Fuses ratings

Chip Fetrow tacos at fetrow.org
Thu Jul 26 16:47:15 CDT 2012

This kind of system is now being used in come car models.  It isn't to  
save weight, but to save the cost of copper, and the space in which to  
run the wires.  Strangely enough, they are still switching grounds,  
not hot side.  They are currently mostly using it for lights.

Regarding higher Voltage in cars -- that is on the table too, and we  
will be seeing that soon.  We will have lights, radios and so on  
running on a higher Voltage.

However, that higher Voltage will be no more than 48 Volts, and some  
are pushing for 36 Volts, but I believe they will lose.  120 Volts is  
totally out of the question (at least that is what I have read in the  
IEEE publication).

The reason for 48 Volts is that it is under 50 Volts, the cut-off  
Voltage for the requirement of armored or protected cable.  Anything  
less than 50 is considered low Voltage, and can be run just like  
todays 12 Volt wiring.  Still, the savings in wire gauge is  
considerable, and the design of conventional starter motors is greatly  

The proponents of 36 Volts have a valid point regarding charging  
Voltage.  Todays 12 Volt batteries are typically supplied with 13.6  
Volts for charging.  If the same ratio for charging of 48 Volt  
batteries holds true, the charging Voltage would be 54.4 Volts, above  
the cut off for low Voltage wiring.  Of course, the loads could be fed  
from a regulator, limiting the Voltage and eliminating the need for  
armored wiring.

I have no horse in that race, I'm just reporting on what I have read.


On Jul 26, 2012, at 1:00 PM, tacos-request at amrad.org wrote:

> Message: 2
> Date: Wed, 25 Jul 2012 13:50:48 -0400
> From: Mike O'Dell <mo at 131.ccr.org>
> Subject: Re: Fuses ratings
> fast and precise response is an advantage of the
> "electronic fuses" now being used in "multiplexed
> power systems".  the name is misleading because
> it is the *control system* which is multiplexed.
> the systems are based on the idea of a large DC
> bus running the length of a boat or RV with taps
> feeding electronic switching hubs. the drops for
> switched loads come off those hubs instead of
> doing the home-run back to a central distribution panel.
> the system is controlled by a bus system - usually
> some flavor of RS-485 - which has control panels
> placed wherever they are wanted.
> the power switching hubs can be programmed so that
> each channel (varies from 2-16 in a hub) listens to
> one or several switches (for N-way control), and
> the overcurrent level and response curve for each
> channel is likewise programmable up to some maximum
> for the channel (usually enforced with a fuse, breaker,
> or polyfuse).
> some of the hubs have manual over-rides on circuits
> so they can be switched even if there is a total
> failure of the control system.
> why add all this complexity just for switching
> DC loads?  the first answer is "flexibility" to
> move switches around and add additional control
> stations very easily, but the *real* reason is
> WEIGHT. a 60 foot bus of a 00 cable pair weights
> appreciably less than all the home-run tails that
> would be missing by just running to a nearby hub
> on the centerline. This is a 12V or 24V system
> and I-squared-R is *not* your friend and most of
> those home-runs are (duplex or triplex) #12 or maybe #14
> (and the wiring standards prohibit anything smaller than #16
> for anything but very low current control signals).
> in my boat, we had to freeze the design before
> the buss-network switching systems were baked enough
> to bet on, so we had to do the home-run thing mostly
> (small sub-panel forward to take care of cabin hotel loads).
> If we had it to do over and could use a buss-net system,
> the electrician that did the wiring estimates that
> we could save at least 3000 pounds of copper,
> not to mention the *cost* of a ton-and-a-half of copper.
> as you might guess, airliners were the first applications
> of this technology and it has existed for some time in the
> "why use lead when gold will do" world of commercial aviation.
> recently with the explosive growth in power semiconductors
> and the drop in cost of very cheap micro-controllers, it is
> now easy to put a spring-head in every switch bank and
> every power switching pod and take advantage of the savings.
>      -mo
> ------------------------------
> Message: 3
> Date: Wed, 25 Jul 2012 16:28:41 -0400
> From: "Bob Bruninga" <bruninga at usna.edu>
> Subject: RE: Fuses ratings
> Very good concept.  DC distribution bus with load control devices to  
> spread
> the power.  I like it....
> But one thing that is driving this is the legacy of 12v DC devices
> throughout these RV and boat systems, so they still have to back  
> convert to
> 12v DC at each and every load.
> A clean-slate approach would not do it that way.  They might do it  
> at 120
> VAC and still save even more in the copper because even the main bus  
> can be
> 1/10th the size.  Then all of the load devices can be conventional  
> 120 VAC
> and can operate equally on shore power or battery power.  With  
> inverters now
> being 95% efficient, it makes no sense to distribute any power at low
> voltage DC.  And with CFL lighting that is a big part of the load.  
> Etc.
> Shucks, since 330 VDC even saves another factor of 10 in copper,  
> that woiuld
> be best except for the main reason of broken connections initiating  
> plazma
> arcs...
> Bob, WB4APR

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