Article 430 Question

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Chris McCutcheon

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I have a test question that I'm having some difficulty answering. The question states: Calculate for the Over Current Protection Device. 25 HP, 3 phase, 208 volt. Design "B" with an Instantaneous Trip Breaker. OK... I understand that I need to go to 430.251(B) and I get a FLC of 404. What I'm confused about is what to do with the information from 430.52. If I multiply I get 4444 amps. Is this the setting on the breaker? If that's the case would the breaker be 600 amps. I have read 430.52 (3) Exceptions 1 and 2 but I must be missing something.
 
augie47

augie47

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If you look at table 430.52 where you select your maximum SCGF device you will see Design B motors have a higher multiplier than non-design B.
 
Smart $

Smart $

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Chris McCutcheon said:
430.250 25HP 208 V FLC = 74.8 x 1100% = 822.8. Te question is asking the size of the over-current protection device. How do you determine the size of a instantaneous trip breaker?
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I hate questions like this...

First off, there is no such thing as an overcurrent protective device regarding motor loads (other than small motors). Article 430 uses the terms short-circuit and ground fault protection (SC&GFP) and overload protection (OLP). We are stuck with assuming the writer means SC&GFP even though OLP is actually closer to what would be considered overcurrent protection.

Additionally, Table 430.52 gives you a maximum rating for SC&GFP. There is nothing which says the SC&GFP cannot be less than that value. If I decided to put that motor on a 20A breaker, it would still be a compliant installation (yes, the usefulness would be nil :p).
 
Jraef

Jraef

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Chris McCutcheon said:
I have a test question that I'm having some difficulty answering. The question states: Calculate for the Over Current Protection Device. 25 HP, 3 phase, 208 volt. Design "B" with an Instantaneous Trip Breaker. OK... I understand that I need to go to 430.251(B) and I get a FLC of 404. What I'm confused about is what to do with the information from 430.52. If I multiply I get 4444 amps. Is this the setting on the breaker? If that's the case would the breaker be 600 amps. I have read 430.52 (3) Exceptions 1 and 2 but I must be missing something.
Click to expand...
By the way, this is kind of a trick question! You might want to see if you can impress your instructor with this... :angel:

Technically, your answer is... "Unless I work for a starter manufacturer, I cannot!" :jawdrop:

YOU cannot "calculate" the size of an Instantaneous Trip (IT) breaker! The reason is, they CANNOT BE USED IN THE FIELD (other than for direct replacement). IT breakers can ONLY be used as a component in a factory built, tested and listed combination starter assembly. In the process of getting such an assembly tested and listed, the mfr of the starter assembly will have chosen the IT breaker for that motor starter, you have NO OPTION to select otherwise, unless you opt to have them change it to a Thermal Mag breaker (which then nullifies the exact wording of the question). You might feel inclined, for the purpose of exercising your brain I suppose, to calculate what you THINK the right size would be, but your calculation is completely irrelevant. What you can get is only what the mfr has designed into that combination starter.

Semantics... it's a bitch sometimes...

(PS: I had to modify even this because of my OWN semantics, adding the qualifier that "Unless I work for a starter manufacturer...")
 
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GoldDigger

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Jraef said:
By the way, this is kind of a trick question! You might want to see if you can impress your instructor with this... :angel:

Technically, your answer is... I cannot! :jawdrop:

YOU cannot "calculate" the size of an Instantaneous Trip (IT) breaker! The reason is, they CANNOT BE USED IN THE FIELD (other than for direct replacement). IT breakers can ONLY be used as a component in a factory built, tested and listed combination starter assembly. In the process of getting such an assembly tested and listed, the mfr of the starter assembly will have chosen the IT breaker for that motor starter, you have NO OPTION to select otherwise, unless you opt to have them change it to a Thermal Mag breaker (which then nullifies the exact wording of the question). You might feel inclined, for the purpose of exercising your brain I suppose, to calculate what you THINK the right size would be, but your calculation is completely irrelevant. What you can get is only what the mfr has designed into that combination starter.

Semantics... it's a bitch sometimes...
Click to expand...
Maybe the poorly worded question can be interpreted in a different way.
What if it is not asking for the size of the IT breaker but is telling you that the IT breaker is there for short circuit protection and so you only need to calculate the size of the overload protection?
A stretch maybe, but it was a bad question anyway.
 
Jraef

Jraef

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GoldDigger said:
Maybe the poorly worded question can be interpreted in a different way.
What if it is not asking for the size of the IT breaker but is telling you that the IT breaker is there for short circuit protection and so you only need to calculate the size of the overload protection?
A stretch maybe, but it was a bad question anyway.
Click to expand...

Smart $ said:
I hate questions like this...

First off, there is no such thing as an overcurrent protective device regarding motor loads (other than small motors). Article 430 uses the terms short-circuit and ground fault protection (SC&GFP) and overload protection (OLP). We are stuck with assuming the writer means SC&GFP even though OLP is actually closer to what would be considered overcurrent protection...
Click to expand...
You know what, you are right, and that makes Smart $'s answer clear to me know, I was not looking at it that way. :slaphead:
 
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