Twisted primary wire or Shielded near compass?

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Dale Isenhoff

I am mounting a new electronics pod on the Edson binnacle and am unsure if simply twisting the ancor wires is sufficient protection against the 12V current deflecting my compass, (also in binnacle), or is a shielded cable a better solution? Given the current draw from the new autopilot, I suspect the shielded stuff is too small a gage. Suggestions?
 
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Ron

Twist vs Shield

Twisted pair negates more of the magnetic field, whereas shielding (e.g., tinned copper braid) mainly affects the electrical field. If the compass is your only concern, used twisted pair. More importantly, you want to keep any source of current flow far away from your compass -- and that includes the fluxgate compass for your autopilot. In a practical sense, the use of an instrument pod near the compass binnacle represents a trade-off for convenience. A magnetic field is generated around any conductor carrying an electric current, and the strength of that mag field is proportional to the amount of current. The strength of the magnetic field drops off very quickly with distance. For this reason, the instruments used on these pods generally carry very low currents, and we try to run wiring at least a meter from these instruments. Most autopilots have only low current signal and lighting wiring to the actual pod-mounted instrument. There is generally a heavier set of power wiring that runs from the circuit breaker to the servo drive connected to your wheel or tiller. Typically, you can keep that wiring far enough from the compass(es) to minimize interference. The wire guage you use should be based upon the current it has to carry, the distance (round trip!) it must carry it, and the acceptable voltage drop in the wires. Consult a good boat wiring guide, or one of Nigel Calder's books, for wiring to the applicable codes. It might be a good idea to swing your compass to establish the deviation table after doing any work that disturbs the wiring. Do it first with all electrical stuff off, then repeat with the max load (e.g., all instrument lights on, radar on, autopilot on and driving a turn, etc.). --Ron
 
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