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Old 07-11-2019, 12:38 AM   #1
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Default Motor Rating Table 29

Looking to size a breaker that is going to feed 3 motors these three motor are on their own skid and they have their own breaker protection and run off VFDS.
In order size the breaker to feed theses three motor in take the fla of the largest motor and times it by the factor of 2.5 from table 29 and then add the fla of the other two motor to get my breaker size.
My question is can a vfd be used as a soft start and de rate the number I times my largest motor by to say 2 or 1.5. I was told you could but I do not the rule involved in this or can confirm there is one.
Any help would be appreciated thx
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Old 07-11-2019, 07:56 AM   #2
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You can always use a smaller overcurrent if you want, and if it allows the motors to start and is able to carry the continuous load. You just can not exceed the values calculated using 28-200 and table 29.
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Old 07-11-2019, 09:11 AM   #3
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Not sure what you mean by "on a skid", but when sizing for a VFD, you follow what the VFD calls for in the manual.

Is/was this bought as a unit ?
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Old 07-11-2019, 10:56 AM   #4
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Not a skid just a separate building but the one post reinforced what I was thinking table 29 is a max can’t be exceeded but free to go under this rating as long as the motors start and i would think operate below 80 percent breaker rating.

My other question is if a beaker is rated at 80 percent and it is a 600 amp breaker it is good to operate
At max 480 amp anything above this and it would start to approach trip level.
If a breaker is rated at 100 percent then it can operate under higher load with out tripping will it trip at 600 anything below it is good
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Old 07-11-2019, 01:16 PM   #5
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Quote:
Originally Posted by MDave View Post
Not a skid just a separate building but the one post reinforced what I was thinking table 29 is a max can’t be exceeded but free to go under this rating as long as the motors start and i would think operate below 80 percent breaker rating.

My other question is if a beaker is rated at 80 percent and it is a 600 amp breaker it is good to operate
At max 480 amp anything above this and it would start to approach trip level.
If a breaker is rated at 100 percent then it can operate under higher load with out tripping will it trip at 600 anything below it is good
The continuous load rating is not what the breaker will trip at, it is the rating that the breaker is designed and tested to operate at continuously.

Also, I doubt you will find a breaker with a 100% continuous load rating.
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Old 07-11-2019, 02:38 PM   #6
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Okay the breaker will still trip the same rate as 80 percent rated breaker it’s just more robust and has continuous duty rating?
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Old 07-11-2019, 08:41 PM   #7
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Quote:
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The continuous load rating is not what the breaker will trip at, it is the rating that the breaker is designed and tested to operate at continuously.

Also, I doubt you will find a breaker with a 100% continuous load rating.
I agree with that and not many breakers you will find it in full 100% continuous rating at all and they are not cheap at all.

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Okay the breaker will still trip the same rate as 80 percent rated breaker it’s just more robust and has continuous duty rating?
No if you have 80% rated breaker so you can sized the continuous load up to 80% of the breaker rating

MDave .,, if you get 100 amp 100% continuous breaker it will stay on until you hit full load of 100% without tripping ( just keep in your mind there is not much margin of overload protection )

80% continuous breaker you can load it up to full 80% without issue but the last 20 % it may be ok for tempory overloads or short high continuous load.

Please refer to your CEC code book for latest detail on the rating of the breaker. it will cover it good.
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Old 07-11-2019, 09:40 PM   #8
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Quote:
Originally Posted by MDave View Post
Not a skid just a separate building but the one post reinforced what I was thinking table 29 is a max can’t be exceeded but free to go under this rating as long as the motors start and i would think operate below 80 percent breaker rating.

My other question is if a beaker is rated at 80 percent and it is a 600 amp breaker it is good to operate
At max 480 amp anything above this and it would start to approach trip level.
If a breaker is rated at 100 percent then it can operate under higher load with out tripping will it trip at 600 anything below it is good
Table 29 doesn't apply when you are using a VFD.

Also, please take a moment to fill out your profile. It would help us to know who we are dealing with.
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Old 07-11-2019, 10:15 PM   #9
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I would fill out my profile were do I go about doing it. Does not seem to work.
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Old 07-11-2019, 11:27 PM   #10
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Quote:
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I would fill out my profile were do I go about doing it. Does not seem to work.
Look at the user CP on the upper left corner of the screen and click on that and scroll down to the profile and you can fill it in .,,

I suggest to run this format in desktop style due I heard some members have issue to set it up on their cell phone apps so just be aware of it in case you have to switch it over to desktop for a moment until you get the profile done then can switch back to conventail app set up.
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Old 07-12-2019, 07:12 AM   #11
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Quote:
Originally Posted by emtnut View Post
Not sure what you mean by "on a skid", but when sizing for a VFD, you follow what the VFD calls for in the manual.

Is/was this bought as a unit ?
A "skid" is an assembly of electrical/mechanical components, prebuilt in a shop and trucked to some location in the oil patch and jigsawed together with all the other skids.
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Old 07-12-2019, 09:58 AM   #12
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How does a person get out of desk top format and get back into cell phone app?
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Old 07-12-2019, 02:43 PM   #13
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very top right of the screen, there should be 3 dots ... that pulls up a menu.

on chrome on my android anyways.
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Old 07-12-2019, 07:21 PM   #14
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i would add all the max amp ratings of the drives (not the motors) together add 20% and call it good. If you have any motors that are on starters then times the largest motor by 2.5. The only time you may run into a problem on super efficient motors and breakers where the inrush current is preset by the factory.(non adjustable).

The Main breaker in your case is only to protect the wire and will offer no protection to the motors (they have there own breakers or fuses).


Larger breakers have amp rating settings. You still have to gauge your wire for max amps but you then turn the breaker down. So a 600 amp breaker can be turned down to a 400 amp

Last edited by gpop; 07-12-2019 at 07:26 PM. Reason: addition
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