On 1/29/2014 11:23 PM, gregz wrote:
Fred Abse wrote:
On Sun, 26 Jan 2014 08:13:56 -0500, Jim Wilkins wrote:

"DoN. Nichols" wrote in message
...
On 2014-01-25, Jim Wilkins wrote:

Why do you need two fuses for a single-phase motor?

In the UK, you don't, because 240 VAC comes with one side grounded.
However, in the USA, 240 VAC is normally supplied with a grounded center
tap, so if you have only one fuse -- or have two but only one blows, you
still have 120 VAC live in the motor's housing, and
potentially available for contact and personal zapping. :-)


I assumed it was wired with a 20A double-pole breaker at the panel
protecting from shorts and a smaller fuse sized (how?) to blow before the
motor burned out.


Panel breakers are there to protect the upstream wiring from excessive
load current, not to protect the load.

Matching the I-squared-t of a breaker, or fuse, to startup and running
conditions of a motor is not trivial. Motors are subject to starting
inrush currents sometimes tens of times the rated full-load current. Be
guided by the data published by reputable manufacturers. There's plenty of
it.


My ac compressor unit says, use 20 amp breaker. The wiring said nothing.


The manufacturer tells you what size breaker will withstand the starting
current and provide protection from manor failures.

Overload protection is very likely a thermal trip on the compressor.
Small fans are "impedance" protected.