Confused on a test prep question

[George Stolz]

What is the MINIMUM size underground service lateral for a copper-clad aluminum single branch circuit serving a controlled water heater?

(A) #14
(B) #12
(C) #10
(D) #8

I got this wrong. Can you get it right? I need a sanity check. This is based on the 2005 NEC.

 

[George Stolz]

Here's another:

What is the MINIMUM size copper EGC which may be used for a circuit wired with size 2/0 copper THW and protected by a 125A circuit breaker?

(A) #6
(B) #4
(C) #2
(D) #2/0

The book's answer is #6. Mine was #4.

 

[charlie b]

My answers are:
First question (C) - Reference is 230.31(B) Exception
Second question (A) - Reference Table 250.122. The "2/0 THW" is meaninless. The breaker is over 100 and not over 200, and the wire is copper. That is all that matters.

 

[George Stolz]

My answers are:
First question (C) - Reference is 230.31(B) Exception

Good call. I didn't pay attention to the exception.

Second question (A) - Reference Table 250.122. The "2/0 THW" is meaninless. The breaker is over 100 and not over 200, and the wire is copper. That is all that matters.

What about 250.122(B)?

 

[iwire]

The breaker is over 100 and not over 200, and the wire is copper. That is all that matters.[/SIZE][/FONT]

Sure sounds like 250.122(B) would come into this one.

2/0 CU @ 75 = 175 amps

 

[charlie b]

In relation to the first question, I would like to ask how the code comes to speak of "service lateral" and "branch circuit" in the same breath. My limited understanding is that there is "service," and there is "feeder," and there is "branch circuit," and never the twain (is there an old English term for three things? "twain" means two.) shall meet.

 

[George Stolz]

In relation to the first question, I would like to ask how the code comes to speak of "service lateral" and "branch circuit" in the same breath.

I'd say the word "load" ties them all together. The load is present on all three. It's goofy, but it works, IMO.

 

[lpelectric]

I got this wrong. Can you get it right? I need a sanity check. This is based on the 2005 NEC.

According to the way I read 230.31 (B) and it's exception, the question is geared for this exception which permits a #12 awg copper. :smile:

 

[charlie b]

What about 250.122(B)?

Obviously this was a trick question, and its author wanted you to invoke the "biggie size the EGC rule." But I will cling to my answer, and will invite attention to the the emphasis given in the question to the word "MINIMUM." What is the bare minimum size that can be justified in any way, without violating 250.122(B) or any other code requirement? The answer is #6. Why? Because we do not know the load.

Suppose the load was calculated to be 170 amps. Suppose we arrived at that value by adding up a large number of things that are really not likely to run at the same time, but for which the code does not give us any demand factors. We absolutely must use a conductor that has an ampacity of 170 or higher. So a 2/0 THW copper is a good choice. However, nothing requires us to use a 175 amp overcurrent device. We can't go higher than 175, but we can go lower. So I could make the design decision of selecting a 125 amp breaker, perhaps on the basis of having one laying idle on my shelf. That makes the installation legal, even if not a great design, and it permits the use of an EGC that is sized per the table. I did not increase the ungrounded conductors in size, so I don't need to increase the size of the EGC.

 

[charlie b]

According to the way I read 230.31 (B) and it's exception, the question is geared for this exception which permits a #12 awg copper. :smile:

Agreed. But the question calls for copper-clad aluminum, so the answer is #10.

 

[George Stolz]

Obviously this was a trick question, and its author wanted you to invoke the "biggie size the EGC rule." But I will cling to my answer, and will invite attention to the the emphasis given in the question to the word "MINIMUM." What is the bare minimum size that can be justified in any way, without violating 250.122(B) or any other code requirement? The answer is #6. Why? Because we do not know the load.

While I give you credit for coming up with a possible explanation for the answer provided by the author, do you really believe this was the case? The thought process you have presented is utterly foreign to me, and I spent some time trying to find an angle to knock you off your perch, and couldn't find one.

It seems to me like a case of the author either forgetting 250.122(B), or misapplying it (since the breaker/wire size are under 200A, then one could wrongly conclude that the smaller EGC is still acceptable.)

 

[George Stolz]

From the same test:

A lighting system limited to 30 volts or less is operate at no more than ____ amperes maximum supplying lights.

(A) 10
(B) 15
(C) 20
(D) 25

After reading the text from the NEC when answering the question, I was surprised to find it was one I seemingly got wrong. When I saw how they came to their answer, I decided it was either a bogus question or the NEC is somewhat contradicting itself.

Their source:

Lighting Systems Operating at 30 Volts or Less. A lighting system consisting of an isolating power supply operating at 30 volts (42.4 volts peak) or less under any load condition, with one or more secondary circuits, each limited to 25 amperes maximum, supplying luminaires (lighting fixtures) and associated equipment identified for the use.

Mine:

411.6 Branch Circuit. Lighting systems operating at 30 volts or less shall be supplied from a maximum 20-ampere branch circuit.

 

[charlie b]

It seems to me like a case of the author either forgetting 250.122(B), or misapplying it . . . .

I suspect you are right. I also would say this person has no business writing test questions.

That said, I will mention that I have gone on record as disagreeing with a number of well-respected members of this forum on the question of whether 250.122(B) applies when you derate a conductor's ampacity due to ambient temperature or due to the number of current-carrying conductors in a raceway. If the author of your test agrees with my position, then perhaps he arrived at a #6 EGC by selecting a 2/0 THW for a high ambient temperature situation.

 

[George Stolz]

I also would say this person has no business writing test questions.

I am glad I just borrowed this book and didn't purchase it.

 

[lpelectric]

Agreed. But the question calls for copper-clad aluminum, so the answer is #10.

Duh! I guess I don't read too well. :smile: What a great example of where I could EDIT the right answer in and appear like I was really smart a couple of hours after the post.

 

[Dennis Alwon]

After reading the text from the NEC when answering the question, I was surprised to find it was one I seemingly got wrong. When I saw how they came to their answer, I decided it was either a bogus question or the NEC is somewhat contradicting itself.

It appears to me that their source (art. 411.2)was stating the max load on the secondary side of the transformer while your source(art 411.6) is stating the maximium size branch circuit that supplies the transformer. Although the wording is somewhat confusing I would have answer 25amps.

 

[charlie b]

It appears to me that their source (art. 411.2)was stating the max load on the secondary side of the transformer while your source(art 411.6) is stating the maximium size branch circuit that supplies the transformer.

I would say that "their source" is not a source of requirements in any sense of the word. 411.2 is a definition. If a circuit meets that definition, then it must fulfill the requirements that follow. If not, then it must fulfill any applicable requirements that are given elsewhere in the code.

Thus, for them to use that article as the basis for a test question, particularly a test question that addresses minimum or maximum limits for circuit loading, is an error on their part. This brings me back to my earlier assertion that he/she/they should not be writting test questions.

 

[Dennis Alwon]

I would say that "their source" is not a source of requirements in any sense of the word. 411.2 is a definition. If a circuit meets that definition, then it must fulfill the requirements that follow. If not, then it must fulfill any applicable requirements that are given elsewhere in the code.

Thus, for them to use that article as the basis for a test question, particularly a test question that addresses minimum or maximum limits for circuit loading, is an error on their part. This brings me back to my earlier assertion that he/she/they should not be writting test questions.

As usually, you make a lot of sense Charlie and as you know most test makers can really confuse the issue. I totally agree the question has much to be desired.

 

[staticcontrol]

conduit fill

conduit fill

Hello,

I'm a new member. Can you help with what should be a simple question with one element missing?

Given: 7 1/2 HP motor 240v single phase and 10HP motor 240v 3 phase. Seperate conductors to each motor. Wires in one conduit. What size galvanized pipe is needed?

Element missing? Which wire style should be selected for volume since it's not stated? Is there a rule? I used conductor property tables for thhn and thwn.

I know RMC is galvanized so I selected this.

Thank you.

Staticconrol.

p.s. I'm an industrial/commercial electrican studying for exam.

 

[radiopet]

ok..here is a quiz for ya.