[Power] [UKids] 12V wiring for XO-1s, XSCE and networking infra om Haiti

[James Cameron]

On Mon, Jun 30, 2014 at 09:12:10AM -0400, Tim Moody wrote:
> As James points out, wire gauge depends on amperage not voltage
> http://www.powerstream.com/Wire_Size.htm
> shows amp rating per gauge and ohms per distance if you want to
> compute voltage drop

good reference, thanks.

> the problem with low voltage is that amps go up

yes ... , but it depends on the load.

for a resistive load like a bar heater, the current falls as the
voltage falls.

for the switching regulators in the AP, 3G, and XOs, the current rises
as the voltage falls.

for the switching power supply in a 12V powered server, the same
trend.

but only up to a point.  as the voltage falls below their minimum
operating voltage, they stop working.  this minimum varies slightly
across the XOs of the same model.

(one could easily create a situation where connecting one more XO-1
laptop to the end of a cable to charge might cause the other XO-1
laptops on the same cable to stop charging.  or connect a whole
classroom and the server turns off.  or connect five XO-1 and the AP
reboots.)

keeping the diameter constant and increasing the length of the cable,
will increase the resistance.

keeping the length constant and decreasing the diameter of the copper
conductor will increase the resistance.

the consequence of increased resistance is an increase in the voltage
drop, so the voltage at the end of the cable will be lower.

another consequence is that some of the power carried by the cable
will be lost in the cable as heat.

this loss makes the system less efficient, but that's not really a
major concern.  the major concern is fire safety.

cables normally heat to some extent, the design question is whether
the heating is within a safe boundary.

cables that are fixed in walls by an electrician have very different
safety factors than cables run across a floor in living space.  humans
drop stuff on top of cables, or coil them up, or kink them, and this
can increase their heating.

it would be so much easier without humans.  ;-)

> as a point of reference, in Canada we use 14/3 gauge for 15A
> household outlet circuits

the safety factor here presumes the cable is not going to be covered
or coiled.  ;-)

> Tim

-- 
James Cameron
http://quozl.linux.org.au/