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what wire size to secondary fuse block?

Desert Rat

Well Known Member
Hey guys-

I've got a bunch of breakers on the main ships buss, but due to panel space constraints, am using a separate fuse block for all my lighting circuits. Total amp capacity on the fuse block will be about 15-ish amps with every light strobing, wigwagging floodlighting, etc.

Main ships power comes in to the buss via a 60 amp ANL and a #8 wire. Then I need to jumper about 24" to the fuse block.

How would you guys go about sizing that jumper wire? #14 would be plenty big enough for the load, but the only circuit protection would be the ANL coming off the master contactor. Seem like if that wire shorted out it would likely melt before popping that 60 amp ANL.

I could go with a #8 awg wire from main buss to fuse block and keep everything the same size, or would a #14 with a fusable link made out of #18 wire be reasonable? Am I over thinking this?
 
The fuse is protect the wire(s) coming out of it, not the loads that the wire is going to. In your case, that’s a 60 amp fuse. I think the proper size wire for a 60 amp fuse is 6 awg minimum, 4 awg preferred (depending on the length of that wire run). #14 is too small, unless you incorporate another - smaller - fuse in that circuit, or the fusible link like you mentioned.
 
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fuses

I'm curious what you have on that fuse block that would keep you in the 15A range.

Also, 18 AWG is generally used for up to 10 Amps, and 14 AWG for 15 Amps. Not sure I would use a piece of 18 for a fuse block that is going to draw 15 Amps...
 
I'm curious what you have on that fuse block that would keep you in the 15A range.

Also, 18 AWG is generally used for up to 10 Amps, and 14 AWG for 15 Amps. Not sure I would use a piece of 18 for a fuse block that is going to draw 15 Amps...

Fly LED lights and a CO detector. The 15amps is just a back of the envelope guess as to inrushing current Sustained will be quite a bit less than that.
 
Just me

I was in the same dilemma as you. In my case I needed to use a 12 AWG wire to handle the overload current before the upstream fuse blew. What I did was use two wires of 16 AWG, which is equivalent to one 12 AWG (2 steps in AWG is twice the size). The 2x 16 AWG was much more flexible than a 12 AWG would have been.
 
The fuse is protect the wire(s) coming out of it, not the loads that the wire is going to. In your case, that’s a 60 amp fuse. I think the proper size wire for a 60 amp fuse is 6 awg minimum, 4 awg preferred (depending on the length of that wire run). #14 is too small, unless you incorporate another - smaller - fuse in that circuit, or the fusible link like you mentioned.

Unless I've misread the 43.13 chart that has been around since I was in A&P school about 1,000 years ago (entirely possible) 8AWG will carry 60 amps continuous 12' on the single wire in free air line.

As far as the fuse link- I'm far from an expert on the magic of electrons. I just know that bob knukolls says in his book that you can make a fuse link with a short section of wire 4 sizes smaller.

Thanks for the replies guys. This is such a simple thing I don't know why I'm getting hung up on it.
 
Which Flyleds? Those Seven Star ones draw a bunch!

leading edge combos 1 amp per- 4 in each wing = 8 amps, Nav lights=2.5 amps strobes = 6amps peak 3 amp average.

the CO detector calls out a 1amp fuse. don't know what it actually draws, but it can't be much.

okay, worst case scenario, call it 18 amps.
 
Wire sizing

I think the problem is trying to figure out the physical interconnections and sizing from an electrical symbolic wiring diagram that just shows the point to point connections while trying to ensure protection for all the potential failure points. The best way to keep it under control is to figure out all the physical distribution components, wire lengths and then draw out a point to point diagram with current capability, current draw for the loads and short circuit current if there were a fault. In some cases a higher current carrying wire with an in-line fuse may be the most practical solution. For example if you have an emergency bus powered directly from the battery through a 20 amp capacity switch then putting an inline fuse rated at 20 amps in the line coming off the battery to protect the switch and the wiring. In this case the typical load on the emergency bus might max out at 10-15 amps and the individual loads would be protected by circuit breakers. I know drawing it all out in a physical drawing is a lot of hassle but it is worth it in the long run - especially when troubleshooting any down the road problems.

KT
 
I was in the same dilemma as you. In my case I needed to use a 12 AWG wire to handle the overload current before the upstream fuse blew. What I did was use two wires of 16 AWG, which is equivalent to one 12 AWG (2 steps in AWG is twice the size). The 2x 16 AWG was much more flexible than a 12 AWG would have been.

I don’t think this is correct. 3 steps (e.g., AWG 16 to AWG 13) gives you a factor of two in cross-sectional area, hence half the resistance. So you need two AWG15 wires in parallel to equal one AWG 12 wire. AWG 16 is a bit too small. Another problem: In the real world, wire resistances are so small that slight differences in the quality of a crimp can lead to a non- 50-50 split of the current.
 
It is not a good idea to have fuses in series. A short circuit can not only blow the individual load fuse, but also the bus fuse even though it is larger. Best to use a fuselink.
 
It is not a good idea to have fuses in series. A short circuit can not only blow the individual load fuse, but also the bus fuse even though it is larger. Best to use a fuselink.

The protection components you insert into your electrical power generation and distribution are there to prevent flames and smoke resulting from a fault condition in the circuit. Typically those components take the form of fuses and circuit breakers but may also be overvoltage and overcurrent electronic components and modules. Short circuit or high current (low resistance) paths may develop as a result of failures within a module, wires becoming detached or chafed and making contact with the airframe return path. How you protect against smoke and flames from an electrical problem is either based on experience and knowledge of good practices and or doing a detailed failure modes and effects (FMEA) study. How you choose to distribute power will inevitably result in how you choose to protect it. For example if you use multiple crimped splices to segregate wires to circuit breakers rather than a screwed connection terminal block with individual wires to circuit breakers you may consider that the possibility of a short circuit is extremely remote and not put a current limiting. device (fuse) in the power line from the power source (battery, generator). Having two fuses or circuit breakers in the circuit from power source to load may be entirely reasonable or overkill. It depends on the analysis and postulated failure modes and probabilities or occurrence being considered. Every time you add protection devices into the mix you increase the probability of failure so in the end it is a judgment call. The worst you can do is just wire it up without a detailed wiring and routing diagram and hope for the best.
KT
 
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