what is the answer to these questions?

[infinity]

For #1 - we use #6 since next size up 65 amps would be 70amps which would equal 70 amps

For #2 - we choose #6 because if we choose #8, next size up would be 45 amps and not 50 amps on table 240.(6)A

And for #3 - we choose # 8 and not #10 as next size up for #10 would be 40 amps and not 50 amps.

1) is correct
2) is correct
3) #8=50 amps there is no next size up so #8 is the answer.

 

[jaggedben]

I get it guys.. thank you.

It's basically next size up rule.
So,
For #1 - we use #6 since next size up 65 amps would be 70amps which would equal 70 amps

For #2 - we choose #6 because if we choose #8, next size up would be 45 amps and not 50 amps on table 240.(6)A

And for #3 - we choose # 8 and not #10 as next size up for #10 would be 40 amps and not 50 amps.

Is my logic now correct? or no? please let me know.. ty

Right answers but logic on #2 and #3 is a bit off.
Since 40A is the 75C ampacity of 8awg and 40A is also a standard breaker you don't get to use the next size up no matter what that size might be. Similarly for the #10, with the additional detail that 240.4(B) generally does not allow the next size up rule for 10awg and smaller.

 

[infinity]

Since 40A is the 75C ampacity of 8awg and 40A is also a standard breaker you don't get to use the next size up no matter what that size might be.

I think thst you meant #8 @ 75° C has an ampacity of 50 amps not 40.

 

[wwhitney]

with the additional detail that 240.4(B) generally does not allow the next size up rule for 10awg and smaller.

I think you meant 240.4(D)?

There's no special interaction between 240.4(B) and 240.4(D), since the standard ampacities of #10, #12, and #14, at both 60C and 75C, are standard breaker sizes. And 240.4(B) can certainly be used when there is significant derating. E.g. (10) 14/2 NM cable crammed together in a hole through a framing member which is insulated. Very likely that's 20 CCC, so 0.5 ampacity adjustment, so an ampacity of 12.5A. Still fine to protect at 15A if they are supplying circuits other than multiple receptacles and with a calculated load of 12.5A or less.

But if any of the test questions were asking about 15A-30A breakers, then 240.4(D) would be something you need to follow.

Cheers, Wayne

 

[jaggedben]

I think thst you meant #8 @ 75° C has an ampacity of 50 amps not 40.

That was regarding question #2 in the OP which said to use 60C

I think you meant 240.4(D)?

Yes. Sorry, that was a confusing typo.

There's no special interaction between 240.4(B) and 240.4(D), since the standard ampacities of #10, #12, and #14, at both 60C and 75C, are standard breaker sizes. And 240.4(B) can certainly be used when there is significant derating. E.g. (10) 14/2 NM cable crammed together in a hole through a framing member which is insulated. Very likely that's 20 CCC, so 0.5 ampacity adjustment, so an ampacity of 12.5A. Still fine to protect at 15A if they are supplying circuits other than multiple receptacles and with a calculated load of 12.5A or less.

But if any of the test questions were asking about 15A-30A breakers, then 240.4(D) would be something you need to follow.

Yes agree, but my point was we can simply state that if you're thinking of using 10awg copper (and not 240.4(G) and taps don't apply) on a 40A breaker you just stop, because you can't.

 

[Dennis Alwon]

@solar man The next size up rule is used only when the load is not over the conductor ampacity. Since none of the questions gives a calculated load then I would go straight from the table.

Eample of next size up. Say you use a #6 NM cable that is rated at 60C. The max the conductor is allowed is 55 amps. Since 55 amps is not a standard size breaker you can use a 60 amp breaker only if the calculated load isn't greater than 55 amps. Does that help

 

[solar man]

@solar man The next size up rule is used only when the load is not over the conductor ampacity. Since none of the questions gives a calculated load then I would go straight from the table.

Eample of next size up. Say you use a #6 NM cable that is rated at 60C. The max the conductor is allowed is 55 amps. Since 55 amps is not a standard size breaker you can use a 60 amp breaker only if the calculated load isn't greater than 55 amps. Does that help

Yeah I understand that it applies to next size up of the OCPD - this is I guess reverse next size up condition...

 

[Carultch]

I understand that there is a round up rule; but isn't it for just when you are sizing the OCPD device? So, I don't know how 240.4(B) is related to these questions.. that's what I am trying to figure out. Thank you

Agreed. The next-size-up rule seems counterintuitive, considering the purpose of the breaker. You'd expect the breaker trip rating to be required to be the "weak link". Likely, the idea is that #6 Cu wire and terminations won't fail exactly at 65A, but there's enough margin of safety built-in to the 310.16 ampacities, to make it perfectly safe to allow for a breaker that doesn't trip until 70A, before it will cause any serious problems. Breakers also trip quickly on high overcurrents of short circuits, and trip slowly on moderate overloads.

In any case, regardless of its physical basis, it's a rule we're allowed to take credit for, when meeting the conditions of the rule. As long as the wire and terminal ampacity is some epsilon above the previous-size-down OCPD, and as long as you're using a standard OCPD, and otherwise meeting the conditions of the rule, you can take credit for it.

How small is that epsilon? It doesn't really say. Half an amp would be a reasonable standard, since the NEC allows standard rounding conventions to the nearest full amp when sizing the OCPD. I've had cases where calculated wire ampacity with derates is less than 1A above the previous size, so this is a significant question. But as long as it isn't an exact match to the previous-size-down, and as long as ampacity is greater than the calculated load with applicable safety factors, it's good to go.

 

[Tulsa Electrician]

I have following this to see what others say.
Pretty sure what they want. The way it's written it can be answered many ways.

What I did was ask these questions at work.
Apprentice and journeymen.
I was surprised at some of the answers.
Mostly what I read here.

Then I ask let's look at the question again.

Hi all -
I am trying to figure out the answers and the logic behind the answer for the following 3 questions.

1) What's the minimum size THHN conductor permitted to terminate on a 70A circuit breaker or fuse if the circuit breaker and equipment terminals are listed for 75C conductor sizing?

Only one gave what I was looking for.
He said it depends on the breaker. He pulled out a 70 amp Eaton and it says minimum is #8, #8-1/0.
He also added you can not terminate a wire on a fuse. He said for the equipment it would be based on the termination within equipment. He did add the breaker and equipment has the same temperature rating at it terminals so no change required.

Then we all laughed and decided to go with a basic answer based on terminal temperature, conductor size using THHN rates @90 c . Sizing conductor based on 60 c and 75 c using the amp listed as a minimum. We felt this was the basis for the questions since nothing else was provided. We used copper since the NEC says to use copper unless otherwise noted (110.5).

As a group we went with table 310.15 only.
#1, B, #4 @75c (70 amp)
#2, C, #6 @60c (50 amp)
#3, D, #8 @75c (50 amp)

Great work discussion, plus the donuts were yummy. We felt this would be basic entry level questions.

Thank for posting questions.

 

[kwired]

240.4(B) is the 'next size up' rule. It says that when the wire ampacity is not a standard overcurrent device rating (see 240.6(A)) then the next size up standard breaker can be used (unless over 800A).

This affects your answer to question 1.

and question 2

 

[kwired]

To be pedantic, the questions are poorly worded. Any of these breakers could be used to supply a tap conductor under 240.21(B). So the answer is really whatever the smallest conductor the breaker is labeled for.

These and other problems would be fixed by changing "THHN conductor" to "THHN branch circuit conductor not subject to 240.4(G)." In a test situation, I think it's reasonable to assume that is the test question writer's intent.

Cheers, Wayne

They could supply motors, refrigeration, or other specific items that allow different method of determining overcurrent protection as well.

We can only assume without any other information that the general rules is what we need to apply.

 

[kwired]

As mentioned in answer by Kwired - all are related to 240.4(B) - this is what I am confused about??

#3 maybe not so much since the initial result lands at a standard size. It still could apply if you had a situation where the ampacity required adjustment though

 

[infinity]

We can only assume without any other information that the general rules is what we need to apply.

Exactly don't read too much into it. The code is filled with various rules and exceptions that could apply to an ampacity question. Just take what's given and respond to that.

 

[Fred B]

@solar man The next size up rule is used only when the load is not over the conductor ampacity. Since none of the questions gives a calculated load then I would go straight from the table.

Eample of next size up. Say you use a #6 NM cable that is rated at 60C. The max the conductor is allowed is 55 amps. Since 55 amps is not a standard size breaker you can use a 60 amp breaker only if the calculated load isn't greater than 55 amps. Does that help

240.6 lists the standard size breakers, 70A and 50A are both standard size thus discussion on next size up related to the test questions are moot.

Also Just because you can't find a particular breaker size in the store doesn't automatically mean you can take advantage of the next size up.

 

[jaggedben]

and question 2

Nope.
You are seemingly the second person to overlook that question 2 specified 60C terminals. OP got the right answer in post #14.

 

[jaggedben]

240.6 lists the standard size breakers, 70A and 50A are both standard size thus discussion on next size up related to the test questions are moot.

...

No, the next size up rule is not moot at all if the question is what is the minimum size conductor that can be protected by such a breaker. You seem unclear on the concept of the rule.

 

[Carultch]

No, the next size up rule is not moot at all if the question is what is the minimum size conductor that can be protected by such a breaker. You seem unclear on the concept of the rule.

So is the question expecting you to account for the next size up rule? Or is it just asking for the conventional answer that has at least as much wire as OCPD?

If a document tells me I need a 70A breaker without further information, I would specify #4 Cu wire in practice. If I have enough information to know that it's only utilizing 65A or less, I'd specify #6 Cu wire, but not until I have that information.

 

[Dennis Alwon]

So is the question expecting you to account for the next size up rule? Or is it just asking for the conventional answer that has at least as much wire as OCPD?

If a document tells me I need a 70A breaker without further information, I would specify #4 Cu wire in practice. If I have enough information to know that it's only utilizing 65A or less, I'd specify #6 Cu wire, but not until I have that information.

This is why the questions suck. Is it a trick to use the next size up to answer or are we to use the table directly. There is no way we can anticipate what the testers were thinking when they made the test.

If they wanted the minimum I would have answered #6

 

[jaggedben]

So is the question expecting you to account for the next size up rule? Or is it just asking for the conventional answer that has at least as much wire as OCPD?

If a document tells me I need a 70A breaker without further information, I would specify #4 Cu wire in practice. If I have enough information to know that it's only utilizing 65A or less, I'd specify #6 Cu wire, but not until I have that information.

The point has been made, and it has been corroborated by the discussion, that the questions are too poorly written to divine their intent. So I'm not going to try further.

What I will point out, and your second paragraph speaks to it, is that the questions basically get the proper code process backwards, but often in the real world it is done backwards as well.

Generally the proper code process, if you want to determine minimum requirements, is as follows:
1> determine circuit current requirement based on connected equipment and code
2> choose conductor size based on circuit current, terminals, and conductor material and temp rating (also derating, but that's a side topic here)
3> choose an OCPD that protects the conductor according to 240

The questions give us the breaker and expect us to work backwards to the 'minimum' conductor size without info. I take your point that often in the real world we may have a breaker and we size conductors on the safe side. But that isn't a minimum like the question asked. Also to flip your hypothetical on its head a bit, if I'm told there's a 6awg cu THHN feeder without being told the load, arguably I should protect that with a 70A breaker unless I know the load is less than 60A.

 

[Birken Vogt]

Agreed. The next-size-up rule seems counterintuitive, considering the purpose of the breaker. You'd expect the breaker trip rating to be required to be the "weak link".

The way I have always described it:

Proper system design protects the conductors from overloading.

The breaker is just there to protect against shorts and such.

We don't run conductors right up on the edge and expect some cheap breaker to provide protection from fire.