nuckollsr(at)cox.net
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 Posted: Sun Jan 14, 2007 8:16 am Post subject: Voltage Regulator Wire Sizes: Do I Really Need 12 AWG? |
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At 07:44 PM 1/13/2007 -0600, you wrote:
| Quote: | Folks,
Per page 4-7 of the "Connection", I'm planning on running the Field and
two ground wires from the B&C LR3C-14 to the battery in the back of my
GlaStar. The round trip distance between the battery location and the
regulator is (generously) about 40' (it may turn out closer to 35'
depending on wire routing and final battery location).
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| Quote: |
Using the 5 Amp rating of the field breaker, my figuring using Figure 11-2
in AC 43.13 says to use a 12 AWG wire. That seems awfully "fat",
especially since the B&C instructions specify 18 AWG. Fig. 11-2 indicates
that 18 AWG will carry 5 Amps continuously for about 16', which sounds
about right for the distance from the alternator to the regulator and from
the regulator to a power buss on the panel.
Do I really need 12 AWG here? I don't recall seeing any information on
the normal load on the Field wire. Since the B&C instructions call for
redundant ground wires, can I use two 15 AWG for the ground wires and a 12
AWG for the Field wire?
(My calculations don't take into account that these voltage regulator
wires would be bundled with the (2) 2 AWG "mains"; the two wires for the
battery contactor and an "always hot" wire.)
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The schematics on 4-7 are a discussion of the effects of voltage
drop in wiring and the advantages of choosing voltage sense
points with understanding. As a practical matter I recommend
the following:
Mount the regulator up front. Either side of the firewall is
fine. Ground the regulator's "ground" terminal and case to
the firewall ground block. Run 20AWG wire for the field supply
and field output leads by the shortest practical route to
alternator and DC PWR Master switch. Since the LR series
regulators now feature remote sensing by a conductor
separate from the field supply wire, concern for voltage
drops described in Chapter 4 have been eliminated by
design.
See Z-figures.
| Quote: |
Unrelated "bonus" question. I there any reason I can't bundle the Field
wire from the alternator back through the firewall with the thermocouple
wires for the CHT and EGT? This bundle will also have another wire from
the alternator to a test point on the panel so I can measure the Field
voltage from the cockpit per Bob's suggestion.)
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There are NO constraints for bundling wires of various
systems together - assuming each of the systems in question have
been designed to live in the real world.
Bob . . .
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