Re: Battery Boxes...What One Company Designed

[Marc F Hult <MFHult@xxxxxxxxxxxxxxxxxxxxx>]

On 21 Sep 2005 05:09:19 GMT, ddl@danlan.*com (Dan Lanciani) wrote in message
<1331592@xxxxxxxxxxxxxxxxxxx>:

>
>I was really trying to frame some simpler questions.  What is it that you
>are allowed to do by remaining < 30V that you would not be allowed to do if
>you used, say, 48V?  Similarly for using DC instead of AC.

Let me tackle the 'AC' part here.

(We need to stick to comparing pomes (as in apples to pears) so large diesel
generators and other solutions beyond the modest ones previously implied are
beyond the scope of the solutions to be discussed here.)

I've recently discussed the aesthetic problem of "wall acne" in a thread in
comp.home.automation, so won't repeat that in this thread which is
cross-posted. Let it suffice that there are advantages to low voltage
distribution that are unrelated to the need for uninterrupted power. It is
the need for UPS that I address here.

Starting from the design objective of making power available ad infinitum,
not just for a few minutes to (eg) allow a computer to shut down. This
(necessarily, methinks) results in a _central_ UPS system the power from
which must be distributed as either DC or AC.

The DC supplies I've described provide about (60 x 14 + 80 x 14) =~ 2000
watts . Not coincidentally this is about what  15 amp (1800 watt) and 20 amp
(2400 watt) AC circuits can provide.

Noting that in the US, the National Electrical Code prohibits supplying a
(eg) a conventional wall outlet with less than a 15 amp source, it follows
that an AC UPS with the same capacity as the DC system I described must be on
a _single_ circuit There can be mo overcurrent protection between the panel
and the outlet less than 15 amps.

So in a NEC-compliant 15-20 amp distributed 120vac UPS wiring system, *every*
outlet goes dead if the trip point is exceeded.

Summary:

1) After putting all the most important devices that should never be allowed
to go dead, a NEC-compliant AC  system is vulnerable to someone plugging in a
vacuum cleaner during Saturday house cleaning, or to a space heater plugged
in during an emergency and shutting everything down. This is a recipe for
certain failure in my opinion. With the DC distribution system I've
described, over-voltatge protection can be used _ab_ libitum_ (although not
necessarily as effectively as desired -- 'nuther topic ;-)

2) Elsewhere in this thread, Dan correctly notes that the DC system described
requires running new/different wires. How does having power from a single ~
2000 watt UPS system distributed throughout the house obviate the need to
have new/different wires? As best I know,  it doesn't.

3) From the efficiency standpoint, DC distribution also has the advantage
when efficiency is most needed, namely when running from batteries. In a DC
distributed system, the power used by (eg) PC goes through exactly one DC-DC
conversion (battery-> DC-DC converter inside PC).  In a AC distributed
system, there are at least twice as many conversions with attendant decrease
in overall efficiency of battery utilization (battery--> DC_AC UPS -->
AC_DC_DC converter in computer).


Marc
Marc_F_Hult
www.ECOntrol.com