Complex Imaginary question 31 NEC problem

[Sparky Bill PE]

Can someone help me connect the dots, I don't know how to know just to automatically go to #3 at 430.24

 

[akyip]

I've done enough questions involving NEC 430.24 that I now know this off the top of my head.

Basically, NEC 430.24 describes the required conductor ampacity for a circuit that serves a group of both motor and non-motor loads (or at least a group of multiple motors).

This required ampacity is the sum of:

125% of the FLC of the highest-rated motor in the group being served

Sum of 100% of all other FLCs of all other motors

125% of the total current of all continuous non-motor loads

100% of the total current of all non-continuous non-motor loads

Basically, this question is asking which of the choices is correct/true, based on the requirements of NEC 430.24. This is why they have B as the correct answer here.

A is wrong because it should be 125% of the sum of currents of continuous non-motor loads.

C is wrong because it should be 125% of the FLC of the highest-rated motor in the group of loads being served.

D is wrong because it should be 100% of the FLCs of all other motors (this excludes the highest rated motor) in the group of loads being served.

 

[akyip]

Also, I'll try to answer what I think you're looking for.

The big hint that tells you to refer to NEC 430.24 is: 2 motors on a single branch.

NEC 430.24 is mainly for multiple motors on 1 branch circuit and/or a motor and a non-motor load on 1 branch circuit.

 

[Byk]

This question is pretty straight forward for someone who uses NEC on daily basis.

I would look for clues in the question here it is "two motors" and non continuous part. It can also very much be  service factor,  temperature rise, start the 
motor (check out 430.32(C)), duty cycle etc.

Regarding this specific question, my rule of thumb is if continuous go for 125% for single motor. For group of motors the largest one is to be calculated with 125%

 

[Sparky Bill PE]

This question is pretty straight forward for someone who uses NEC on daily basis.

I would look for clues in the question here it is "two motors" and non continuous part. It can also very much be  service factor,  temperature rise, start the 
motor (check out 430.32(C)), duty cycle etc.

Regarding this specific question, my rule of thumb is if continuous go for 125% for single motor. For group of motors the largest one is to be calculated with 125%

Your last sentence is exactly why I am not connecting the dots of how i know to use C. I still don't understand why its not 125% of first motor and 100% of second motor. 

It's so damn frustrating that I, as well, have worked enough NEC problems I knew exactly where to go. and I'm just looking at the 4 options having no idea which one they are wanting me to pick or why. 

 

[DilutedAr18_PE]

Your last sentence is exactly why I am not connecting the dots of how i know to use C. I still don't understand why its not 125% of first motor and 100% of second motor. 

It's so damn frustrating that I, as well, have worked enough NEC problems I knew exactly where to go. and I'm just looking at the 4 options having no idea which one they are wanting me to pick or why. 

The question is a bad question. The answer is not how you would actually size this load for a two motor circuit, as it does not indicate that there are any other loads on the circuit beyond those motors. Answer B is correct b/c it is the only one that would comply with the rules for sizing a multimotor circuit regardless of what loads were on the circuit.

Answer A is not correct b/c it is 125% of the continuous non-motor load, not 100%. Answer C is not correct b/c it is 125% of the FLA of the highest rated motor, not 100%. Answer D is not correct b/c it is 100% of the FLA of other motors beyond the first motor, not 125%. 

 

[Byk]

Your last sentence is exactly why I am not connecting the dots of how i know to use C. I still don't understand why its not 125% of first motor and 100% of second motor. 

It's so damn frustrating that I, as well, have worked enough NEC problems I knew exactly where to go. and I'm just looking at the 4 options having no idea which one they are wanting me to pick or why. 

I hear you but that is the NEC for you my friend. Like I said in my original post, have to look for clues in the question.

The purpose of this question is to make you go to code and look at it (the answer taken straight from code) instead of going with you intuition.  

 

[Sparky Bill PE]

The question is a bad question. The answer is not how you would actually size this load for a two motor circuit, as it does not indicate that there are any other loads on the circuit beyond those motors. Answer B is correct b/c it is the only one that would comply with the rules for sizing a multimotor circuit regardless of what loads were on the circuit.

Answer A is not correct b/c it is 125% of the continuous non-motor load, not 100%. Answer C is not correct b/c it is 125% of the FLA of the highest rated motor, not 100%. Answer D is not correct b/c it is 100% of the FLA of other motors beyond the first motor, not 125%. 

I'm having trouble finding where in the NEC it says the rules for sizing a multimotor circuit regardless of what loads are on the circuit. 

 

[DilutedAr18_PE]

I'm having trouble finding where in the NEC it says the rules for sizing a multimotor circuit regardless of what loads are on the circuit. 

Those are the rules listed in 430.24. Notice the title “Several Motors or a Motor(s) and Other Load(s). I only have my 2014 version here at home, but those rules haven’t changed in the newer versions. 

 

[Sparky Bill PE]

Sigh. 

Light bulb moment.

is it really "B", because that's the only one out of these 4 that is in the answers?

SO that isn't the "only way to size a conductor" It is the "only way listed" 

For example if A said "125 percent of full-load current rating of the highest rated motor", then you wouldn't have had enough information to know to chose A or B correct?

 

[DilutedAr18_PE]

Sigh. 

Light bulb moment.

is it really "B", because that's the only one out of these 4 that is in the answers?

SO that isn't the "only way to size a conductor" It is the "only way listed" 

For example if A said "125 percent of full-load current rating of the highest rated motor", then you wouldn't have had enough information to know to chose A or B correct?

Correct. Although, that would have actually been relevant to the question, since they listed that there were two motors. I would say that if that answer were also included, I would select that one as it would be most correct. That’s why I say it is a bad question. The answer isn’t really relevant when it only lists motors on the circuit.

 

[Byk]

That is correct however, in that case they wouldn't be able to offer B as a answer (IMO).

I am of the opinion that code questions that do not involve any math "tricky" one.

Here is a tricky example NEC question: — Electrical equip. is being replaced in the existing building. What is the minimum clearance required for 800A, 480V,3Ph,4W panels with exposed live parts on both sides of the working space? 

1)3 ft

2)3½ft

3)4 ft

4)6 ft

 

[Sparky Bill PE]

That is correct however, in that case they wouldn't be able to offer B as a answer (IMO).

I am of the opinion that code questions that do not involve any math "tricky" one.

Here is a tricky example NEC question: — Electrical equip. is being replaced in the existing building. What is the minimum clearance required for 800A, 480V,3Ph,4W panels with exposed live parts on both sides of the working space? 

1)3 ft

2)3½ft

3)4 ft

4)6 ft

Yeah luckily the NCEES practice exam NEC questions aren't a joke like this. This really frustrated me, this made me feel like engineering school. I don't feel this is testing you to see how good of an engineer you are or how you can utilize the NEC to keep people safe this. This was a "hehehe I got my students with a trick b/c I'm so smart" type of joke question. 

I agree that is a very good example of a NEC question. Excited to take this test for the first time in a couple of weeks. Been a LONG time coming. 

 

[Dothracki PE]

Light bulb moment.

I miss hearing that every week in Zach's live class!

 

[DLD PE]

I need a new light bulb.

 

[rburns18 PE]

That is correct however, in that case they wouldn't be able to offer B as a answer (IMO).

I am of the opinion that code questions that do not involve any math "tricky" one.

Here is a tricky example NEC question: — Electrical equip. is being replaced in the existing building. What is the minimum clearance required for 800A, 480V,3Ph,4W panels with exposed live parts on both sides of the working space? 

1)3 ft

2)3½ft

3)4 ft

4)6 ft

So is the answer A. 3ft because we don't know if this install is Condition 1 or 3?

 

[Dothracki PE]

That is correct however, in that case they wouldn't be able to offer B as a answer (IMO).

I am of the opinion that code questions that do not involve any math "tricky" one.

Here is a tricky example NEC question: — Electrical equip. is being replaced in the existing building. What is the minimum clearance required for 800A, 480V,3Ph,4W panels with exposed live parts on both sides of the working space? 

1)3 ft

2)3½ft

3)4 ft

4)6 ft

Condition 3 says for 277V to ground, 4 feet is required. It is under the 1200A threshold to require double clearance (assuming one egress). Also assuming the exposed live parts are not guarded by insulating material per condition 1.

 

[Byk]

Condition 3 says for 277V to ground, 4 feet is required. It is under the 1200A threshold to require double clearance (assuming one egress). Also assuming the exposed live parts are not guarded by insulating material per condition 1.

The trick here was to look for the exception for the existing building 110.26.(A)(c) which allows you to use condition 2, which is 4 ft.

 

[DLD PE]

Sigh. 

Light bulb moment.

is it really "B", because that's the only one out of these 4 that is in the answers?

SO that isn't the "only way to size a conductor" It is the "only way listed" 

For example if A said "125 percent of full-load current rating of the highest rated motor", then you wouldn't have had enough information to know to chose A or B correct?

Yes, I think you're getting it now!

Disclaimer:  The example problem I'm giving below is something I completely made up just now.  It is not a problem I saw in a previous sample exam nor any exam for that matter.  Just a theoretical.

Sometimes the exam will give you just enough information to give allow you to solve the problem.  Sometimes it will give you 10 pieces (ok I'm exaggerating a bit) of information but you only need 1 or 2 to solve it (red herrings as Zach likes to call them).  Sometimes it might appear you need more info to specifically answer the question, but then you look at the answer choices and 3 will always be wrong, leaving only 1 correct possible choice (as in the problem @SparkyBill posted).  

For example, let's say (for theoretical purposes) the NCEES gives you properties of a motor, and you're asked to select fuses for overcurrent protection based on those properties.  They might not tell you everything you need to know about the motor, but let's say the choices are:  (A) 112A, (B) 224A, (C) 100A, (D) 160A.

Without going through the exercise of finding the FLC of the motor, you can immediately see from the answer list that only one choice is a standard fuse size, therefore leaving only one possible correct answer.

This is a test taking skill that can really help.  Get in the habit of reading the questions carefully, know specifically what the question is asking for, and review the choices carefully before solving the problem.  Sometimes just glancing at the answers can give a clue about what the question is asking (especially when it comes to units).  Also sometimes you can easily eliminate 1 or 2 obvious incorrect choices right off the bat, increasing your chances of getting the correct answer even if you don't know or are fuzzy about the question being asked.  Just some food for thought. 

 

[Sparky Bill PE]

Yes, I think you're getting it now!

Disclaimer:  The example problem I'm giving below is something I completely made up just now.  It is not a problem I saw in a previous sample exam nor any exam for that matter.  Just a theoretical.

Sometimes the exam will give you just enough information to give allow you to solve the problem.  Sometimes it will give you 10 pieces (ok I'm exaggerating a bit) of information but you only need 1 or 2 to solve it (red herrings as Zach likes to call them).  Sometimes it might appear you need more info to specifically answer the question, but then you look at the answer choices and 3 will always be wrong, leaving only 1 correct possible choice (as in the problem @SparkyBill posted).  

For example, let's say (for theoretical purposes) the NCEES gives you properties of a motor, and you're asked to select fuses for overcurrent protection based on those properties.  They might not tell you everything you need to know about the motor, but let's say the choices are:  (A) 112A, (B) 224A, (C) 100A, (D) 160A.

Without going through the exercise of finding the FLC of the motor, you can immediately see from the answer list that only one choice is a standard fuse size, therefore leaving only one possible correct answer.

This is a test taking skill that can really help.  Get in the habit of reading the questions carefully, know specifically what the question is asking for, and review the choices carefully before solving the problem.  Sometimes just glancing at the answers can give a clue about what the question is asking (especially when it comes to units).  Also sometimes you can easily eliminate 1 or 2 obvious incorrect choices right off the bat, increasing your chances of getting the correct answer even if you don't know or are fuzzy about the question being asked.  Just some food for thought. 

I'm very good at "looking at what they are asking" before "looking at what's given" but wow thank you for taking the time to write this. That is a skill I never realized is to after "looking at what is asked" go to the answers to see if any don't make sense, before even solving, JUST IN CASE 3 don't make sense.