Which wires are fat, which are skinny?

To the many fans of AeroElectric connection, you know Bob Nuckolls advocates separation as a primary means of reducing the problems associated with Electromagnetic Compatibility. For example, he says to "keep fat wires (which tend to have strong, noisy currents) seperated from the skinny ones (which tend to be avionics and instrumentation). Half a page later he says "Avionics and audio systems should not share wire bundles with DC power distribution".

In the interests of trying to follow this advice, I'm trying to plan how I'd bundle the wires leaving the panel aftward, and in modern RVs, there's a lot of them. Here's a few:

• trim servo power and position sensor
• autopilot servo power and contol
• transponder antenna
• comm antenna
• GPS antenna
• strobe lights
• nav lights
• landing lights
• pitot heat
• magnetometer, OAT, fuel senders

Down the fuselage, you basically only get two bundles. Down each wing, you might get two bundles. Which wires are victims susceptible to interference? Which are the antagonist wires? Mr. Nuckolls mentions some, but his list is hardly exhaustive. For example, are the wires to the roll AP servo victims or antagonists? I've heard it both ways. Are they DC power distribution, or are they avionics? Should you be seperating the AP power wires from the AP control wires? Where do antennas fit into this? Would these go into the fat bundle or the skinny bundle?

More generally, how would you plan your bundles to minimize noise issues? What general rules of thumb do you follow? I dare say Mr. Nuckolls' book leaves more questions than answers on the topic.

I have a 99% electric noise free RV7 so I must have done something right.

As I recall, there are three types of wires. Antagonists, victims and neither.

As I recall, in mine there are 5 general wire paths out of the panel area.
-two going FWF, one through on the left for sensors (they are prone to be victims) and one gps antenna, one through on the right for power distribution.
-two going to the wings, (lLeft and right side of lower fuselage) and these also include some DC wires going aft.
-the antenna wires come from the radio stack directly towards the firewall, down the Center of the firewall, along side (but securely separated) the fuel lines, through the Center of the fuselage to the antenna locations. One comm under each outer seat pan, transponder about 24 inches aft of the baggage bulkhead, VOR in the top of vertical stab.
-the elevator autopilot control wires also take this center path but are separated by at least 2 inches all the way from the antenna wires. I don’t think my AP wires are antagonists or victims. They are twisted so that probably helps.
-the GPS antenna wires don’t really leave the panel area as the gps antennas are on the glairshield with one exception that a white gps antenna is FWF under the fibreglass engine cowl in the center. I didn’t want the white gps puck on the glairshield causing bad reflections.

The strobe power supply could be noisy so it’s located by itself behind the baggage bulkhead and the high voltage strobe wires go to the wing tips near the aft spar, and to the tail on one side of the fuse by themselves. They are highly antagonistic.

Overall, the only noise I get is a slight ticking from the wigwag landing lights when recording audio on a gopro camera. This is expected to go away when I switch to LED landing lights (due to lower inrush current draw).

I've got the same question, for an RV-6A, which has even more limited wiring path options for crossing the main wing spar from fwd to aft. I'm also trying to follow the physical bundling and mounting guidelines in AC43.13-1B Section 9.

The Dynon harness includes autopilot power and control wires within the same D-SUB connector, these wires are all 20-22 gauge and I assume they can be run together to their destination, otherwise they would have separate connectors?

Generally, my simplistic thoughts are:

Wing tip lighting wires (strobe, landing, wig-wag) will be separated from avionics system wires.

Audio wiring will be separated from all other wiring.

Antenna wiring will be separated from all other wiring.

I don't have much "heavy" wiring planned behind the firewall and this will all be routed via a fuse block and panel switches direct to equipment e.g. radio.

While I agree that the more current that a wire carries, the larger and stronger the offending magnetic field will be, I do not agree that all high current wires are noisy. Most of what we find as noise on the headsets is not a simple magnetic field, but noisy current signals (interrupted magnetic patterns) from things like strobes, LEDs, cheaply made USB ports, PWM, etc. Some of these are actually putting noise on the ground bus or power distribution bus and it is not bleed over across wires that cause it.

strong magnetic fields and their bleed over effect are more likely to cause problems on high speed data circuits than produce noticeable noise in a headset.


Larry