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Colyn Case
Joined: 12 Oct 2009
Posts: 13
Location: Thetford Center
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 Posted: Fri Apr 10, 2020 11:36 am Post subject: Minimum ANL size of 35A vs. power feed AWG |
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I wanted to protect all my big power wires that go through the firewall so I put ANL's on all of them.
So for example, I put a 35A ANL on a #8 feed.
The data sheet shows the 35A ANL will basically sustain about 90 amps indefinitely.
I had hoped to set it up so that the ANL would interrupt about at the 2 minute intermittent max for the wire size. In this case a #8 wire is good for 70 - 90 amps for 2 minutes at 20' run.
Why did I do that? I wanted to make sure that if anything shorted anywhere behind the firewall, that the ANL would interrupt before the wire insulation melted.
The problem is ANL's only go down to 35A. So for any smaller wire size there is no ANL that will blow at the 2 minute max for the wire.
You could argue that's a non problem because if you have a dead short the 35A ANL will blow anyway. On the other hand, if you have a path with some resistance but still allowing more current than the wire can handle but less than 90 amps, the wire insulation is going to burn.
So right now i have a #6 and some #8's going through my firewall that otherwise could be much smaller.
What is the right solution to this problem?
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Colyn Case
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user9253
Joined: 28 Mar 2008
Posts: 1925
Location: Riley TWP Michigan
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| Posted: Fri Apr 10, 2020 1:34 pm Post subject: Re: Minimum ANL size of 35A vs. power feed AWG |
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The way that many type certificated aircraft are wired (and most home-built)
is with no current limiter at all on the main power bus feeder. If it is not
installed, it can not fail. If the feeder does short to ground, the sheet metal
will burn away, thus removing the short circuit. The best solution is to install
the main bus feeder so that it is very unlikely to short to ground.
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Colyn Case
Joined: 12 Oct 2009
Posts: 13
Location: Thetford Center
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| Posted: Fri Apr 10, 2020 3:30 pm Post subject: Re: Minimum ANL size of 35A vs. power feed AWG |
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I can see the logic of that. However, if you abide that logic you probably wouldn't put any current limiters in your airplane at all, right?
I think the guys that made my Nissan Stanza thought something like that. One day for no reason that I could find, the electrical system fried itself, burned through a fender and destroyed the entire wiring harness in the car. Fortunately, shortly into the episode, someone had the presence of mind to grab one of the cables connected to the battery and simply pull it off.
Not something I would enjoy doing at 20,000'
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Colyn Case
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johnbright
Joined: 14 Dec 2011
Posts: 165
Location: Newport News, VA
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| Posted: Fri Apr 10, 2020 4:17 pm Post subject: Re: Minimum ANL size of 35A vs. power feed AWG |
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| Colyn Case wrote: | | ... The problem is ANL's only go down to 35A... |
Hi Colyn,
Renogy offers ANLs down to 20A but I don't know the quality. I don't know if they are a manufacturer or a vendor.
"Mini ANLs": They are slow but they blow faster than ANLs. Bussman calls them AMI and has them down to 30A. Littlefuse calls them MIDI and has them down to 23A. I can send you the data sheets if you email me.
With a Z dwg, versus the Nissan Stanza case, the master contactor will disconnect the battery.
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John Bright, RV-6A, at FWF, O-360
Z-101 single batt dual alt SDS EM-5-F.
john_s_bright@yahoo.com, Newport News, Va
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Colyn Case
Joined: 12 Oct 2009
Posts: 13
Location: Thetford Center
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| Posted: Sat Apr 11, 2020 4:45 am Post subject: Re: Minimum ANL size of 35A vs. power feed AWG |
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Thinking about this a little differently: When the wires are sized, they are sized for voltage drop vs. current and distance. Tables for calculating that are widely available. Typically we apply a fuse or breaker to allow the current used in that calculation.
That breaker size is way way lower than the current required to compromise the insulating properties of the insulation. What I would like to know, is if I have a #10 wire, at what current is the temp rise high enough to start compromising the insulation? e.g. ETFE melts at 255 degrees centigrade which in my application would be about a 200 degree C temp rise.
So I might set the anl interrupt current to allow a 100 degree temp rise for that gauge wire. ....looking for the table that shows what that current is.
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Colyn Case
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