# emt fill



## chenley (Feb 20, 2007)

Adjustment Factors for More Than Three Current-Carrying Conductors in a Raceway or Cable: Table 310.15(B)(2)(a)


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## Speedy Petey (Jan 10, 2007)

The adjustment factors start at three conductors. Because of 240.4(D) which limits the ampacity of #14-#10, you don't have to worry about derating until you get to ten conductors. 

This is because 240.4(D) typically limits #12 to a 20A circuit. When derating though you use the actual ampacity of the conductor itself, which for #12 THHN is 30 amps. This is in T310.16.

You then go to Table 310.15(B)(2)(a) which states that between 7-9 conductors the adjustment factor is 70%. 70% of 30 amps is 21 amps. 

So you can tell your boss that a 3/4" EMT can hold sixteen #12 THHN (Table C.1 Annex C), but ten or over you no long can use a 20A breaker because of derating.

Get it?


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## heel600 (Oct 31, 2007)

Speedy Petey said:


> The adjustment factors start at three conductors. Because of 240.4(D) which limits the ampacity of #14-#10, you don't have to worry about derating until you get to ten conductors.
> 
> This is because 240.4(D) typically limits #12 to a 20A circuit. When derating though you use the actual ampacity of the conductor itself, which for #12 THHN is 30 amps. This is in T310.16.
> 
> ...


Even though I'm sure you know way more than me, I think you're wrong on this one.

You don't derate for breaker size, but for load.


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## heyhi (Jun 25, 2008)

I understand it now.So, 16 ,# 12 thhn , is allowed in 3/4 emt .But since i have more then 3 current carrying conductors Im not gonna use the 30 amp rating. U said 7-9 derates at 70% but since im using 10 3.10(15)(b)2.(A) says to derate to 50 % so my ampacity is now 15 amps. Just to go one step further, say this was for recepts,I would throw this on a 20 amp breaker? 


Thanks alot in advance this is a great forum and really does help me out alot..........


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## Speedy Petey (Jan 10, 2007)

heel600 said:


> Even though I'm sure you know way more than me, I think you're wrong on this one.
> 
> You don't derate for breaker size, but for load.


Uh, no, the load has nothing to do with derating. Derating comes into play for things like conduit fill, bundled cables, ambient temperature, etc.

I am not suggesting to "derate for breaker size", whatever that means. I am suggesting that a certain conductor can only serve a certain size circuit after adjustment factors are considered.

I should have clarified in my last reply. Derating only comes into play for current carrying conductors as Chenley mentioned.


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## jbfan (Jan 22, 2007)

heyhi said:


> I understand it now.So, 16 ,# 12 thhn , is allowed in 3/4 emt .But since i have more then 3 current carrying conductors Im not gonna use the 30 amp rating. U said 7-9 derates at 70% but since im using 10 3.10(15)(b)2.(A) says to derate to 50 % so my ampacity is now 15 amps. Just to go one step further, say this was for recepts,I would throw this on a 20 amp breaker?
> 
> 
> Thanks alot in advance this is a great forum and really does help me out alot..........


 
You could not put a 20 amp breaker on these conductors. The derating comes to 15, and 15 is the max breaker you can use in this situation.


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## billsnuff (Dec 29, 2007)

#12 THHN = 16 for EMT 3/4" and 9 for 1/2" so, answer is 3/4". Now, unless you know how many are considered current carrying conductors you can't anaswer the derating problem. How many grounded conductors and how many grounding conductors and any MWBCs........unless they are all CCCs and the conduit is used for the EGC. If there are 5 individual branch circuits, you count them all. Then derate for the 10 CCCs and the answer given is correct IMHO. (You could have 6 CCCs, 3 gounded and 1 grounding = 10 total. then derate 80%)


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## billsnuff (Dec 29, 2007)

also, there are other factors. continuous load and non-continuous. temp. rating of the terminals 60d or 75d. also, ten #12 AWG THHN @ 50% won't go on a 20A breaker, derating would force you to upsize the CCCs unti you covered that ampacity.

I may not have said this all correctly, perhaps someone will clarify.


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## JohnJ0906 (Jan 22, 2007)

Just remember, *ALL *conductors count towards conduit fill, but only Current carrying conductors count for (and require) derating.


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## heel600 (Oct 31, 2007)

Speedy Petey said:


> Uh, no, the load has nothing to do with derating. Derating comes into play for things like conduit fill, bundled cables, ambient temperature, etc.
> 
> I am not suggesting to "derate for breaker size", whatever that means. I am suggesting that a certain conductor can only serve a certain size circuit after adjustment factors are considered.
> 
> I should have clarified in my last reply. Derating only comes into play for current carrying conductors as Chenley mentioned.


I don't have a nec at home, some bear with me (some number's from memory, might be wrong).

If you have 100 current carrying conductors (12 thhn) in a raceway, and each conductor feeds a 1 amp 120v load, then all the conductors can be on a 20A breaker.

12 ga cu in the 90 deg column is 25 amps (I think). With 100 conductors, you derate to 30% (I think) so that's 7.5 amps.

So those conductors can carry 7.5 amps. You are still allowed to use a 20 Amp OCPD.

Was this a little more clear?


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## Speedy Petey (Jan 10, 2007)

Clear, yes. Correct, no.

In your example you would theoretically need a 7.5A OCPD. 
You need to PREVENT the conductor from exceeding the adjusted value. 
The load is irrelevant. The potential load is what's important.


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## heel600 (Oct 31, 2007)

Speedy Petey said:


> Clear, yes. Correct, no.
> 
> In your example you would theoretically need a 7.5A OCPD.
> You need to PREVENT the conductor from exceeding the adjusted value.
> The load is irrelevant. The potential load is what's important.


In my 2008 NEC HANDBOOK, they explain it the way I did. I took a code class a few months ago, and same thing.

You size a feeder neutral based on COMPUTED load, not OCPD.


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## Speedy Petey (Jan 10, 2007)

HOW did we get from a theoretical 100 - #12 in a raceway, to a feeder neutral???

Did I miss something?


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## JohnJ0906 (Jan 22, 2007)

heel600 said:


> In my 2008 NEC HANDBOOK, they explain it the way I did. I took a code class a few months ago, and same thing.
> 
> You size a feeder neutral based on COMPUTED load, not OCPD.


What page of the NECHB?

In your example above, those sound like branch circuits, not feeders.


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## JohnJ0906 (Jan 22, 2007)

Example - (6) 120v, 20 amp circuits in conduit. All 2-wire circuits (no MWBCs). 1 common wire EGC. All THHN.

I have 13 conductors total - 6 hots, 6 grounded, one EGC. All count towards conduit fill.
I have 12 current carrying conductors - 6 hots, 6 grounded.
#12 cu THHN - 30 amps @ 90°
30 amps @ 50% is 15 amps. I cannot use a 20 amp breaker, I either use a 15 amp breaker, or upsize the wire.

If I were to use (2) MWBC for the 6 circuits - 6 hots, 2 neutrals, one EGC
9 conductors towards conduit fill.
6 hots only count towards derating.
30 amps @ 80% is 24 amps.
#12 and 20 amp breakers are OK - _and_ a smaller conduit is allowed.


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## heel600 (Oct 31, 2007)

Speedy Petey said:


> HOW did we get from a theoretical 100 - #12 in a raceway, to a feeder neutral???
> 
> Did I miss something?


just another example of the OCPD being larger than the wire ampacity.
Maybe wasn't the best example.

I'll bring the handbook home tomorrow.


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## JohnJ0906 (Jan 22, 2007)

heel600 said:


> just another example of the OCPD being larger than the wire ampacity.
> Maybe wasn't the best example.
> 
> I'll bring the handbook home tomorrow.


There certainly are cases where the OCPD can be larger than the conductor ampacity - motors for instance.


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## nyerinfl (Dec 1, 2007)

heel600 said:


> just another example of the OCPD being larger than the wire ampacity.
> Maybe wasn't the best example.
> 
> I'll bring the handbook home tomorrow.


I believe he's referring to the next size up rule? I could be wrong, but I do know speedy petey is right. Like has been said if your adjusted load from the 100 #12s is 7.5 amp, your breaker can not exceed that 7.5 amp because at that point you would be overloading your adjusted value, understand?


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## rlc3854 (Dec 30, 2007)

Heyhi, are you confused yet? Your boss gave you a good question to play with. He sounds like a smart fellow you might want to stick with for a while. See if he will teach you a little about the business side too.


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## Bkessler (Feb 14, 2007)

Since were on the subject could we review sizing an ocpd for a large motor? For example if I had two 100hp 480 volt motors on one circuit what is the required wire size and ocpd. We will say that these are a continues duty. I know the wire size is is smaller than the "normal" size you would find for the ocpd. I also know you have to reference a few different points in the nec. Thanks bk


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## heyhi (Jun 25, 2008)

I gotta admit I am confused but not as much as i was a few days ago.The nec is very hard to understand the wording. i HAQVE A RAY MULLIN RESIDENTIAL WIRING BOOK AND ORDERED COMMERCIAL WIRING AND THEY EXPLAIN WHAT THE NEC MEANS AND GIVES U THE CODE TO REFRENCE TO SO IM LEARNING ALOT AND HAVE ALOT MORE TO LEARN.

NONE OF THE GUYS I WORK WITH ARE LICENSED, SO, WHEN I ASK THEM SOMETHING I NEVER GET A STRAIGHT ANSWER. THEY DO EXPLAIN THINGS BUT I GET MORE OF THIS IS THE WAY I WAS TAUGHT TO DO IT, AND THE WAY I BEEN DOING IT FOR 20 YEARS KIND OF ANSWER.
I COME ON HERE AND LOOK AT SOME THREADS AND MOST OF THE TIME IM CLUELESS.I CANT WAIT TO GET A QUESTION I CAN ACTUALLY ANSWER.IM GONNA ANSWER IT EVEN IF IT WAS ANSWERED ALREADY. BUT IM LEARNING FROM U GUYS AND APRICIATE TAKIN UR TIME OUT AND ANSWERING MY QUESTIONS


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## heyhi (Jun 25, 2008)

billsnuff said:


> #12 THHN = 16 for EMT 3/4" and 9 for 1/2" so, answer is 3/4". Now, unless you know how many are considered current carrying conductors you can't anaswer the derating problem. How many grounded conductors and how many grounding conductors and any MWBCs........unless they are all CCCs and the conduit is used for the EGC. If there are 5 individual branch circuits, you count them all. Then derate for the 10 CCCs and the answer given is correct IMHO. (You could have 6 CCCs, 3 gounded and 1 grounding = 10 total. then derate 80%)


yOUR SAYING TO DERATE 80% FOR 10CCS I THAUGHT IT WAS 50 % FOR 10 - 20 NOW IM CONFUSED


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## Speedy Petey (Jan 10, 2007)

heyhi said:


> yOUR SAYING TO DERATE 80% FOR 10CCS I THAUGHT IT WAS 50 % FOR 10 - 20 NOW IM CONFUSED


No, in his last sentence he is saying you can have ten conductors in the conduit, but only six of them would be CCCs allowing the 80% adjustment. 
His example is one of the reasons MWBCs are beneficial.


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## Speedy Petey (Jan 10, 2007)

heyhi said:


> NONE OF THE GUYS I WORK WITH ARE LICENSED, SO, WHEN I ASK THEM SOMETHING I NEVER GET A STRAIGHT ANSWER. THEY DO EXPLAIN THINGS BUT I GET MORE OF THIS IS THE WAY I WAS TAUGHT TO DO IT, AND THE WAY I BEEN DOING IT FOR 20 YEARS KIND OF ANSWER.


Yes, this is a very unfortunate fact in many places.


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## billsnuff (Dec 29, 2007)

*bk*

i use UGLY's for motors. off the top of my pea brain, 100% of all mtrs + 125% of the largest, the OCPD depends on one of four types = Instant Trip = ???% etc. Can someone fill in the blanks for me.......book at work.

and for heyhi..what speedy petey said. :notworthy: it's all about the money. if i use 5 individual circuits i have to upsize the wire and possibly conduit, to carry the load (more money). but if i use MWBCs i can meet the load at 80% (24A) and use #12s. (save the money). 1st case 10 CCCs, but with MWBCs only 6 CCCs to count for derating. grounded conductors in individual circuits carry the same load as the ungrounded. but with a MWBC, the grounded conductor carry only the unbalance of the load and is not a CCC for derating purposes. neither is the grounding conductor. Clear as mud, right. :thumbsup:


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## leland (Dec 28, 2007)

rlc3854 said:


> Heyhi, are you confused yet? Your boss gave you a good question to play with. He sounds like a smart fellow you might want to stick with for a while. See if he will teach you a little about the business side too.


 
Perfect Answer!!!!:thumbsup:


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## leland (Dec 28, 2007)

heyhi said:


> Today my boss caught me reading the nec, so he decides to test me. I just started studying and i don't know a whole lot.I actually been studying 210.52 and 314 so I'm prob not gonna figure this out in a code book.
> He asked me how many number 12 thhn can i fit in a 3/4 in EMT ,and if i put 10, number 12, wires in 3/4 whats the ampacity. I found that i can fit 9 in 1/2 in. 16 in 3/4 in and 26 for 1 in.
> 
> I told him id have the answer for him tomorrow. I really don't want a answer so much as an explanation on the ampacity part. I know number 12 is usually 20 amp breaker and i think 17 amp max ampacity, but thats romex or m.c .One chart i saw said 30 amps for # 12 thhn in a conduit.
> how would i get this calculation...oh and i actually would like the answer but i also wanna learn for the future


 
After all this, All very good and valuable information.

Ask the boss "what will this conduit be serving?", perhaps 1 3 phase motor for a sump pump,and a few general purpose rec. and some lights. maybe several motors, some continuouse and some not.

after all, this will ultimately determine all of the above influences and only then will you be able to answer the question.


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## Magnettica (Jan 23, 2007)

I learned about derating in school. If I hadn't done that I'd be very confused right about now. The best advice I could give is to think about inductive heating within the conduit itself and also the temperature of where this conduit is located. For instance, if the conduit goes through a boiler room or high up in a warehouse than you have to account for that as well (ambient temperature). This is the primary reason why #10 THHN is often run in an industrial setting to account for both.


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## nick (Feb 14, 2008)

*310.16 table ul listing of equipment*

well i disagee with the way you are saying 90 c . heres why you must use the 75 c on conductors in 310.16 even when you derate , because mostly all panels or gear we buy today are only rated for 75 c , the lug terminals or breaker lugs the device terminal meaning switches or plugs you connect to has a rated ul listing at a 75 c . they make 90 degree stuff but its rare and its special order stuff , most electricians use the wrong chart when picking wire size most inspectors dont even look at it but it is a fact , ask your inspector about 75 degree rated equipment ? temp is important to terminal connections wire must match lug or terminal temperture connection if not your connection will get over heated as the metal that is at a lower temp will drawn the heat from the metal or wire with the higher temp . now ask your instructor at school or your local inspector about this see what he tells you ? better read art[ 110.3b] , and [ 110.14 c 1 ] 100 amps or less 60 c / 100 amps or more 75 c wire rated not to offend any one iam just pointing out that there is more to what we do everyday then what we see in the code book . we do it with thhn but is it correct air temp in room or conduit ? lugs rated at ? breaker is now limited to only what temp correction alows to get by to use that load ? what if the owner of that circuit in the future adds load to circuit or takes out the breaker and installs a new 20 amp ? who controls the people plugging stuff into the outlet ? meaning we now have insulation that can handle the temp in the conduit but does the total temp of all the wires in that conduit added to that temp increase the temp of the copper conductors on them 75 degree lugs !!! i say it transfers heat to the lugs even when derated and the code is wrong in that area of heat or contact of the metal to the metal connections . the code is mis leading and someone needs corrected parts of it at least its just my thinking Speedy. you are correct in your example take no offense i respect your know how. ive have also use derating in the manner you have explained , but we need to change the code i think its a fine line . and look at the instant itself meaning what load what temp of why we need to derate when we could just run the correct wires in a conduit and use the correct breaker thats what iam trying to give input on . i do not like the nec on some issues its 2008 and the code book its way behind the times ! power today is harmonics the code is not thinkin what we plug in today in our outlets . id like to go to the nfpa and sit down with them boys once . thers alot id like to go over with them id make some changes to better us as electricians. my calculations came out 16 current carriers at 17 amps ? 9 current carriers at 21 amps i hope you understand take care best to ya


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## billsnuff (Dec 29, 2007)

nick: i agree on your points on terminations

bk: on motors, 50 hp 480 vac design B 75d terminal = 65A @ 125% = 81.25A conductor t430.250 and 430.22

ocpd 430.52 C1 non-time fuse = 300% * 65A = 195A (EX1) 200A fuse

feeder for 50 hp and 30 hp 480vac 3ph is 125% of largest and 100% of the rest (125 *65) + 40 = 125.25A feeder 430.24

feed ocp 430.62A lagrest ocpd plus the rest (fla) 
50hp = 200A fuse (65flc)
30hp = 125A fuse (40flc)

200A + 40A = 240A (do not exceed on feeder) use 225A fuse. '05 NEC


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## nick (Feb 14, 2008)

ya billsnuff , nice job on the motors ya beat me to it .take care best to ya :thumbsup:


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## Misha1980 (Sep 24, 2008)

I thought all branch circuits under 100A had to be figured for 60c regardless of terminal ratings per 110.14(C)(1)

I'm new here so cut me some slack if I'm wrong


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## billsnuff (Dec 29, 2007)

*110.14 c 1*

in ref to my post see 110.14 C 1 a 4


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## Misha1980 (Sep 24, 2008)

*110.14 c*

I was referring to what Nick posted, but then I re-read what he posted and he actually said this. That's what I get for speed reading after being on a job all day :/


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## heyhi (Jun 25, 2008)

billsnuff said:


> also, there are other factors. continuous load and non-continuous. temp. rating of the terminals 60d or 75d. also, ten #12 AWG THHN @ 50% won't go on a 20A breaker, derating would force you to upsize the CCCs unti you covered that ampacity.
> 
> I may not have said this all correctly, perhaps someone will clarify.


I understand the temp rating factor,but can u give me an example of the terminal rating.and how it affects the circuit. 
Can some one also explain ,N.E.C 110.14(C) says equipment rated 100 amps or less or 14 awg thru 1 awg, the allowable ampacity is 140(F)column. Are they just saying ,14 thru 1 awg ,the minimum temp rating for the wire should be 140 f?

I am also confused with the ,100%continuous load plus 125% non continuous load rule..

Since I'm asking stupid questions. Is a raceway, outside ,18 in below grade level ,considered a wet location?


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## nick (Feb 14, 2008)

i think what there saying is example = a 12# thw copper ambient temp at 124 degs f == 12# thw x .67 =16.75 amps reduced overcurrent protection is 15 amps max but if you use 12#thhn = 30 amps x.76 = 22.8 amps and a 20 amp breaker can be used but provide the circuit is not a continous load . thats code !if you had a continous load the load on that wire would have to be multipled by 125% of the 100% you started with .the 18 inch below grade is a wet area . if ya look at 310.16 == 60 c and 75 c 14 and 12 are the same amps 10 is the odd one at 30/ 35 amps so on 14 /12 you cant screw it up . but you must not exceed the 60 c you really cant and if ya look at the temp derating in code book bottom of wire charts ,it kind of helps the way they place the centigrade ratings in the derated columns your always one step behind the limit of temp. but this is why i do not agree with the code book as people today have stuff plugged in and running more than 3 hours so think about the loads on most outlets today lots more electronics lots more power used its 2008 not 1969 ? did this help best to ya take care :thumbsup:


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## stillirnin (Jan 24, 2008)

*confused*

:blink: I was about to post on same subject seen this post and replies now let me see.... adjustment factors start after 3ccc in raceway , but figure conduit fill on all ccc , gc, egc ? use column that your conducter type falls under adjust from that point for ampacity. Now do you use temp at top of columns table 310.16 for anything other than choosing proper conducter for installation needs? or does this value have another use? also do fuses get sized like a breaker as in percetage 20 amp breaker i6 amp load...


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## nick (Feb 14, 2008)

The temp at the top of 310.16 is the temp for that insulation type at the top of column of that row . you can not exceed that when you use not more than 3 conductors at that insulation type , but if you use more than three conductors the temp is higher at that insulation type so you derate these temp gos up , also you must include air temp of area around conduit . the temp correction on the bottom of that same table 310 .16 , if you use a different insulation type a higher insulation type then you must still derate it but you can not exceed ampacity rating a example like wire size meaning 12 awg size copper in that top column at that 60 c when you get done derating it . if its 100 amps or less , if its 100 amps or more you can not exceed the 75c column , you must base all your wire ampacity/ derating by the column that pertains to the type wire size and insulation that you are using at that time when you pick your ampacity/ insulation type after you picked the size wire or your type of insulation and worked it out then go to the correct 60c or 75c column which one pertains to the original before you started to derate if you only had three conductors .does this make sense ? i left out the part of percent of derating to help with just the table columns you know already about the derating on the conduit fill part just add this to it . just a example if your at 50% from conduit fill and then you derate for ambient air both are used at same time. best to ya


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