Proper wire size for land/taxi lights

[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]

Originally Posted by 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'.

[Dean_aeroleds]

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]

Originally Posted by 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'.

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]

Originally Posted by alpinelakespilot2000

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]

Originally Posted by alpinelakespilot2000

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]

Originally Posted by Captain Avgas

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.