motor breaker sizing, inspector issues

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kyle wattmeister

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I have a problem here. We have 2 motors on a cooling tower both pull 36 amps. we sized the wire by taking 36 x 1.25 + the sum of the other motor and we got 81 amps so we used # 4 wire. then we sized the breaker by taking 36 x 2.50 plus the sum of the other motor and we got 126 amps. so we used a 150 amp breaker. the inspector said that the wire was to small for the breaker and that we needed to put a 90 amp breaker in place of the 150 amp breaker. we tried to tell him that it would not hold the inrush of the two motors but he refused to reason with us. so we had to install the 90 amp breaker. which led us to the problem we are having now, the breaker keeps on tripping. the inspector called us back over to the job and said that it is the wire size that it is making it trip, so he had us install # 2 wire in place of the # 4 because of the derating of the ambient temperature. we tried to tell him that it was not going to work still because of the improperly sized breaker. he refused to listen to us. now the breaker is still tripping and he is now saying that he thinks the wires have a direct short. so we have to go back over there and meg them out.
can anybody tell me if im missing something
thanks in advance
kyle
 
Re: motor breaker sizing, inspector issues

Kyle
You are close. 430.24 requires the conductor have the ampacity equal to 125% of the FLA of the largest motor + FLA of the other motor.
You should get the FLA of the motors from table
430.150 based on HP. You did not specify the voltage, single phase or three phase motors.
Assuming that the 36 amps is correct, the conductor must have the ampacity of
36 x 1.25 + 36 = 81 amps or #4 thhn 75C.
430.62 A specifies that the protective device shall have a rating not to exceed the max rating for the largest motor per 430.52 plus the FLA of the other motor. OD device per table 430.52 =
36 x 2.5 = 90 + 36 = 126 amps. I do not see permission to go to the next size. Do the MFG spec a min circuit size and a max OC device?
 
Re: motor breaker sizing, inspector issues

You are correct exactly per code and your inspector does not have the foggiest clue as to how to apply the code in this instance and he also doesnt have a clue for sure how a motor operates as pertains to inrush and full load ratings. Your wire size is absolutley correct. yout breaker of 150 is also exactly correct as you have applied the correct mutiplier and gotten a current which does not correspond with a device. The code in section430.52B says that the short cicuit device SHALL be cpable of carrying the starting current of the motor. PERIOD. 430.52 C1 says to use the table to make that selection YOU HAVE. 430.52 c1 exception 1 states that the next higher size or rating may be used. exception 2 further states in c also states that the rating of an inverse time breaker shall be permitted to be increased to "400" percent for full load currents of 100 or less and or 300 percent for currents greater than 100 amps. All of this is to ensure that you can finally comply with 430.52 B and get the motor started. I think you should go to the chief inspector of your area and complain and quote these code sections after you have read them well and maybe the inspection agencey should be billed for your time and efforts. You are 100 percent correct and he is blowing smoke and hasnt a clue. Sorry to be so upset but I feel inspectors should know wherof they speak and be able to interpret the code and use a cosde reference for what they feels is incorrect instead of trying to be the troubleshooter of someting that he obviously knows nothing about. Even the subnotes in table 430.52 refer you to exception 1 and 2 to help you get rating that will start the load. I have this argument on a regular basis with new inspectors who arent up to speed on motor or ac equipmet and have to explaiuns this over and over and believe me I hold my ground. I guess I am bragging a bit but Ive been doin this for mpore years than some of thes guys have been inspecting. Way more over 35 to be exact. I dont know it all but motors I do. Owned an electric motor repair shop up to 1000 hp for 17 years and grew up on this area of the code.

[ July 14, 2005, 01:38 AM: Message edited by: stew ]
 
Re: motor breaker sizing, inspector issues

kyle
I agree with you and everyone else.(100%) You have you OL's on your starter protecting the current draw and the breaker is for your short circuit protection. Must be pretty big motors on these tower fans. I just wired up 2 towers fans and they had 7 1/2hp motors on each one(480 3 phase)14 Amps per fan. Does these motors start at the same time? If they do, try starting them up one at a time and see if your breaker holds. Your breaker should hold for at less one motor to get going. Then kick on the other one after the first is running and see if it holds. If it does you might be able to put a delay from 1 to the 2nd starter to get this system going if you don't want to deal with the inspector. Also take a amp reading on both motors to see if they are OK. Again I agree that your breaker for this setup should be larger. Try explaining one more time to the inspector how this works and have papers to show him. Maybe he needs a picture drawing. A lot just looks but doesn't read things. Let us know what happens.
Bye now,
Jim
 
Re: motor breaker sizing, inspector issues

Stew
I was aware of the exception listed in 430.52C1.
But I had forgotten note 2 at the bottom of the table referring to 430.52C1.
Thanks for the correction.
 
Re: motor breaker sizing, inspector issues

thank you all for the replies. this morning we went over to the same job and had to install time delays to prevent the breaker from tripping. i tried to teach the inspector the code but he still thinks he is gods gift to the electric world. by the way the motors were 480v, i dont remeber the hp but i know they were 36 amps apiece. thanks again for your help. by the way we did meg the wires and they were all clear. it really frustrates me that these guys have so much stroke and some of them dont know what they are even talking about.
kyle

[ July 14, 2005, 07:17 PM: Message edited by: kyle wattmeister ]
 
Re: motor breaker sizing, inspector issues

Bob I apologize for not being a little less frank. My objection to this guy stands and I wish I could get my hands on him and wring his neck for these poor guys. They still need to go over his head. Time delays are not the permanent solution as the starting current will eventually strees the breaker and I can virtually garantee this guy is setting this sytem up for future failure to occur. One of the other reasons to use the larger frame breaker is to not put undue stress on the internal workings of th e deveice . Thats why the code leads us along a path that allows us to use the larger wsizes on motors. Inrush current can cause lots of stress on the breaker.This dude is wrong wrong wrong and has caused these fellows a very large amount of money and time on this when they were right on in the first place.
 
Re: motor breaker sizing, inspector issues

Stew
No need to apologize. I had no problem with the response.
 
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