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johnt(at)comp-sol.com
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 Posted: Mon Feb 25, 2008 11:36 am Post subject: wire sizing |
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Bob,
A couple questions concerning wire sizing.
Given: Aeroelectric FAQ, it was mentioned that typical field current draw for Alternators (20, 40, 60A) is 1 to 1.5A, with 3A max...
What is the design Vd for Alternator field wiring?
What is the design I for the Alternator B lead? i.e. SD-20, L-40, L-60?
In Z14, 10 awg is suggested for the Aux alternator, and while fine Ampacity wise, I am wondering about voltage drop meeting 2% -
Given a rated power output of 20A (SD-20), a 2% Vd - wire length allowed is approx. 5.5 ft (one way) if 10 awg was used throughout...however, as the return path is a 2 awg suggested wire....??
FAA AC 43-13-1B on page 11-21 Par 11-66
"b. Voltage Drop in Wires. The voltage
drop in the main power wires from the generation
source or the battery to the bus should not
exceed 2 percent of the regulated voltage when
the generator is carrying rated current or the
battery is being discharged at the 5-minute
rate. The tabulation shown in table 11-6 defines
the maximum acceptable voltage drop in
the load circuits between the bus and the utilization
equipment ground."
Also - on Z14, 4awg is suggested for the Aux battery - can you provide some insight into that suggestion?
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mprather(at)spro.net
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| Posted: Mon Feb 25, 2008 12:39 pm Post subject: wire sizing |
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Alternators are often grounded to the engine case. From there they are
grounded to the battery with a cable large enough to support starting
loads (hundreds of amps). So, I don't think you have to worry about
round-trip wiring length.
If the one-way wire run from the aux alternator to its buss is
significantly greater than 5.5ft, I'd up-rate the wire one size.
I believe the components listed on the Z-diagrams are suggestions for
architecture layout based on a large set of assumptions. If the circuit
appropriate for your particular airplane requires something different,
deviation from the Z-diagrams makes sense.
Regards,
Matt-
| Quote: |
Bob,
A couple questions concerning wire sizing.
Given: Aeroelectric FAQ, it was mentioned that typical field current draw
for Alternators (20, 40, 60A) is 1 to 1.5A, with 3A max...
What is the design Vd for Alternator field wiring?
What is the design I for the Alternator B lead? i.e. SD-20, L-40, L-60?
In Z14, 10 awg is suggested for the Aux alternator, and while fine
Ampacity wise, I am wondering about voltage drop meeting 2% -
Given a rated power output of 20A (SD-20), a 2% Vd - wire length allowed
is approx. 5.5 ft (one way) if 10 awg was used throughout...however, as
the return path is a 2 awg suggested wire....??
FAA AC 43-13-1B on page 11-21 Par 11-66
"b. Voltage Drop in Wires. The voltage
drop in the main power wires from the generation
source or the battery to the bus should not
exceed 2 percent of the regulated voltage when
the generator is carrying rated current or the
battery is being discharged at the 5-minute
rate. The tabulation shown in table 11-6 defines
the maximum acceptable voltage drop in
the load circuits between the bus and the utilization
equipment ground."
Also - on Z14, 4awg is suggested for the Aux battery - can you provide
some insight into that suggestion?
|
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nuckolls.bob(at)cox.net
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| Posted: Mon Feb 25, 2008 3:31 pm Post subject: wire sizing |
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At 01:30 PM 2/25/2008 -0600, you wrote:
| Quote: |
Bob,
A couple questions concerning wire sizing.
Given: Aeroelectric FAQ, it was mentioned that typical field current draw
for Alternators (20, 40, 60A) is 1 to 1.5A, with 3A max...
What is the design Vd for Alternator field wiring?
|
It depends. If your regulator has a separate bus
sense lead like the later LR series regulators
from B&C, then you can practically downsize the
field current feeder to 22AWG for most installations.
HOWEVER: if your regulator's field supply input
-AND- bus voltage sense share the same wire, then
VERY SMALL (order of 0.3 volts) of drop in this lead
combined with fluctuations of field current in response
to loads and rpms can cause alternator system instability.
This is common on many spam cans and produces the
"galloping ammeter" effect.
This is why the Z-figures show 20AWG in feeders
from bus to the alternator regulator. If you dropped
to 18AWG, it wouldn't give me any heartburn.
| Quote: | What is the design I for the Alternator B lead? i.e. SD-20, L-40, L-60?
|
20, 40 and 60 Amps . . .
| Quote: | In Z14, 10 awg is suggested for the Aux alternator, and while fine
Ampacity wise, I am wondering about voltage drop meeting 2% -
Given a rated power output of 20A (SD-20), a 2% Vd - wire length allowed
is approx. 5.5 ft (one way) if 10 awg was used throughout...however, as
the return path is a 2 awg suggested wire....??
FAA AC 43-13-1B on page 11-21 Par 11-66
"b. Voltage Drop in Wires. The voltage
drop in the main power wires from the generation
source or the battery to the bus should not
exceed 2 percent of the regulated voltage when
the generator is carrying rated current or the
battery is being discharged at the 5-minute
rate. The tabulation shown in table 11-6 defines
the maximum acceptable voltage drop in
the load circuits between the bus and the utilization
equipment ground."
Also - on Z14, 4awg is suggested for the Aux battery - can you provide
some insight into that suggestion?
|
I would caution you and all OBAM aircraft builders
to approach AC43.13 with caution. It's more of an
engineering document than a cookbook. To make matters
worse, it was not crafted by individuals with a wide
range of practical experience that talked to each other
will writing their respective sections.
Unfortunately, folks charged showing us how much
more they know about airplanes that we do will snatch
things out of AC43.13 and flog the unwary designer
with it. Unless you plan to take up a career in
getting your handiwork certified, I'll suggest your
time is better spent doing a nice job on your
canopy installation or upholstery than slogging
your way through AC43.13.
Now to your question. If you size wires per the
suggestions in Figure 8.3 of the 'Connection
(See http://aeroelectric.com/articles/Wire_Chart.pdf )
. . . then you'll be fine. But know also that these
ratings are VERY conservative and sized for relatively
long runs where voltage drop MIGHT become an issue.
These ratings do not apply to FAT wires subject to
intermittent heavy loads. These would include battery,
starter and ground leads.
I show 4AWG for all these leads which are good values
if you're building a tractor a/c with battery
within 5 ft or so of engine. If you're building a
canard pusher with battery on other end of airplane,
a large composite a/c with aft mounted battery, or
a seaplane with 30 foot battery feeders, then the 4AWG
will serve you well.
Which, if any, wire sizes suggested in the z-figures
are of concern to you based on the special cases
cited above?
Bob . . .
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johnt(at)comp-sol.com
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| Posted: Tue Feb 26, 2008 3:22 pm Post subject: wire sizing |
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>> Which, if any, wire sizes suggested in the z-figures
are of concern to you based on the special cases
cited above?
I am interested in sizing the B-leads for the alternators.
I am building a Velocity XL-FG. I will be using an L-40, and an SD-20 alternator. The distance from the SD-20 to the battery box is 18.67 feet, and the contactor will be within 6" of the battery. I will use 2 awg from the battery to the firewall - and 2 awg eq. bonding straps to the engine (Gnd).
If the design I for the SD-20 Alternator B lead is 20A, we must still decide upon a Vd to size the wire. As you know, Figure 8-3 in Chapter 8 of your book shows the Ampacity of the wire. This is chosen for safety, not Vd.
I have chosen 2% bus feed loss, and 3% device feed loss (5% max loss from device to battery) as a design goal.
Calculating awg required for a 2% loss (at) 18.67 feet = 4.85 awg, so 4 awg would be suggested - however, we have a 2 awg return path. This suggests downsizing to a 6 awg or possibly 8 awg - - I am unsure how to calculate awg required when multiple awg's are used.
With the L-40, the distance is 19.33 feet. (at) 40A 2% Vd, awg required is 1.7 - using 2 awg throughout, we should be about 2.15% Vd.
[quote][b]
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nuckolls.bob(at)cox.net
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| Posted: Tue Feb 26, 2008 6:12 pm Post subject: wire sizing |
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At 05:17 PM 2/26/2008 -0600, you wrote:
| Quote: | >> Which, if any, wire sizes suggested in the z-figures
are of concern to you based on the special cases
cited above?
I am interested in sizing the B-leads for the alternators.
I am building a Velocity XL-FG. I will be using an L-40, and an SD-20
alternator. The distance from the SD-20 to the battery box is 18.67 feet,
and the contactor will be within 6" of the battery. I will use 2 awg from
the battery to the firewall - and 2 awg eq. bonding straps to the engine (Gnd).
If the design I for the SD-20 Alternator B lead is 20A, we must still
decide upon a Vd to size the wire. As you know, Figure 8-3 in Chapter 8
of your book shows the Ampacity of the wire. This is chosen for safety,
not Vd.
I have chosen 2% bus feed loss, and 3% device feed loss (5% max loss from
device to battery) as a design goal.
Calculating awg required for a 2% loss (at) 18.67 feet = 4.85 awg, so 4 awg
would be suggested - however, we have a 2 awg return path. This suggests
downsizing to a 6 awg or possibly 8 awg - - I am unsure how to calculate
awg required when multiple awg's are used.
With the L-40, the distance is 19.33 feet. (at) 40A 2% Vd, awg required is
1.7 - using 2 awg throughout, we should be about 2.15% Vd.
|
Consider Z-12 and feed both the L-40 and SD-20 into
the system at the starter contactor on the firewall.
Now you get to use 2AWG starter and ground feeders
as feeders for the alternators as well. Your small
wires from the b-leads are only a few feet long
and drops are insignificant.
Further, voltage drops in alternator output leads
do not fall into the 5% rule of thumb for drops
that feed loads. Consider the fact that your
alternator's REGULATOR adjusts field votlage
in the alternator such that the BUS is maintained
at the desired setpoint.
If the drops in alternator leads exceeds 5%,
the airplane never knows it . . . because the
regulator will cause your alternator to put out
15 or more volts as needed to accommodate the
desire for a 14.2 volt bus.
But if it were my airplane, I'd take advantage
of those two FAT wires that have to be there
already to accommodate the starter.
Bob . . .
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