On 12/19/25 17:05, Rene Churchill wrote:
The runtime rating @ 900W is only 2.3 minutes. Resistance of 12AWG is
~1.6 ohms/1000ft. Be generous and say there's 1ft of lead, so 0.0016 ohms.
P=I^2 * R
Use 60 amps to get your full 1500VA, that works out to 5.76 watts for
2.3 minutes. Not enough heat for long enough.
René, nice analysis. Great idea to look up the wire's resistance and
calculate the power dissipation (and correct you can still pull 1500VA
at 900W with a bad power factor).
So you can go much higher than 20A for 12 AWG a good combination of
short wires, short times, and/or adequate cooling. There must be
guidelines out there for doing that (and still getting the products
through UL certification).
Meditating on that wire (and the circuit board traces) makes me think
that one should be cautious about running with a full load when
replacing UPS batteries with higher AH ones or modifying the UPS to add
external batteries in parallel, since the limited capacity of the
original batteries and the length of the wires is probably a factor in
the cooling design. For example limiting this unit to 500VA (vs. 1500VA)
would to get that 60A number down toward the magic 20A for continuous
current in 12AWG wire.
--
Anthony Carrico
