Circuit breaker size (5HP, 3ph, 460V + 10KW heater)
- Thread starter Loki
- Start date
- Status
Hmmm..... your profile says you are an electrical engineer... but yet you don't know how to size a circuit breaker for a motor...??? Though it's just my opinion, I believe it's time for you to learn how.
If you are vaguely familiar with the NEC, you can start in Article 430...
- Location
- Logan, Utah
Loki,
Why don't you post your calculations and we will take a look and help you out.
Chris
Why don't you post your calculations and we will take a look and help you out.
Chris
ronmath
Senior Member
- Location
- Burnsville, MN
Remember,
Heat pumps will run along with the electric heat so there is no diversity here.
Usually best to use nameplate/catalogue ratings.
Heat pumps will run along with the electric heat so there is no diversity here.
Usually best to use nameplate/catalogue ratings.
Looks good for minimum overcurrent/short circuit protection rating... now what's the maximum rating for type used....???
...and why do we need to know the maximum?
For maximum overcurrent protection
locked Rotor Current @460V (values obtained from NEC 430.250(B))
5HP = 46A
10KW heater = 10000/(1.732*460V) = 12.55A
Total current = 58.55A
Therefore maximum breaker size is 60A. We need to know the maximum to protect the motor from getting damaged due to any surge in current (from a short circuit of some kind)
Am i correct?
locked Rotor Current @460V (values obtained from NEC 430.250(B))
5HP = 46A
10KW heater = 10000/(1.732*460V) = 12.55A
Total current = 58.55A
Therefore maximum breaker size is 60A. We need to know the maximum to protect the motor from getting damaged due to any surge in current (from a short circuit of some kind)
Am i correct?
Afraid not. Wouldn't a lower rated overcurrent protection device (ocpd) afford better protection against a surge or short circuit? What other reason might there be for using a larger ocpd?
Perhaps the following EC&M articles?written by Mike Holt?will help:
http://ecmweb.com/mag/electric_motor_calculations_part/
http://ecmweb.com/mag/electric_motor_calculations_part_2/index.html
Clue 1: maximum ocpd is based on motor data only
Clue 2: LRC is not the motor data to be used
Come on back if you want more info or help... sorry about putting you through it this way, but this is no different than someone studying for a test and wanting us to just give them the answer without them demonstrating any effort or learning on their part.
Smart$, I do understand what you are trying to do here and I do appreciate your help. I will study these articles and respond as soon as I can.
Loki
Loki, It's a good idea to follow Smart$ directions, but also remember that, for the most part, the pertinent circuit requirements for heat pumps with auxiliary heat are predetermined by the manufacturer per 440.4(B) and 440.22 directs us to comply with that information.
- Status