I have a dust collection system that I am connecting. It has a 3 phase 7.5 HP motor connected at 208 volt. cont. duty, des B, max amb C 40, nema nom EFF 89.5, the FLA is 21amp we have a Sq D qob panel feeding into a fused disconnect 30a time delay fuses, then to a stop start contactor. My quetion is what is the max breaker size I can put on this from the panel? at start up the motor jumps to 106 amps then starts to come down as speed builds up. Sq D has to fast of a trip setting to hold at 30amps. Thanks in advance motors are not my strong point.
motor breaker size protection
- Thread starter acrwc10
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- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
acrwc10 said:
First, you could use a Square D type QO330HID breaker which does not include the fast trip feature.
Second, you can only put in as large a breaker as what your wire is sized for. This breaker is for branch circuit protection (your fused switch provides the motor protection) so you can not use any of the "motor upsizing OCPD" options.
noonan
Member
- Location
- Bucks County, PA
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- Project Manager
what is the size and length of the feeder?
Thanks for the clarification Jim on the ocp. The branch circuit feeder can be anything I need because it is right next to the panel, connected with an offset nipple. What size would you think I should use to feed the fused disconnect with so as not to have a trip problem on the breaker?
jim dungar said:
Since I am going from the distribution panel to a fused disconnect wouldn't 430.52 allow me to up-size the breaker in the panel to 84 amp then to the next standard size 90 amp ( FLA is 21amp ) This give me ground fault protection for the conductors. The fuses in the disconnect after the panel are 30 amp time delay, which would protect the motor and conductors against over heating.
Edit to add -all conductors are #10 thhn
- Location
- Illinois
- Occupation
- retired electrician
I wanted to say that you couldn't use the motor rules for the sizing and protection of the conductors between the panel and the disconnect, but the code doesn't say that. 430.62 permits a motor feeder to serve a single motor and be protected per Table 430.52. In this application the conductors between the panel and the disconnect are feeder conductors and the ones between the disconnect and the motor are branch circuit conductors.
It apprears that you can use #10 between the panel and the disconnect, but the maximum permitted OCPD is 250% of the FLA from Table 430.250. That would make the maximum size of the breaker in the panel 70 amps. (250% of 22.4 amps rounded up to the next standard size)
It apprears that you can use #10 between the panel and the disconnect, but the maximum permitted OCPD is 250% of the FLA from Table 430.250. That would make the maximum size of the breaker in the panel 70 amps. (250% of 22.4 amps rounded up to the next standard size)
don_resqcapt19 said:
I feel better now, Thank you for confirming that table 430.52 was o.k. to use in this situation. It is suprizing to me that a motor that has a 16 amp ,normal load, jumps up to over 105 amps at start up.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
acrwc10 said:
8x inrush is not uncommon. Motor overloads relays are designed at 6x for either 10, 20, or 30 sec.
QES
Senior Member
- Location
- California
why not use MCP breaker with upsteam overload?
- Location
- Illinois
- Occupation
- retired electrician
You are only permitted to use MCPs when they are part of a listed combination starter. 430.52(C)(3)QES said:
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