Okay - but I wouldn't think you need that. Perhaps I can do a better job explaining.
My discussion is limited to 3ph, industrial/commercial continuous loading applications. If you are looking to size cbs for single phase light duty, such as a box store air compressor, I can't help - this discussion won't help.
jraef, Ham, and electures all have give an excellent discussion on the issues.
Here is my distillation:
You have asked for a "rule of thumb" to size a thermal-magnetic CB for a motor circuit based on the motor FLA. You are certain that most times the CB selected is far larger than is necessary to start the motor. You wish to size the cb as small as possible to allow bidding the lowest number you can.
If this part is in error - cancel the rest. However, if this is correct, then:
Just having the motor FLA is not enough information to reliably size a Thermal-Mag CB for a motor circuit.
At least four additional pieces of information are needed to accurately size the CB and the overloads as well:
Code letter
Design letter
Energy efficiency status
Load acceleration characteristic
I realize you wish to shave the CB size right down to the minimum - but most customers will want the motor to start reliabily. If so, then this is the data required to get a good approximation of the motor starting curve and to insure that the starting curve will fit under the CB trip curve.
cf
No, this is not correct. I'm having a difficult time trying to express myself here. As a PE I'm sure you are given way more information than I typically
receive. I ask for all power requirements,preferably manufacturers pdfs of
the motor they (mechanical) intend to use. I typically get 20 HP 460,3ph, and
always 60HZ. When I ask for Design letter (if I don't receive a pdf) i get
a blank stare. This is very typical unless it's a large project then I will
get a Mech Package. But, since construction has slowed and is focused
on smaller projects I don't get the "packages" I normally do. So I have to
adapt to very little information I receive,yet still design a system that will
perform the tasks the customer needs and be compliant.
So I think my mistake is,"in the details."
How about this, If only HP,voltage,phase, is known would there be a "rule
of thumb" you would use when sizing the OCPD (not conductors,or OL's)?
I personally will lean to 250% every time, but sometimes that can be expensive
when there's no data to back up that expense. The Lack of data for my MAX
limit is due to my lack of data from Mech. That's just the way it is sometimes
and in today's economy it's becoming more frequent. So I'm try to adjust,
adapt but still be as close as possible to a favorable outcome. I am not
currently sizing a system for a 20HP motor BTW. I'm just trying to get
some resources before the next "no information" motor project comes
across my desk. BTW I didn't disagree with you one bit Cold Fusion.
Thanks
