Robert Bruninga via EV wrote:
The only time high voltage helps is when you need to have long wire
runs...
The operative word is "long" And when you wire a house for every room
and for every appliance and for every outlet (whether used fully or not)
then every wire is "long".
What is "long" to you may be "short" to others. Let's put numbers on it,
Robert.
In my house, the longest run from circuit breaker panel to the farthest
outlet is about 35 feet. Current wiring practices use #12 wire on a 15
amp circuit. #12 has a resistance of 1.588 milliohms per foot. So this
35-foot run has a total resistance of 70 feet x 0.001588ohm = 0.111
ohms. Google tells me that #12-2 with ground is currently selling for
about $0.28/foot; so 35 feet costs $10.
You're only allowed to load a 15 amp circuit to 12 amps continuously
(1440 watts). So the worst-case voltage drop is 12a x 0.111ohm = 1.33v,
and power loss is I^2R = 12^2 x 0.111 = 16 watts. 16w/1440w = 0.011 or
just over 1% of your power is lost.
24vdc is a common PV panel voltage. Let's assume we simply distributed
this power directly, through the same 35 feet of #12 wire. (It would be
silly to do it this way, but just for the sake of argument...)
We have 24v/120v = 1/5th of the voltage, so we need 5 times the current
to get the same power. The voltage drop will be 5 times more; 1.33v x 5
= 6.65v. The current is 12 x 5 = 60 amps, and I^2R losses would 25 times
more, so 16 x 25 = 400 watts. We'd be wasting about 25% of our power in
wire losses. Besides, #12 wire would overheat and isn't safe at this
current.
So realistically, we'd increase the wire size; probably to #6. We only
need 2 wires (not 3) because you don't need a separate ground wire on
24v systems. Google says #6-2 is $0.87/foot, so the 35-foot run is
$30.52 ($20 more).
#6 resistance is 0.3951 milliohms/foot, so a 35-foot run is 70 feet x
0.0003951ohm = 0.01383 ohms. The worst-case voltage drop is 60a x
0.01383ohm = 0.829v, and power loss is I^2R = 60^2 x 0.01383 = 49.7
watts. 49.7w/1440w = 0.0345 or about 3.5% of our power is lost.
Bottom line: The 24v approach works fine, but costs $20 more and is 2%
less efficient. Whether you consider that a little or a lot is a matter
of opinion.
And... this is the EV list; no one is going to have a 35-foot run in
their car. And PV panels (or more likely batteries) are a distributed
source, so you have many small individual wires to each of them anyway;
not one big cable. Design the system around the situation you've got;
not with rules of thumb that don't apply to the situation.
--
"IC chip performance doubles every 18 months." -- Moore's law
"The speed of software halves every 18 months." -- Gates' law
--
Lee Hart, 814 8th Ave N, Sartell MN 56377, www.sunrise-ev.com
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