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nuckolls.bob(at)aeroelect
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 Posted: Sat Apr 25, 2020 11:19 am Post subject: Preliminary tests on modern fuse link wire |
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At 01:18 PM 4/25/2020, you wrote:
| Quote: | --> AeroElectric-List message posted by: "markfw" <markwheelermd(at)icloud.com>
Bob,
Thank you for doing this experimental research on fusible links.
They appear to be an elegant and lightweight solution to my
problem of where to mount an MANL fitting for my alternator b-lead in my cub.
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Agreed
| Quote: | I will be using a 40 amp externally regulated B&C alternator.
My average load will be 30 amps.
I am using 8 AWG wire for the b-lead. The normal rule for link
sizing is apparently to go up 4 steps in AWG size. This would
suggest using a 12 AWG link. However the ampacity of 12 AWG
wire is only 30 amps according to the National Electrical code. |
NEC has no applicability here . . .
I'd go with a 10AWG b-lead feeder and 14AWG fusible link.
They will be just gine.
| Quote: | This seems a little low, but I realize that fusible links will
usually significantly exceed their nominal ampacity before "blowing". |
Yes, that 20AWG link I tested took perhaps 3-4
SECONDS at 100A before there was observable
reaction. A 22AWG Tefzel wire will carry
20A indefinitely without stressing Tefzel
insulation at room temperature.
http://aeroelectric.com/Pictures/Wire/22AWG_20A.pdf
3 AWG steps doubles the cross section, so a
17AWG wire would perform approximaterly the
same at 40A.
| Quote: | | What do you think in terms of link size? |
10/14 will be just fine
| Quote: | Can you suggest a brand for Hypalon jacketed fusible links?
It doesn't seem to be called out in many of the listings for fusible links. |
Most search engines on automotive parts
site suck for air. Go into an autoparts
store and cruise the selection of fuses.
They'll have fusible links in the same
section.
Here's and exemplar part:
https://tinyurl.com/y9677jrz
You might want to replace the butt splice
with a PIDG. Given the currents involved
on this path, REALLY gas-tight joints are
called for.
| Quote: | How long should the link be? I assume that it should be
as close the battery as possible? |
Yes . . . AS SHOWN on z-figures
How ever long comes in the package . . . I have
read recommendations for 9" minimum when
using bulk wire . . . but I've not yet
seen a justification for that number based
on physics.
Based on a bit more study and perhaps some ShopNotes
additions to aeroelectric.com, I will recommend
that B&C acquire bulk fusible link wire. Those
pre-fab assemblies from Smiley Jack's Car Parts
are pretty pricey, however, perhaps worth it
when compared to cost-of-ownership for installing
an MANL or ANL currently limiter.
Bob . . .
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bcone1381
Joined: 25 Apr 2017
Posts: 42
Location: Southeast Michigan
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| Posted: Mon Apr 27, 2020 4:47 am Post subject: Re: Preliminary tests on modern fuse link wire |
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Humm. Concerning the B lead wire size.
In the above case, the expected load is 30A, the Alternator is 40A.
A typical wire size on your Z-diagrams is 4 to 6 AWG
The proper size from the wire table for 30A is 10 AWG, but for 40A its 8 AWG.
Since you say use 10 AWG for the B-lead, can I say the authoritative rule for B-lead wire is the Load Analysis, not the Z-diagram or the Alternator output?
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Brooks Cone
Bearhawk Patrol Kit Build |
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user9253
Joined: 28 Mar 2008
Posts: 1925
Location: Riley TWP Michigan
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| Posted: Mon Apr 27, 2020 7:50 am Post subject: Re: Preliminary tests on modern fuse link wire |
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If debating two sizes, I would go with the larger size. Do not forget battery charging current. And larger wire might slightly reduce alternator whine in vulnerable loads.
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Joe Gores |
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nuckolls.bob(at)aeroelect
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| Posted: Mon Apr 27, 2020 8:26 am Post subject: Preliminary tests on modern fuse link wire |
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At 07:47 AM 4/27/2020, you wrote:
| Quote: | --> AeroElectric-List message posted by: "bcone1381" <bcone1964(at)gmail.com>
Humm. Concerning the B lead wire size.
In the above case, the expected load is 30A, the Alternator is 40A. |
understand
| Quote: | A typical wire size on your Z-diagrams is 4 to 6 AWG
The proper size from the wire table for 30A is 10 AWG,
but for 40A its 8 AWG. |
'Proper' is not a term carved in stone.
As explained in chapter 8 and illustrated
in Figure 8-4 and demonstrated in the
bench experiment at
https://tinyurl.com/6qr72fr
backed up with specifications offered
at:
https://tinyurl.com/y88kyypu
The operating current for a wire is based
on satisfactory insulation temperatures
in the application being configured.
Take a peek at qualification temperatures
in the cited wire spec . . . the wire
has to stand off various abuses at 200C.
As shown in Figure 8-4, a 10AWG wire
will rise 30C with 45A of current flow.
With 150C wire, this means that our
b-lead would not be pushed beyond
recommended limits with an ambient temperature
of 120C.
There are no sustained conditions under
the cowl that would produce risk to the 10AWG
b-lead at 45A. From a voltage drop
perspective 10AWG is 0.001 ohms per foot;
a 3-foot b-lead would drop 0.045 volts
at 45A . . . also no big deal.
Further, the only time you would even come
close to loading this alternator to near
full output would be in cruise . . . buckets
of cooling air coming in.
Wire tables are exceedingly conservative
recommendations unlikely to create hazards
for situations where the system integrator
is on solid, low-risk ground. But this
does not preclude one from considering departures
from the table values.
In this case, I considered the availability
of commercial off the shelf fusible links
and found that 14AWG was a pretty common size
for the largest link. Okay, that protects a 10AWG
wire. Question: Would a 10AWG wire be at risk
when used in the situation we're discussing?
I concluded that it would not.
| Quote: | Since you say use 10 AWG for the B-lead, can I say the
authoritative rule for B-lead wire is the Load Analysis,
not the Z-diagram or the Alternator output? |
The tables are not 'authoritative' for
anything except the special case which
created the tables. The plots in Figure
8-4 are helpful for examining other cases.
I've always been a bit queasy working on cars
and taking note of the wire and fuse sizes
selected for various tasks. Sometimes the
fuses seem just a bit 'too big' and/or
the wires just a bit 'too small'. But
electrical fires having root cause in
insulation failure are rare if not
non-existent.
The folk in Detroit have to be pretty
careful with their trade-offs between
cost/performance/risk. They produce
cars by the millions . . . their exposure
for professional malfeasance is pretty
sobering!
Bob . . .
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nuckolls.bob(at)aeroelect
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| Posted: Mon Apr 27, 2020 9:02 am Post subject: Preliminary tests on modern fuse link wire |
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At 10:50 AM 4/27/2020, you wrote:
| Quote: | --> AeroElectric-List message posted by: "user9253" <fransew(at)gmail.com>
If debating two sizes, I would go with the larger size.
Do not forget battery charging current. |
Battery charging current is part of the alternator's
load at the b-terminal. Battery demands are not
a component of the design calculation.
| Quote: | | And larger wire might slightly reduce alternator whine in vulnerable loads. |
There are no "loads" particularly vulnerable
to alternator whine. The vast majority of
alternator noise issues are conductivly
coupled to the victim's low level signal
paths via ground loops.
A tiny percentage are magnetically coupled
in wire bundles. See my narrative on
alternator whine in my newly acquired
Plymouth Voyager back about 1990.
The b-lead on my 87 GMC pick-em-up truck
is 10AWG. It'a about 18" long and runs
from the b-term right to the battery(+)
terminal. The alternator options INCLUDE
machines up to 90A in output. At 90a,
and 0.0015 milliohms of wire, the voltage
drop would be 135 millivolts and dissipate
12 watts. No doubt the wire would warm up
pretty good . . . but the thing is hanging
out in the air right behind the cooling
air flow out of the radiator. This lead
doesn't event have a fusible link . . . it
just hangs out in the air.
Bob . . .
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