he going to find 17 guage wire ? It makes much more sense to choose something from 20 guage to 16 guage, (20,18,16) which are readily available.

And it's 16.6 mOhms/m, not 17. (rounding error increases over length)

Thank you all for your answers, 18 guage is what i shall use. The led strip comes with soldered wire on both ends, should i remove this and solder the 18 guage wire to the strip? I am hoping to get away with just wiring the end closest to the psu as i want to run the led strip in a aluminium profile along the side of the stairs and any extra wiring would need to run inside the profile or trunking underneath it. I understand if i separate the strip into sections i can fuse each section with a 5 amp fuse but i need to think about how to hide the wires. I also have to put in two pir sensors and a LDR sensor. The first sensor will be at the bottom of the stairs with the ldr and i was thinking of powering the top pir off the 5v at the end of the led strip and just having the data line for it run back to the arduino in the aluminium profile.

Im not sure if i can post links but this is the sort of thing i want to build

Oh cool. I've got to build one of these.

First, you do realize that the purpose of the fuse is to protect the power supply and the wires from it to the fuse? Your power supply may already have a fuse in its output. (You haven't said which Power Supply you are considering).

You need to power the LED strip at both ends, which is why you need the 18ga wire from the PS to the far end of the strip. That long of a strip will have a voltage drop from one end to the other.

Have you decided on a housing for the LEDs?

Could i use a car fuse or does it need to be a glass one

Ugh! Car fuses are horrible! Use a 20mm glass fuse. As to the comments about breaking high current, probably does not matter at 5V or 12V, but the simplest solution to extinguishing the arc is to use a HRC (high rupture capacity) fuse. HRC fuses are typically filled with (I think quartz) sand, which quenches the arc. In the UK common mains plug fuses are of this type. Look for HRC in the specification.

I am assuming that in a short circuit situation the power supply draws more amperage than what the circuit can handle and that is what blows the fuse, is that correct.

The power supply does not draw current on its output, it supplies current. You might or might not be correct, depending on the power supply. A lot of power supplied use foldback current limiting, which means that when you short the output instead of supplying their maximum current the current drops to a very low level until the short is removed.

I understand if I separate the strip into sections i can fuse each section with a 5 amp fuse but I need to think about how to hide the wires.

Maybe I missed it somewhere so apologies if this has been said already but to be clear the fuses need to be close to the PSU, not close to the LEDs.

Quite frankly, I don’t see how you can feel comfortable telling this to a newbie who would have no way of knowing (without the AWG) that 1 mm^2 is 17 guage (1.04 to be exact)(AWG) which is in between 16 and 18 guuage and which almost nobody sells. This is useless information for a newbie.

How strange. 1mm^2 wire is easily available in the UK, I have 2 rolls of it right next to me.

Stebod, using thicker wire than calculated is not a problem, just a bit more awkward to work with.

Looks reasonable.

I am assuming that in a short circuit situation the power supply draws more amperage than what the circuit can handle and that is what blows the fuse, is that correct.

From the data sheet:

Protection type :Shut down O/P voltage,re-power on to recover

I take that to mean that if you short the output the current will be very low, and probably not blow a fuse.

I take that to mean that if you short the output the current will be very low, and probably not blow a fuse.

Does that mean i dont require a fuse in the circuit?

PerryBebbington: I take that to mean that if you short the output the current will be very low, and probably not blow a fuse.

That's called a Soft Fuse. The Power supply shuts down to protect itself, and when you remove the load and turn the supply back on, all is good.

Does that mean I don't require a fuse in the circuit?

Good question.

If you short the power supply it will shut down, quite possibly before any fuse can blow. The danger is 40A into a partial short could easily start a fire. I think I'd go with the fuses; 5A to individual sections LEDs. A fuse won't do any harm and it might do some good.

You could short the furthest point of the tape from the PSU, then measure the resistance of the short at the PSU, with the wires disconnected from the PSU. If the resistance is more than: 5V / 40A = 0.125 Ohms, then you are at risk because the PSU could easily supply the 40A into a short at the end of the tape, the tape would go up in smoke and possibly your house with it.

You could short the furthest point of the tape from the PSU, then measure the resistance of the short at the PSU, with the wires disconnected from the PSU. If the resistance is more than: 5V / 40A = 0.125 Ohms, then you are at risk because the PSU could easily supply the 40A into a short at the end of the tape, the tape would go up in smoke and possibly your house with it.

Im not sure how to do this and i think im in way over my head, i just want it to be safe and not burn my house down :confused:

I'm not sure how to do this and I think I'm in way over my head, I just want it to be safe and not burn my house down.

Just put the fuses in, 5A, close to the power supply.