How would you work derating of this branch circuit run?

[bjp_ne_elec]

Say you have a 200 foot run - with ten (10) - #10 AWG THHN and these feed five (5) 120 volt circuits. The run goes through an area of attic area that is 125 degrees Farenheit. What is the would be the ampacity of maximum overload that should be set for these circuits?

 

[roger]

Show how you would do this first.

Roger

 

[pierre]

Roger
Are you cooking dinner tonight? I bring the meat, you bring the veggies.

 

[roger]

Pierre, yes I am and you know you are always welcome.

 

[pierre]

I will have to skip tonight, Thanks anyway.
I am helping with the cooking tonight, I am responsible for the meat... I am cooking a roast. Your veggies look really good... I will have to make it next week. :wink:

 

[roger]

Okay, but be aware that my veggies are all hot peppers.

Roger

 

[bjp_ne_elec]

Here's how I'd do it:

First derate for the ambient temperature of the attic (120 F) - which is 0.82 under the 90 C column of 310.16 - this is for #10 AWG THHN. Under this column, #10 AWG THHN is rated 40 A.

So: 40A * 0.82 = 32.8 A

Now derate for number of conductors in conduit - and this is what I'm not totally clear on - and we have ten (10) conductors. Using Table 310.15(B)(2)(a) - and I figure I'm counting the all the neutrals - as there is no mention of "shared" neutrals - it's 50% for 10-20 conductors.

So: 32.8 A * 0.50 = 16.4 A

Therefore, this circuit would have to be fed with a 20A breaker - as 240.4(D) allows you to move to next higher breaker.

So that's how I'd do it - am I right or wrong? Is taking all five of the neutrals in to account when derating for conductor fill correct?

Thanks,

Brett

 

[jbwhite]

310.15 (B) (4) is the answer.

In your example you must count the Grounded Conductors.

They only become Neutral if they are carrying only the imbalance, as in a multiwire circuit.

 

[bjp_ne_elec]

So - jb - bottomline, is my scenario correct?

I also don't know how to interpret the Exception in 310.15(A)(2). Do you go to the higher ampacity if the length that it travels through a higher temperature area if the total length through the higher temperature area is less than 10%, but over 10 feet. In other words - if the total run is 300 feet, and the run goes through a higher temp of 15 feet - do you still go with the higher ampacity? It's over 10 feet, but less than 10%. I've re-read that exception several times, and it's not totally clear to me.

Thanks


Brett

 

[infinity]

So - jb - bottomline, is my scenario correct?

Thanks


Brett

You scenario is correct if the circuits do not supply multiple receptacles on each circuit. You can not go to the next higher standard sized OCPD if the circuit is "part of a multioultlet branch circuit supplying receptacles for cord and plug connected portable loads." 240.4(B)(1). Also I have been told that in New Jersey ambient temperature corrections for attics are based on the average year round temperature of the air that they are in.

 

[roger]

Brett, given no more information in the question, and assuming the loads are less than the adjusted ampacity of the conductors, you are right in your method.

However, scince we don't have the specifics, you may still have a problem, for instance, if any of these five circuits are part of "Multi-Outlet Branch Circuits" the provissions of 240.4(B) could not be used for these particular circuits.

If they are hard wired you would be fine

Roger

 

[jbwhite]

If they are hard wired you would be fine

Roger

He is fine as is. He did not go up to the next higher OCPD, He went down from 16.4 to 15 amp breaker.

Edit : ooops I either read that wrong earlier, or he changed it..

you are right roger. has to be 15 amp breaker

 

[jbwhite]

So - jb - bottomline, is my scenario correct?

I also don't know how to interpret the Exception in 310.15(A)(2). Do you go to the higher ampacity if the length that it travels through a higher temperature area if the total length through the higher temperature area is less than 10%, but over 10 feet. In other words - if the total run is 300 feet, and the run goes through a higher temp of 15 feet - do you still go with the higher ampacity? It's over 10 feet, but less than 10%. I've re-read that exception several times, and it's not totally clear to me.

Thanks


Brett

I read the exception this way. Assuming the same attic but the wiring is run through the floor and up the wall to a switch. "From the switch the wire is run up into the attic space and to a light. If the length of the wire from the point where you enter the attic to the light is either less than 10% of the total length or 10 feet, which ever is less. You would not have to adjust for temperature. In no case could this length be more than 10 feet.

Here is another example: Commercial building all in conduit. Less than 4 current carrying conductors in each conduit to a troff mounded above a panel. Less than 10 foot of wire bundled from troff through large nipples to panel and breakers. There is no need to adjust for ampacity for number of conductors in the conduit.

 

[infinity]

More than 30 conductors in a trough requires derating.

 

[bjp_ne_elec]

jb - I want to see what the consesus is - but I see it as you said it:
10 feet or 10% - which ever is ever is less - but in no case more than 10
feet

Does any one else want to weigh in. I'm prepping for the Master's Exam, and they had a similar question - and I want to make sure I nail any question related to ampacity, derating, etc. There is supposedly many questions pertaining to this - and they put casual twists on it.

Thanks

Brett