scada wrote:
"John Hudak" wrote in message
...


Ignoramus26750 wrote:


On Thu, 10 Nov 2005 09:36:44 -0800, Esther & Fester Bestertester

wrote:

NEC reference says that 12ga THHN wire can carry 30A max
current. Motor load is currently drawing constant 20A (230v
1ph circuit). Breaker is 30A.


It is my understanding that wire must be matched to the breaker. So,
if the breaker is 30A, the wire should be rated for 30A as well. That
means 10 gauge.


NO, not according to NEC and wiring protection practices. You size the
breaker to be 80% of the rated FLA of the conductor. You are protecting
the wire with the breaker, not the motor...If the wiring overheats you
got a fire.
John


True, but why would anyone undersize the wire? If it's a 30A breaker - you
best be using at least 10AWG wire! Even larger if youre 50' or more from the
source. Why would you Jeopardize burning out the motor from a large voltage
drop in the wire? Stall current (starting current to break inertia) is
typically 20* running I...



i



(A bit of history: Previous motor (running from 208v) was
drawing less (15A?) current until machine broke. New
machine's motor (running from 230v via boost
autotransformer) is drawing 20A (measured at breaker).
Motor spec plate says 16A.)

Is 12ga wire sufficient, or should I bump it up to 10ga?

Would the autotransformer account for the increased current
draw?

FBt






scada wrote:
"John Hudak" wrote in message
...


Ignoramus26750 wrote:


On Thu, 10 Nov 2005 09:36:44 -0800, Esther & Fester Bestertester

wrote:

NEC reference says that 12ga THHN wire can carry 30A max
current. Motor load is currently drawing constant 20A (230v
1ph circuit). Breaker is 30A.
(A bit of history: Previous motor (running from 208v) was
drawing less (15A?) current until machine broke. New
machine's motor (running from 230v via boost
autotransformer) is drawing 20A (measured at breaker).
Motor spec plate says 16A.)

Is 12ga wire sufficient, or should I bump it up to 10ga?

Would the autotransformer account for the increased current
draw?

FBt







It is my understanding that wire must be matched to the breaker. So,
if the breaker is 30A, the wire should be rated for 30A as well. That
means 10 gage.


NO, not according to NEC and wiring protection practices. You size the
breaker to be 80% of the rated FLA of the conductor. You are protecting
the wire with the breaker, not the motor...If the wiring overheats you
got a fire.
John


True, but why would anyone under size the wire? If it's a 30A breaker
- you
best be using at least 10AWG wire! Even larger if your 50' or more
from the
source. Why would you Jeopardize burning out the motor from a large
voltage
drop in the wire? Stall current (starting current to break inertia) is
typically 20* running I...


Motor Circuits are unique animals. In general the Over Current
Protective Device for a motor branch circuit is sized to permit the
motor to start without nuisance tripping of the breaker or fuse in
question. The OCPD serves only to provide fault protection to the
circuit. Overload protection is provided by the Motor Overload
Protective Device. This may be a thermal protector on the motor or it
may be heaters sized to the motor running current and installed in the
motor starter. The circuit conductors are sized at 1.25 times the full
load current of the motor it serves based on the ampacity in the NEC
table. The note to the table reads "except as otherwise permitted by
this code" and it is that phrase that allows for full current ampacity
of conductors to be used in sizing motor branch circuits and feeders.

In other words the requirement to protect number twelve copper
conductors with an OCPD not greater than twenty amperes does not apply
to motor circuits.
--
Tom Horne