[alpinelakespilot2000]

I'm in the process of learning about electricity and am stumped about what should be a basic question.

In examining other people's RV-9A schematics, I see many people using 10A fuses for the 100W landing and 100W taxi light circuits with 14AWG or 16AWG wire. Based on my reading (of Van's and Bob Nuckolls), this seems problematic. Can someone please point out where my logic is breaking down (and I humbly assume that it is):

1. Since 100W bulbs will draw 8 amps (according to Van's section 5 chart), I will need to use a 10A fuse, since it is the next higher size.

2. Since my RV-9 Wings are 12 feet long and because it will take another 7 feet to route the wires up to the switches and fuses, I need to plan for a run of about 19 feet.

3. Using the other chart in Van's section 5, I find that a 10A circuit with a 19 foot run requires 12AWG wire. (The chart shows an approximate 11-16 ft range for 14AWG and an approximate 16-25 ft range for 12AWG at 10A.)

Thus my question. How are other RV-9A builders (with longer wings) getting away with 14AWG wire on 10A circuits?

Perhaps they found some way to limit their wire run to < 16 ft? (Is this possible with Duckworks lights in their normal location? I can see how it would be on the short-wing RVs, but not so on the longer-winged -9)

I also understand that if they had lower wattage bulbs in their landing lights, they would be drawing less current, in which case the 14AWG would work just fine under normal circumstances. However, Bob Nuckolls argues that light fixtures when they burn out can either open and draw no current, or remain closed and draw way more than their intended current. Thus, according to Nuckolls, the wire must be appropriately sized for the 10A fuse, regardless of what the fixture normally draws.

Given this, on a 10A circuit with a run of approximately 19 feet, isn't 12AWG the most appropriate wire size for 100W lights?

Any help on this? Thanks.

 

[vlittle]

Hi Steve, I hope I can help.

Rather than using other sources, go to AC 43.13 and the wire selection chart for continuous flow. (Sorry, I don't have the figure number handy).

Use the normal load (not the circuit breaker size) as your current. Also, try to estimate the wiring length to the nearest foot. 19 feet sounds close.

With an 8 amp load, my chart shows between 12 and 14 guage wiring is appropriate (not including derating for bundles with heavy loads, such as pitot heat).

A 12 guage wire can be protected with a 30 amp breaker or 20 amp fuse (AC 43.12 Table 11-3). 14 guage is 20/15. Breaker/fuse ratings are to protect the wire, not the load! Don't fall into the trap of sizing the breaker to the load current. This means that you could use larger breakers than you are planning, which helps to reduce voltage drops in the circuit. Beware, however, that the switches may not be able to handle the fault loads with large breakers.

Also, lamps draw a lot more current when cold, and you don't want nuisance trips of the breaker. A practical size is 15A. My 9A uses 14AWG, 15A breakers and CreativeAir 75W lights.

If you want a little more background, check here: http://raa85.b4.ca/story/index.shtml

Click on Avionics (Vern Little) - Part 3 for the Powerpoint.

Vern

 

[FredMagare]

Steve, I'm not at my schematic right now but it looks like you've crawled through the proper wickets to arrive at your decision. You could throw in a safety factor and go with 10awg... yet keep the same breaker/fuse.

 

[alpinelakespilot2000]

However, Bob Nuckolls argues that light fixtures when they burn out can either open and draw no current, or remain closed and draw way more than their intended current. Thus, according to Nuckolls, the wire must be appropriately sized for the 10A fuse, regardless of what the fixture normally draws.

Thanks for the help guys. Does anyone know where I got this idea above. I'm (99.9%) sure I didn't dream it up (I'm not yet dreaming about electricity! ), but I can't remember where in Aeroelectric I read it. In any case, this is really what is confusing me, even though Vern's suggestion above also sounds very reasonable, but a little contradictory. Do I size the wire for what the fixture will pull (8A) or do I size the wire for what the fuse (next size up is 10A) would potentially allow if there was a failure like that described by Nuckolls?

Thanks also to the reference to 43.13, Vern. I looked up the table and I think it is Figure 11-2 that you were referring me to. However, even on that table, if my lamps are pulling 8A continuous, and I'm using a 19 foot run, the curve ends up between 14AWG and 12AWG, meaning I'd HAVE to use 12AWG, no?

 

[Rupester]

Picking lint jus' a little .... I think your 19' calculation is plenty. The wings may be 12' long to the outside tips, but your landing lights are 2'+ closer to the fuse. When I measured mine ... allowing LOTS of slop for multiple mistakes at the end connections, I got 18'.

 

[AeroLEDs_Support]

Here is a good table showing current load vs. AWG:
http://www.powerstream.com/Wire_Size.htm

Note that the Maximum Amps for Power Transmission figure is VERY conservative.

For the AeroSUN 1600 which draws close to 2 Amps at 12VDC, we recommend 20 gauge wire, although some people might want to use 18 gauge if they want to be very conservative...

For a 100W halogen, you would want to use 12 to 14 gauge wire.

Dean Wilkinson
AeroLEDs LLC

 

[alpinelakespilot2000]

Picking lint jus' a little .... I think your 19' calculation is plenty. The wings may be 12' long to the outside tips, but your landing lights are 2'+ closer to the fuse. When I measured mine ... allowing LOTS of slop for multiple mistakes at the end connections, I got 18'.

Thanks Terry. You're probably right that it's less than 19'. I did a quick measurement. That said, I had originally planned on running my wire out all the way through the conduit to the outboard wing rib and then run it inboard to the lights. My Duckworks cutout is in the outboardmost bay, so the 12 feet I cited was for the 11' of wing (without wingtips) plus one foot back into the outboard bay. In any case, if I want to get the run below 16ft so that I can use 14AWG, I'll have to do some real careful or creative routing.

 

[vlittle]

Thanks for the help guys. Does anyone know where I got this idea above. I'm (99.9%) sure I didn't dream it up (I'm not yet dreaming about electricity! ), but I can't remember where in Aeroelectric I read it. In any case, this is really what is confusing me, even though Vern's suggestion above also sounds very reasonable, but a little contradictory. Do I size the wire for what the fixture will pull (8A) or do I size the wire for what the fuse (next size up is 10A) would potentially allow if there was a failure like that described by Nuckolls?

Thanks also to the reference to 43.13, Vern. I looked up the table and I think it is Figure 11-2 that you were referring me to. However, even on that table, if my lamps are pulling 8A continuous, and I'm using a 19 foot run, the curve ends up between 14AWG and 12AWG, meaning I'd HAVE to use 12AWG, no?

Size the wire for the actual load, size the breaker for the wire size (or the maximum rating of the switch, whichever is lower). The breaker is there to protect the wire (and switch).

As for having to use 12AWG, read AC43.13 carefully to understand the reasoning. You may be able to justify a landing light as an intermittent load with appropriate compromises.

 

[Captain Avgas]

Since my RV-9 Wings are 12 feet long and because it will take another 7 feet to route the wires up to the switches and fuses, I need to plan for a run of about 19 feet.

The length of the wire run also has to include the length of the ground lead. Some people take all grounds back to a common ground on the firewall (to avoid comm noise caused by ground loops) or because they have composite planes.

You may be opting for a local ground. Even so, that distance of wire should be added to your total wire length.

 

[airguy]

You may be opting for a local ground. Even so, that distance of wire should be added to your total wire length.

I'm assuming you mean the length of ground wire from the load to the connection point on the frame - the frame itself serving as ground would be equivalent to a VERY large wire size, and can be treated (for our purposes) as having infinite load-carrying capacity. The only other limitation would be the final ground strap from the frame back to the battery, which should be sized large enough to handle everything up to and including starter draw.

 

[alpinelakespilot2000]

The length of the wire run also has to include the length of the ground lead. Some people take all grounds back to a common ground on the firewall (to avoid comm noise caused by ground loops) or because they have composite planes.

You may be opting for a local ground. Even so, that distance of wire should be added to your total wire length.

Yes, I was planning on a local ground and did include an extra 4-6" in my 19' estimate.

That said, has anyone had any issues with noise emanating from locally grounded landing and taxi lights. I plan on having mine wig-wagging (while in busy airspace) if that makes any difference regarding the ground location.

 

[Captain Avgas]

I'm assuming you mean the length of ground wire from the load to the connection point on the frame.

Yep, when I say "wire", I mean "wire".

 

[kentb]

Vernon, I am not sure I agree.

Size the wire for the actual load, size the breaker for the wire size (or the maximum rating of the switch, whichever is lower). The breaker is there to protect the wire (and switch).

As for having to use 12AWG, read AC43.13 carefully to understand the reasoning. You may be able to justify a landing light as an intermittent load with appropriate compromises.

I had to read this two or three times, before I decided that this is not quite correct.
It is true that you need to protect the wire with the fuse, but any time the circuit draws more amps then the device under power, then you should shutdown the circuit. If you size the wire correctly then the wire is never in danger.

I guess that if you ever added more devices to the wire and didn't exceed the rating of the wire, then the fuse would need to be adjusted upwards, but I don't think that this happens very often after the plane is wired. If you were to do this and had fuse blocks, you could just change the fuse to a higher one.

Kent

 

[Kevin Horton]

There are several different aspects to consider when selecting wire size and fuse rating.

1. The fuse (or CB) rating should be high enough so you don't get nuisance trips during the initial in-rush current. If you screw this up, it is not a safety issue, but it would be an operational and maintenance issue.
2. The wire must be big enough to handle continous current at the fuse rating without overheating. This ensure that there is no electrical fire if there is a short. Or, put another way, the fuse must be small enough so it would blow before the wire overheated. This aspect is a safety issue, so pay attention to it.
3. The wire should be big enough so that the voltage drop due to resistance in the wiring under normal loads is not too high. This is so that you have enough voltage available to provide to the lamp (or whatever is being powered by this wire). This consideration is what the tables that give max wire length for a given current are all about. These tables typically ensure that no more than 5% of the voltage is lost in the wiring resistance. The world isn't going to end if you violate this rule - the only consequence is that your landing light will be a bit dimmer than it otherwise would be.

The only consideration that has a safety aspect is #2. You can ignore items #1 or #3 if you want without compromising safety.

 

[apatti]

There are several different aspects to consider when selecting wire size and fuse rating.

1. The fuse (or CB) rating should be high enough so you don't get nuisance trips during the initial in-rush current. If you screw this up, it is not a safety issue, but it would be an operational and maintenance issue.
2. The wire must be big enough to handle continous current at the fuse rating without overheating. This ensure that there is no electrical fire if there is a short. Or, put another way, the fuse must be small enough so it would blow before the wire overheated. This aspect is a safety issue, so pay attention to it.
3. The wire should be big enough so that the voltage drop due to resistance in the wiring under normal loads is not too high. This is so that you have enough voltage available to provide to the lamp (or whatever is being powered by this wire). This consideration is what the tables that give max wire length for a given current are all about. These tables typically ensure that no more than 5% of the voltage is lost in the wiring resistance. The world isn't going to end if you violate this rule - the only consequence is that your landing light will be a bit dimmer than it otherwise would be.

Is is "safe" to assume that if the wire is sized according to the load/distance tables (that assume no more than 5% drop) that the wire won't overheat? If not, how do you size for heat (err... to not overheat).

 

[gmcjetpilot]

Wait a second lets not get crazy

Is is "safe" to assume that if the wire is sized according to the load/distance tables (that assume no more than 5% drop) that the wire won't overheat? If not, how do you size for heat (err... to not overheat).

Whooo, 10 awg is getting out of hand, in my opinion. 16 awg or 18 awg would work.

You have three criteria (you set/good practice/as needed):
Weight
Temp rise
voltage drop

Factors are:
volts
length
current
Wire open or in bundle
continuous/intermittent ops
(last factor is sizing CB/fuse for wire gauge)

This criteria you set. I would not assume what a chart is based on unless the chart says so.

You can accept more than 5% voltage drop, in fact I would accept more with high current items, unless there is a good reason. A bulb can run on 12 or 13 volts can't it? (yes) Other wise you will add lots of weight. 10% or 15% volt drop can be acceptable (as you determine).

Temp rise, is not an issue with a wire out in then open within reason. Warm to touch 120F is OK, So 28F or 15C rise is fine.

So using 18 awg for 10 amps over 20 feet - You get 28F or 15C rise and 1.27 volt drop. Lets assume the alternator puts out 14.5 volts and you have 14.3 volts at the buss, (14.3-1.27)/14.3 >> 9% volt drop. The light will work fine on 13 volts (14.3-1.27), yes? On a 100F day, 100F+28F = 128F, is safe temp.

NOW LOOK AT WEIGHT 10, 14, 16 and 18 awg for 20 feet:
18 awg = 0.10 lbs
16 awg = 0.16 lbs
14 awg= 0.25 lbs
10 awg = 0.63 lbs

Don't go crazy with wire gauge. If there is one place to run a heaver wire than do it for the Alternator B-lead and Battery to Buss wires. You don't want to start off with excess loss for the whole plane. Individual circuits can handle voltage drop.

18 awg will work, but be conservative and use 16 awg if you want. The weight penalty is not bad. Going to 10 awg will add over 1/2 lb of weight per landing light. Even 20 feet of 14 awg at 0.25 lbs is getting fat. Use what you need, but don't go crazy about min voltage drop or a few degrees temp rise. However it depends on the using device and its voltage requirement. A bulb can run on 13 volts or less.

Any of these wires 10, 16 or 18 can be protected with 10 amp CB/fuse.

Here is a excel spread sheet you can plug numbers into:

http://www.rst-engr.com/rst/jimsdata/wirecalc.xls

 

[Kevin Horton]

Is is "safe" to assume that if the wire is sized according to the load/distance tables (that assume no more than 5% drop) that the wire won't overheat? If not, how do you size for heat (err... to not overheat).

That depends on what type of wire you are using, and the details of the installation. The voltage drop is the same no matter what the insulation is made from, but the allowable temperature (and thus the allowable current) depends on the insulation material.

The voltage drop table is meant to be used with the actual current used by the device in continuous operation, not the fuse rating. You also need to look at the current capacity of the wire gauge and compare that to the fuse rating. See table 11-9 in AC 43.13-1B. It has different values of current capacity depending on the max allowable temperature of the wire. If you used typical MIL-W-22759/16 aviation wire, it has a 150 deg C rating, and you would find current capacities of:

22 AWG - 5 amp
20 AWG - 7 amp
18 AWG - 9 amp
16 AWG - 11 amp
14 AWG - 14 amp
12 AWG - 19 amp
10 AWG - 26 amp
etc

There are some conditions that are attached to the above values, but they should be applicable for typical installation. You need to read the fine print on that table and refer to section 11-69 if you have an atypical installation (wire in abnormally hot location, or abnormally large number of wires in a wire bundle, or many wires in the bundle operating at close to their capacity, etc).

If you have a long wire run, I suspect the limiting factor on wire size will be the voltage drop (assuming you have wire that is rated for at least 150 deg C). If you have a short wire run, the limiting factor will probably be the wire current capacity vs fuse rating.

I just looked at AC43.13-1B. I was a bit wrong on the voltage drop table - the table for continuous operation on is based on a 0.5v drop on a 12v system, not 5%.

 

[vlittle]

I had to read this two or three times, before I decided that this is not quite correct.
It is true that you need to protect the wire with the fuse, but any time the circuit draws more amps then the device under power, then you should shutdown the circuit. If you size the wire correctly then the wire is never in danger.

I guess that if you ever added more devices to the wire and didn't exceed the rating of the wire, then the fuse would need to be adjusted upwards, but I don't think that this happens very often after the plane is wired. If you were to do this and had fuse blocks, you could just change the fuse to a higher one.

Kent

Some people size the fuse/breaker according to the load. This is not a problem per se, but it's not the job of the fuse to protect the load. Think of your household circuits-- they have 15A breakers, but feed all kinds of tiny loads.

You may say "yeah but I know the maximum load". I respectfully suggest that you don't. What is the maximum load of a 100W landing light... 8 Amps?
No, it's probably much more. If you don't understand why, then I've proven my point. Putting a 10A breaker in this circuit may cause nuisance trips.

So, the standard practice is to protect the wire, not the load. If you then get nuisance trips, you REALLY have a problem that needs attending to.

I didn't make this up... study AC43.13 and you'll be fine.

Vern

 

[Captain Avgas]

You may say "yeah but I know the maximum load". I respectfully suggest that you don't. What is the maximum load of a 100W landing light... 8 Amps?
No, it's probably much more. If you don't understand why, then I've proven my point. Putting a 10A breaker in this circuit may cause nuisance trips.

Vern

Vern,
I think that what you are suggesting here is that circuits containing incandescent lamps can draw an initial current that is up to 15 times greater that the continuous current.

Following up on that, I note in table 11-4 (Switch derating factors) of AC43.13 that switches for lamps are supposed to have a derating factor of 5. I take that to mean that if Steve Moore's landing light is drawing approx 8 amps continuously the switch should have a rated capacity of 40 amps. Could that possibly be right. What switch rating would others recommend in this particular application.

 

[alpinelakespilot2000]

This has all been very interesting to read, but it sounds like there is some significant disagreement happening, even among those who seem to know a lot about electricity.

Van's outlines an approach to choosing fuse and wiring sizes in Section 5, pp. 5-17 through 5-20 (3/31/04 edition) that appears pretty logical and also pretty conservative. Thus, aside from the extra weight penalty of a few ounces by using larger wire on these one or two long runs to the wing tips, is there anything wrong with us non-E.E. types just following Van's approach? Is Van's wrong here?

I'm not giving up on learning, so keep educating!

 

[szicree]

Steve,

Don't know if it helps or not, but the instructions I got with my wig-wagger from Nuckoll's say to use 16awg for lights up to 100 watts, 14awg for larger. The circuit diagram shows this wire being used for both the flash mode and the constant mode.

 

[vlittle]

This has all been very interesting to read, but it sounds like there is some significant disagreement happening, even among those who seem to know a lot about electricity.

Van's outlines an approach to choosing fuse and wiring sizes in Section 5, pp. 5-17 through 5-20 (3/31/04 edition) that appears pretty logical and also pretty conservative. Thus, aside from the extra weight penalty of a few ounces by using larger wire on these one or two long runs to the wing tips, is there anything wrong with us non-E.E. types just following Van's approach? Is Van's wrong here?

I'm not giving up on learning, so keep educating!

Thanks for the sanity... no Van's isn't wrong. If we had to analyze every recommendation that Van's gave us we'd never get the airplane built. We engineers will sometimes carry on arcane arguments until everyone falls asleep.

V

 

[Captain Avgas]

Anyone care to take a stab at answering my question on an appropriate switch rating as per post #19.

 

[vlittle]

Anyone care to take a stab at answering my question on an appropriate switch rating as per post #19.

I'm not a switch expert, and switch ratings depend on whether the circuit is being opened/closed or continuous operation. Other can chime in.

What I did was put inrush current limiters into my landing light circuits (aka NTC Thermistors). This saves the switches from overloads. My particular application uses 8 amp limiters (for a 75W lamp).

Available from Digi-Key in a number of ratings.

 

[Lars]

This is slightly tangential but I think worth noting (and yes, I'm also an engineer)...

In the case of the landing light circuit, it would be nice to get something close to the rated light output of the bulb when it's turned on. Otherwise, why bother with high-output lights? It turns out that bulbs are very sensitive to voltage drop, and it's not linear. The following link addresses the use of relays in automotive lighting circuits, but there is some relevant information contained in it: http://www.danielsternlighting.com/tech/relays/relays.html

My own experience with voltage drop and lighting was on my old truck. I had previously run 14 gage wire conventionally, through the headlight switch, powering headlights with 100 watt high beam filaments. The headlights seemed to work fine, but said truck, being a hobby, got treated to a relay upgrade using 12 gage wire. I had no way to quantify the results, but based on the eyeball test the difference was astonishing. Note that I'm not advocating the use of relays in an aviation application.