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  #1  
Old 09-17-2018, 11:32 AM
Zazoos Zazoos is offline
 
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Default Wire Size Chart

I need to run a wire 15' carrying 3 amps. If I am using the Continuous Circuit Chart from AC 43 1B correctly then 20 gauge would suffice. However, when I check it using the Wire Barn Calculator I get a different answer. I figure I am doing something wrong. Your help is appreciated.



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  #2  
Old 09-17-2018, 11:39 AM
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BoilermakerRV BoilermakerRV is offline
 
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Default Apples to Oranges?

New to Wire Barn, but is it possible you are using two different criteria. When looking at the AC are you using temperature rise whereas Wire Barn is using voltage drop? Looks like the max voltage drop in the Wire Barn calculator defaults to 2%, which isn't much.
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  #3  
Old 09-17-2018, 12:19 PM
rv7charlie rv7charlie is offline
 
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2% of 13.8v is 0.276V (half the drop of the other chart, and waay too conservative), so that's one difference. Another *possible* difference is that some charts/calculators expect you to enter a one-way distance (the calculator doubles it for out/back), and others expect you to enter the total round-trip distance.

I don't recall how the AC43 is written, but be aware that using the airframe as ground return greatly affects the calculation. Airframe resistance is *effectively* zero for calculations (massive 'wire' cross section), while if you run an isolated ground return, it must be included in the length of the path.

Having said all that, 20ga is plenty big enough for 3A, even with 15' each way.
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  #4  
Old 09-17-2018, 01:56 PM
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snopercod snopercod is offline
 
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Default

There are a couple of considerations. First of all, you don't want the wire to get so hot that it melts the insulation, possibly causing a fire or a short. That depends upon the temperature rating of the insulation. The cheap PVC stuff can be rated as low as 140F but the Tefzel insulation is rated at 300F! You certainly wouldn't want to run it that hot, but you need to know what wire you're going to be using before calculating the wire size. Also, you need to consider whether this will be a continuous load, or intermittent. If it's intermittent, you can fudge a little on the wire size.

The second consideration is voltage drop. You don't want to lose all your volts in the wire itself. As others have said, 2% drop is very conservative for most devices. OTOH, I went WAY conservative on my starter wires...just because.

...and weight is always a consideration.
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  #5  
Old 09-17-2018, 02:18 PM
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az_gila az_gila is offline
 
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Smile All consistent

They are the same when you correct the inputs...

All for 20 g wire -

Use the Wire Barn calculator for 5% drop and you get 22.66 ft.

The FAA 1/2 volt drop for 14 volts is 3.6%

Everything is linear for these calculations so the Wire Barn number corrects to 16.18 ft for a 1/2 volt drop.

Just what the FAA chart says.

If you want to get more accurate, using 14 volts (FAA) vs. 13.8 volts (Wire Barn) makes a small correction down to 15.95 ft. max.

16 ft. is just about as accurate as you can read the logarithmic scale of the FAA chart.
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Last edited by az_gila : 09-18-2018 at 12:26 PM.
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  #6  
Old 09-18-2018, 12:05 PM
Grumpy-Hodge Grumpy-Hodge is offline
 
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Default CB Rating and Bundle Derating

Donít forget to size the wire to the rating of the Circuit Breaker. The Circuit Breaker should be appropriately sized for the load.

Also have a look at AC 43-13 Bundle Derating curves. The other wire charts are for performance of a single wire in free air.

If the wire is to be routed in a bundle, consider the Derating Factor from the Bundled Wire Derating curves.
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  #7  
Old 09-18-2018, 12:46 PM
rv7charlie rv7charlie is offline
 
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Respectfully, that's backward. Size the *wire* for the load, with acceptable voltage drop and heat rise. Then size the circuit protection to protect the wire. If that doesn't make sense, look at your house wiring. You won't find any 0.5A breakers in the panel to protect your wall wart cell phone chargers.
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  #8  
Old 09-18-2018, 03:52 PM
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az_gila az_gila is offline
 
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Exclamation

Quote:
Originally Posted by rv7charlie View Post
Respectfully, that's backward. Size the *wire* for the load, with acceptable voltage drop and heat rise. Then size the circuit protection to protect the wire. If that doesn't make sense, look at your house wiring. You won't find any 0.5A breakers in the panel to protect your wall wart cell phone chargers.
Perhaps not always... it depends sometimes on the actual load being protected. House wiring uses a minimum 14 g wire, no skinnier 120 volt wiring around.

From the FAA bible - AC43.13 - my highlight -

11-48. DETERMINATION OF CIRCUIT
BREAKER RATINGS. Circuit protection
devices must be sized to supply open circuit
capability. A circuit breaker must be rated so
that it will open before the current rating of the
wire attached to it is exceeded, or before the
cumulative rating of all loads connected to it
are exceeded, whichever is lowest.
A circuit
breaker must always open before any component
downstream can overheat and generate
smoke or fire. Wires must be sized to carry
continuous current in excess of the circuit
protective device rating, including its timecurrent
characteristics, and to avoid excessive
voltage drop. Refer to section 5 for wire rating
methods.


I read that as taking the manufacturer's recommended breaker size even if you happen to use a larger wire size.

As an example, the Garmin G5 calls for a 5A breaker and #22 wire. I personally prefer working with #20 wire for convenience but would still use the same 5A breaker.
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  #9  
Old 09-18-2018, 05:42 PM
rv7charlie rv7charlie is offline
 
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That's one of those cases where it could just be FAA-speak semantics, but it's either deceptive or wrong (it's probably both).

Take a look at virtually any certified a/c, and you'll see one protection device on a circuit feeding multiple devices. Consider examples where multiple relatively low current devices are on one circuit. If a 2A device develops a serious issue, it can easily make smoke without tripping a 5 or 7A breaker that's protecting multiple devices. In fact, virtually any device needing more than a few amps can make smoke without tripping its rated protection device. So AC43 has given mfgrs and maintenance folks a task at which all have failed, and is likely impossible.

I recognize that many mfgrs dump the responsibility of device current 'protection' on the systems guy, but this is really bad form. For the Garmin device you cite, does it spec *minimum* breaker & wire size, *maximum* breaker & wire size, or does it just say '5A & 22ga? The distinctions are important.

The wire conducts the current; the circuit protection protects the wire.
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  #10  
Old 09-18-2018, 06:33 PM
Grumpy-Hodge Grumpy-Hodge is offline
 
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rv7charlie is correct in many of his statements. One does have to size wiring with respect to the anticipated load, distances, wire types in terms of temperature rating, ambient temperatures, altitude (not so much an issue for unpressurized aircraft), and bundle design. All of it matters and needs to be considered. At times, a given factor may be determined to be negligible - but considered nonetheless.

I especially agree with rv7 charlie in his statement that circuit protection protects the wire. And this is really the point of my original comment. I did not know if Zazoos was connecting this 3A load to an existing CB branch or creating a new one. Had the 3A load been attached to a 5A CB branch for example, the wire size might need to be upsized to be compatible with the 5A CB. If the wire was upsized, it would remain a compatible for the 3A load. If not, the wire may begin to overheat in a current fault condition before the 5A CB tripped and opened the circuit. Not making a declaration that Zazoos needs a 5A CB. I don't know. But depending on the circuit protect implemented to source the 3A load, a wire that is sized only with respect to the 3A load and resistive losses might not be large enough to be compatible with the installed circuit protection. Such is why I made my comment about not forgetting to size the wire for circuit protection. It was intended as a both not either or statement.

The combination of these requirements have to be met. The wire has to meet load, loss (voltage drop), circuit protection and the other environmental and installation conditions.

Gil pulled the reference I had in mind. Read specifically the next to last sentence in Gil's AC43-13 reference beginning with "Wires...".

Just like the house wiring rv7charlie mentioned, 12-2 Romex is usually found on 20A branches and 14-2 Romex is usually found on 15A branches. If a wiring fault occurs, the CB should trip and arrest the flow of current in the wiring before the wiring overheats, catches fire and cause potentially catastrophic damage. An aircraft CB may not be able to prevent many fault conditions in an Line Replaceable Unit (LRU) from continuing unless it is of the magnitude of the CB rating or much greater as in a short condition, but the aircraft wiring should be protected if sized correctly not only for the load but for the circuit protection as well.
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