Today's Message Index:
----------------------
1. 06:08 AM - Re: [OT] boat wiring specificity (h&jeuropa)
2. 09:49 AM - voltage drop calulations (Ben Westfall)
3. 03:22 PM - Re: wire sizing (John Tvedte)
4. 04:33 PM - Re: voltage drop calulations (Robert L. Nuckolls, III)
5. 05:22 PM - Re: Re: [OT] boat wiring specificity (Robert L. Nuckolls, III)
6. 06:12 PM - Re: Re: wire sizing (Robert L. Nuckolls, III)
7. 07:59 PM - Re: voltage drop calulations (academic exercise) (raymondj)
8. 09:17 PM - Re: voltage drop calulations (academic exercise) (Ron Quillin)
9. 11:48 PM - Re: voltage drop calulations (academic exercise) (raymondj)
Message 1
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Subject: | Re: [OT] boat wiring specificity |
I've been considering switching to RG batteries in my sailboat. Batteries are
used for all power and the engine isn't run very often. There is a power charger
used in harbor. The lead acid batteries go 3-4 days without recharge. Would
RG do better? The conventional batteries are just auto batteries - supposedly
deep cycle. Would I need the same size RG batteries?
Thoughts, opinions, experiences?
Jim Butcher
Europa XS N241BW
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=166316#166316
Message 2
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Subject: | voltage drop calulations |
When computing the voltage drop for items grounded locally to the airframe
on a metal airplane what length of wire do you use? Do you add any
additional length to the wire for the ground wire? Does the chassis provide
a measurable amount of resistance? Example. Let's say I am grounding my
wingtip nav lights locally to the airframe. The hot wire is 20ft in total
with 1 ft wire grounding to the airframe. Do you use 21ft as your length
value when calculating voltage drop? If you did have to account for the
return through the chassis how would you decide on an ohm/ft value?
Sorry if this seems basic but I have never run across specifics of this in
all my readings.
Ben Westfall
RV-10
Message 3
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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 alte
rnator. 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 decid
e 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 d
evice to battery) as a design goal.
Calculating awg required for a 2% loss @ 18.67 feet = 4.85 awg, so 4 awg
would be suggested - however, we have a 2 awg return path. This suggests d
ownsizing 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. @ 40A 2% Vd, awg required is 1.
7 - using 2 awg throughout, we should be about 2.15% Vd.
Message 4
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Subject: | Re: voltage drop calulations |
At 09:45 AM 2/26/2008 -0800, you wrote:
>When computing the voltage drop for items grounded locally to the airframe
>on a metal airplane what length of wire do you use?
What ever that length is. The airframe adds a negligible
amount of resistance to the total.
> Do you add any additional length to the wire for the ground wire? Does
> the chassis provide a measurable amount of resistance?
No.
> Example& Let s say I am grounding my wingtip nav lights locally to the
> airframe. The hot wire is 20ft in total with 1 ft wire grounding to the
> airframe. Do you use 21ft as your length value when calculating voltage
> drop? If you did have to account for the return through the chassis how
> would you decide on an ohm/ft value?
If you really want a value, use .002 ohms as a
ballpark.
>
>
>Sorry if this seems basic but I have never run across specifics of this in
>all my readings.
This is because we generally don't have to worry about voltage
drop. Our airplanes are small. Even when figuring the round-trip
drop for feeder and ground in a composite airframe, the
suggested wire sizes in the tables are conservative enough
to accept at face value.
It's an interesting academic exercise but not very
fruitful in practice. If I were working on very long
runs of wire in a 767 or B52, I'd have to get out
the calculator and consider the need for up-sizing
a wire for voltage drop considerations.
Bob . . .
----------------------------------------)
( . . . a long habit of not thinking )
( a thing wrong, gives it a superficial )
( appearance of being right . . . )
( )
( -Thomas Paine 1776- )
----------------------------------------
Message 5
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Subject: | Re: [OT] boat wiring specificity |
At 06:02 AM 2/26/2008 -0800, you wrote:
>
>I've been considering switching to RG batteries in my sailboat. Batteries
>are used for all power and the engine isn't run very often. There is a
>power charger used in harbor.
Is it a "smart" charger? Know when to quit
charging and hold the batteries at their
appropriate float voltage?
>The lead acid batteries go 3-4 days without recharge. Would RG do better?
Capacity is capacity is capacity . . . irrespective
of the technology wherein that energy is stored.
Unless you change technologies, i.e. swap out
lead-acid for lithium-ion, then the amount of energy
store per pound our cubic foot of battery isn't
going to improve much by swapping out flooded
batteries in favor of RG batteries.
> The conventional batteries are just auto batteries
> - supposedly deep cycle. Would I need the same size RG batteries?
If it's time to replace your existing batteries,
an set of RG batteries might offer a bit more capacity
for the weight and size but not much. They are safer
in closed spaces. Flooded batteries ALWAYS out-gas
during charging. Battery spaces aboard boats and airplanes
for flooded batteries need deliberate attention to
venting.
RG batteries may be a bit cleaner. Being sealed
the tend not to grow ugly fuzz on the terminals.
But for deep cycle service, you're probably going
to get a better return on investment on batteries
intended for deep cycle service like wheelchair
and golf-cart batteries.
RG batteries are the obvious choice for aircraft
for weight reduction and better cranking performance
at low temperatures. Unlike batteries in your boat,
the aircraft battery may run for years never being
called upon to support a deep-cycle load. I'm not
sure I can recommend you go out and replace your
existing set of batteries with RG devices. If you
perceive a good return on investment for the batteries
you've been using, you may want to stay with them.
Bob . . .
Message 6
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At 05:17 PM 2/26/2008 -0600, you wrote:
> >> 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 @ 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. @ 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 . . .
Message 7
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Subject: | Re: voltage drop calulations (academic exercise) |
Bob,
Academic exercise question.
Why does the resistance in the return path (assuming DC and resistive load
element) affect the voltage seen by the element being provided power? Or am
I thinking '60 British (positive ground).
Thanks in advance,
Raymond Julian
Kettle River, MN
"Hope for the best,
but prepare for the worst."
do not archive
----- Original Message -----
From: "Robert L. Nuckolls, III" <nuckolls.bob@cox.net>
Sent: Tuesday, February 26, 2008 6:29 PM
Subject: Re: AeroElectric-List: voltage drop calulations
> <nuckolls.bob@cox.net>
>
> At 09:45 AM 2/26/2008 -0800, you wrote:
>
>>When computing the voltage drop for items grounded locally to the airframe
>>on a metal airplane what length of wire do you use?
>
> What ever that length is. The airframe adds a negligible
> amount of resistance to the total.
>
>> Do you add any additional length to the wire for the ground wire? Does
>> the chassis provide a measurable amount of resistance?
>
> No.
>
>> Example& Let s say I am grounding my wingtip nav lights locally to the
>> airframe. The hot wire is 20ft in total with 1 ft wire grounding to the
>> airframe. Do you use 21ft as your length value when calculating voltage
>> drop? If you did have to account for the return through the chassis how
>> would you decide on an ohm/ft value?
>
> If you really want a value, use .002 ohms as a
> ballpark.
>>
>>
>>Sorry if this seems basic but I have never run across specifics of this in
>>all my readings.
>
> This is because we generally don't have to worry about voltage
> drop. Our airplanes are small. Even when figuring the round-trip
> drop for feeder and ground in a composite airframe, the
> suggested wire sizes in the tables are conservative enough
> to accept at face value.
>
> It's an interesting academic exercise but not very
> fruitful in practice. If I were working on very long
> runs of wire in a 767 or B52, I'd have to get out
> the calculator and consider the need for up-sizing
> a wire for voltage drop considerations.
>
> Bob . . .
>
>
> ----------------------------------------)
> ( . . . a long habit of not thinking )
> ( a thing wrong, gives it a superficial )
> ( appearance of being right . . . )
> ( )
> ( -Thomas Paine 1776- )
> ----------------------------------------
>
>
>
Message 8
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Subject: | Re: voltage drop calulations (academic exercise) |
Because the load 'sees' the supply voltage referenced to it's, the
load's, local ground. If the local ground isn't really 'ground', that
there is a voltage differential between 'true' ground and local ground
caused by ground currents, the local ground is offset by that voltage
from the true supply ground. So the actual supply voltage the load
sees is reduced by the voltage in the return path and the load doesn't
really see the full supply voltage, as it is reduced by losses in both
the supply and return wiring. It's all just like a resistor in series
with the supply, be it in either conductor.
> Why does the resistance in the return path (assuming DC and resistive load
> element) affect the voltage seen by the element being provided power? Or am
> I thinking '60 British (positive ground).
>
Message 9
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Subject: | Re: voltage drop calulations (academic exercise) |
Ron,
So, the circuit is functionally 3 resistors in series and the middle
resistor (load) can only see the voltage drop across it as part of the total
voltage drop in the entire circuit. If that's correct than I understand.
Thanks for the explanation.
Raymond Julian
Kettle River, MN
"Hope for the best,
but prepare for the worst."
----- Original Message -----
From: "Ron Quillin" <rjquillin@gmail.com>
Sent: Tuesday, February 26, 2008 11:09 PM
Subject: Re: AeroElectric-List: voltage drop calulations (academic exercise)
> <rjquillin@gmail.com>
>
> Because the load 'sees' the supply voltage referenced to it's, the
> load's, local ground. If the local ground isn't really 'ground', that
> there is a voltage differential between 'true' ground and local ground
> caused by ground currents, the local ground is offset by that voltage
> from the true supply ground. So the actual supply voltage the load
> sees is reduced by the voltage in the return path and the load doesn't
> really see the full supply voltage, as it is reduced by losses in both
> the supply and return wiring. It's all just like a resistor in series
> with the supply, be it in either conductor.
>
>> Why does the resistance in the return path (assuming DC and resistive
>> load
>> element) affect the voltage seen by the element being provided power?
>> Or am
>> I thinking '60 British (positive ground).
>>
>
>
>
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