Karl Townsend wrote:


Putting two parallel connected motors on two panel breakers makes zero
sense to me.

i

its for failures. lets assume a wire insulation fails and contacts
ground. what do you want to have happen?.

Or a bearing burns out, or or or

These are two different fault condition and are protected by two different
types of devices.

First of all, for motor branch circuit and breaker sizing, consult the NEC,
table 430.250 for the proper FLA. Don't use the nameplate. For a 220 VAC
3phase 5 hp motor, use 15.2 FLA. For a 220 VAC 3phase 20 hp motor, use 54
FLA. The

The motor overload protection should be set based upon the nameplate FLA.
The reasoning behind this is that, should you need to replace the motor,
the table value is conservative. So the conductors will be sized for a
worst case. On the other hand, the nameplate value is the limit for that
specific motor and needs to be reset in the event that the motor is changed
out.

Overload protection is for failures where the motor conductors themselves do
not suffer a fault but the motor seizes mechanically. The breaker is sized
to protect the branch circuit from an electrical fault.

Also, one must consider the need for a disconnecting means and their
requirements when designing a code compliant installation.

All of the above addresses three phase motor protection from a three phase
source. Phase converters are addressed by NEC Article 455. But this assumes
a device designed and listed as a phase converter. The calculations needed
to home brew such a system can be done, but that won't necessarily make for
a code compliant installation. Whether an inspector's buy-off is an issue
or not, I'll leave up to the individual.

--
Paul Hovnanian
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Shoot low, sheriff. They're riding Shetlands!