ECN Electrical Forum - Discussion Forums for Electricians, Inspectors and Related Professionals
ECN Shout Chat
ShoutChat
Recent Posts
UL 508A SPACING
by ale348 - 03/29/24 01:09 AM
Increasing demand factors in residential
by tortuga - 03/28/24 05:57 PM
Portable generator question
by Steve Miller - 03/19/24 08:50 PM
Do we need grounding?
by NORCAL - 03/19/24 05:11 PM
240V only in a home and NEC?
by dsk - 03/19/24 06:33 AM
New in the Gallery:
This is a new one
This is a new one
by timmp, September 24
Few pics I found
Few pics I found
by timmp, August 15
Who's Online Now
1 members (ale348), 302 guests, and 14 robots.
Key: Admin, Global Mod, Mod
Previous Thread
Next Thread
Print Thread
Rate Thread
Joined: Jan 2006
Posts: 8
I
Junior Member
In accordance with NEC 440.22, the rating of fuse or circuit breaker for motor compressor shall not exceed 175% of nameplate FLA of motor or brach circuit selection current, whichever is greater.

1) My main question is that we usually set the rating of circuit breaker of 250% of FLA for usual induction motor but why 175% rule is applied for motor used for compressor.

2) What is branch circuit selection current?

3) Why does NEC require whichever is greater between 175% of FLA and branch selection current?

Stay up to Code with the Latest NEC:


>> 2023 NEC & Related Reference & Exam Prep
2023 NEC & Related Reference & Study Guides

Pass Your Exam the FIRST TIME with the Latest NEC & Exam Prep

>> 2020 NEC & Related Reference & Study Guides
 

Joined: Dec 2003
Posts: 751
E
Member
There are two currents used in motor calculations: FLA and FLC. One is found on the nameplate, the other from the charts in the NEC. One is used to determine the heater size, the other for determining the size of the conductors, the rating of the OC protection, the disconnect, and the controller.
The rational goes like this: The particular motor current draw will vary with the motor chosen and the load, but when you replace the motor, the branch circuit OC device, wires, disconnects and controller will remain. So the equipment and wiring are sized generically, but the OL heaters are fitted to the exact needs of the motor actually installed. If necessary, the fuse or CB is allowed to be bumped up to an even larger size if the motor chosen so warrants.


Earl
Joined: Jul 2001
Posts: 599
J
JBD Offline
Member
It may have to do with the service factor of standard motors versus compressor motors. Hermetically sealed motors do not always have the ability to handle overloading very well, so our protection has to be tighter.


Link Copied to Clipboard
Powered by UBB.threads™ PHP Forum Software 7.7.5