Code question of the Month

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I got this off the internet, Whats your answer, and give code referrence. I got mine, but im not sure.

What is the maximum rating for an inverse time circuit breaker used to provide
motor branch-circuit short-circuit and ground-fault protection for a residential 240 VAC, single-phase, ?
horsepower well pump motor, controller, and branch-circuit?
A) 30 amps, B) 25 amps, C) 20 amps, D) 15 amps.
 
NEC 2005

1. Start at 430.52(C) for OCP of motor circuits

2. The above sends us to Table 430.52 Max Rating of Overcurrent Device for Motors.

3. Discover that the table says that inverse time breakers with single phase motors can be a max of 250 percent of the Full Load Current of the motor

4. We need the FLC of the motor. Go to 250.248 for single phase motor FLC

5. Discover that 3/4 HP 240 volt single phase motor FLC is 6.9 amps.

6. Multiply 6.9 by 2.5 = 17.25 amps for breaker setting

7. Read 430.52 Excp 1 - it is okay to go to the next standard size breaker if the calculated max doesn't match a standard breaker size.

8. Got to 240.6(A) for standard OCP sizes.

9. Next higher size is 20 amps.

10. Now, if the motor start-up current causes this breaker to trip, 430.52 Excp 2 allows us to upsize the breaker. This can go all the way to 400 percent in the worst case... it cannot exceed 400 percent in any case.

11. Multiply 6.9 by 4 = 27.6 amps

12. 27.6 does not match a standard size in 240.6. However, we are not allowed to size up to the next standard size because Excp 2 limits us to 400% or less. So, 25 amp breaker is max if we have tripping of the 20amp breaker on start-up.

So, answer is:

20 amp breaker is max as long as the motor will start, but 25 amp breaker is max if the 20 amp breaker trips on start-up. It is a toss-up as to what the question writer's intent is concerning this situation.
 
crossman said:
It is a toss-up as to what the question writer's intent is concerning this situation.
Click to expand...

Maybe it is not this situation? Maybe it is an indirect question to get the wheels turning to go down a different road and see that a 1/2 hp well pump is limited to a 15 amp breaker if it starts and you don't get to use 430.52 Exception 2?
 
430.248 flc = 6.9

430.52 c 1 exception 2 c 6.9 x 4 = 27.6, next lower is 25A (shall not exceed)

I go with 25A

(2005)
 
since the question asked "what is the max" I would say "B' 25amps.

Because - according to the previous calculations which are correct - 27.5amps rounding down to 25amps would be the max you could use.
 
I would say the MAX is the 250 percent and 20 Amps, the other is only the case in the event it does not hold . In the question you need to know what they are asking for and not get into the famous LOOP.
 
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when someone asks "what is the maximum rating", I would have to believe that they are asking for the maximum rating, not the maximum rating if you don't need the maximum, but the absolute maximum. Hence I would say that the 25 is the maximum (jmsho)
 
RE: It is a toss-up as to what the question writer's intent is concerning this situation.

volt102 said:
Maybe it is not this situation? Maybe it is an indirect question to get the wheels turning to go down a different road and see that a 1/2 hp well pump is limited to a 15 amp breaker if it starts and you don't get to use 430.52 Exception 2?
Click to expand...

Yuor use of "maybe" pretty much validates my claim that the writer's intent is a toss-up.:smile:

Code test writers really need to exactly define what they are asking. There are so many "loopholes" and exceptions, painstaking care needs to be taken!
 
I'm with "nuckelwack" ,..I mean,.. "nickelback" no,..I mean,.. nakulak ..whew I got it :smile:

They asked a simple question and I think it is direct,.. what is the maximum rating allowed ? the answer is 25 amps
 
This is a "paper" question and the maximum "paper" answer is 20 amps. You have to have a "physical" installation and prove that the 20 amp device won't start the motor without tripping before you can go up to 25 amps.
 
Without a doubt, the answer is "the max is 20 amps, but if that won't allow the motor to start, then the max is 25 amps."

The writer must give more info. If the 20 amp breaker will allow the motor to start, then the 20 amp breaker is indeed the maximum allowed.
 
don_resqcapt19 said:
This is a "paper" question and the maximum "paper" answer is 20 amps. You have to have a "physical" installation and prove that the 20 amp device won't start the motor without tripping before you can go up to 25 amps.
Click to expand...

I'm with you Don.
 
don_resqcapt19 said:
This is a "paper" question and the maximum "paper" answer is 20 amps. You have to have a "physical" installation and prove that the 20 amp device won't start the motor without tripping before you can go up to 25 amps.
Click to expand...

maximum (encarta)




max?i?mum [ m?ksiməm ]


noun (plural max?i?ma [ m?ksimə ] or max?i?mums)

Definition:

1. greatest possible amount: the largest amount, number, extent, or degree possible or allowed
The stadium seats a maximum of 60,000.


2. largest amount or highest level reached: the largest amount or value or highest level that something variable can reach or reaches during a period
Even at its maximum, the noise did not exceed legal levels.


3. largest number: the largest number in a mathematical set


4. greatest value of function: in mathematics, the greatest value that a continuous function can attain over a specific interval


5. time of greatest brightness of star: the interval during which a variable star is most luminous


6. greatest magnitude of star: the magnitude of a variable star at its greatest




adjective

Definition:

greatest possible: of the greatest possible or permitted amount or value
visual effects with maximum impact
Maximum occupancy in this building is 235.




adverb

Definition:

at maximum: at the greatest extent
The hall seats 400 maximum.


[Mid-16th century. Directly or via French< modern Latin, form of Latin maximus "greatest" < magnus "great"]
 
don_resqcapt19 said:
This is a "paper" question and the maximum "paper" answer is 20 amps. You have to have a "physical" installation and prove that the 20 amp device won't start the motor without tripping before you can go up to 25 amps.
Click to expand...

It still looks like 25 is the maximum cuz if it still trips there is no way to increase it,..it has been maxed out,..there is no more ,..end of da road , the fat lady has sung and she is walking away with the flowers,. there ain't no more folks go home.
 
What is the maximum (go ahead and apply all your definitions to the word) allowable setting for the OCP if the 20 amp breaker will hold when the motor starts?
 
crossman said:
What is the maximum (go ahead and apply all your definitions to the word) allowable setting for the OCP if the 20 amp breaker will hold when the motor starts?
Click to expand...

now that's a different question entirely, and not what was asked.

it may be worthwhile to note, that if, perchance, someone was to come up to me and ask me what the maximum allowed was, I would assume (by my very nature, not necessarily yours), that they were already having an issue, and NEEDED to know the MAX.
 
nakulak said:
now that's a different question entirely, and not what was asked.
Click to expand...

And where in the question does it state the breaker is tripping?

nakulak said:
it may be worthwhile to note, that if, perchance, someone was to come up to me and ask me what the maximum allowed was, I would >>>>>assume<<<<<<< (by my very nature, not necessarily yours), that they were already having an issue, and NEEDED to know the MAX.
Click to expand...

This is the point. On this question, we must assume what they are asking. The fact that several of us are seeing it one way, and several are seeing it another way is proof that the question is misleading. If we have to make assumptions, then the question is poorly written.

Look at Table 430.52. The very title of the Table is "Maximum Rating...."

Notice that in exception number 2, the word "maximum" is not even used! So, if we are talking Code, then the Maximum Rating is coming from Table 430.52.
 
brother said:
What is the maximum rating for an inverse time circuit breaker used to provide motor branch-circuit short-circuit and ground-fault protection for a residential 240 VAC, single-phase, ?
horsepower well pump motor, controller, and branch-circuit?
A) 30 amps, B) 25 amps, C) 20 amps, D) 15 amps.
Click to expand...

Okay. Maybe we can put this to rest. The question asks for the MAXIMUM RATING.

The MAXIMUM RATING is found by using Table 430.52.

Exception 2 does not even contain the word maximum and certainly does not say MAXIMUM RATING.

The MAXiMUM RATING is defined by Table 430.52 and is a precise and specific term. It is not an open-ended generic term.
 
Well it is pretty obvious that this is a bad question. With all the knowledge this forum has no one can truly say what intent the writer had. Personally the max means the max given all possibilities. If indeed the writer wanted 20 amps as an answer than I say the question should be trashed and the writer stoned. :grin:
 
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