# Branch Circuit Derating



## Achilles (Sep 11, 2014)

Now that code requires separate neutrals for each branch circuit (unless 2P/3P breaker) how many circuits do you install in a conduit?

Per Table 310.15B3a more then 10-20 current carrying conductors require a 50% adjustment factor. Since the individual neutral conductors are no longer carrying an unbalanced load I believe they count as current carrying conductors.

Which limits me to 4 Hot/4 Neutrals per conduit per code. Which would be a significant amount of additional conduit.

Thoughts?


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## Pete m. (Nov 19, 2011)

If you are using #12 for 20 amp circuits the magic number of CCC's is 9 and still be able to use 20 amp breakers.

More conduit is one solution but another could be larger wire.

Pete


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## EC2253 (Mar 7, 2008)

were is the exception for control wires?


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## Achilles (Sep 11, 2014)

9 circuits used to mean ABC+N*3 ... now since it's A+N, B+N, C+N, A+N ... am I limited to 4 circuits per code. Inspectors seem to all 6-Hot/6-N.

Just wondering how you interpert code, and/or what you will actually install.

and yes I'm referring to 20A #12 THHN


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## 480sparky (Sep 20, 2007)

Achilles said:


> ...... now since it's A+N, B+N, C+N, A+N ..........


Says who?


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## Pete m. (Nov 19, 2011)

Achilles said:


> 9 circuits used to mean ABC+N*3 ... now since it's A+N, B+N, C+N, A+N ... am I limited to 4 circuits per code. Inspectors seem to all 6-Hot/6-N.
> 
> Just wondering how you interpert code, and/or what you will actually install.
> 
> and yes I'm referring to 20A #12 THHN


Thankfully interpretation is not necessary for this. Table 310.15(B)(3)(a) and table 310.15(B)(16) are all you need.

12 awg cu THHN has a rated ampacity of 30 at 90 degrees. If you have 6 branch circuits (hot + neut) you have 12 CCC's. This would require you to derate to 50% and that would limit your #12 to a 15 amp OCPD.

If your inspectors are allowing 6 hots + 6 neut's and 20 amp OCPD's for #12 I'd love to know how. I'm sure many others would as well.

Pete


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## Achilles (Sep 11, 2014)

Ok - maybe I'm getting off track here,

I was under the impression that ABC can only share a neutral if they all trip at the same time.


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## Achilles (Sep 11, 2014)

The change from 2008 240.20(B)(1) to 2011 240.15(B)(1)


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## Pete m. (Nov 19, 2011)

Achilles said:


> Ok - maybe I'm getting off track here,
> 
> I was under the impression that ABC can only share a neutral if they all trip at the same time.


Essentially that is correct. Look at 210.4(B) and also 240.15(B).

Maybe I got off track... I thought you were asking about deration and number of CCC's.:blink:

Pete


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## don_resqcapt19 (Jul 18, 2010)

Achilles said:


> Ok - maybe I'm getting off track here,
> 
> I was under the impression that ABC can only share a neutral if they all trip at the same time.


They don't have to trip at the same time, they have to be able to be disconnected at the same time. To trip at the same time you need to have a multi-pole breaker with common trip. To disconnect at the same time you can used handle ties and single pole breakers. That may or may not result in more than one breaker tripping for a fault on a single pole breaker. In most cases, even with the handle ties, only the faulted breaker trips.


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## 480sparky (Sep 20, 2007)

Achilles said:


> The change from 2008 240.20(B)(1) to 2011 240.15(B)(1)


That doesn't disallow MWBCs, and makes no mention of neutrals.


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## Achilles (Sep 11, 2014)

My question is regarding how do you approach your installations since the code required MWBC to have a common disconnecting means (multi-pole breaker or ties)

Do you simply run ABC on single pole breakers and tie the breakers together?


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## Achilles (Sep 11, 2014)

don_resqcapt19 said:


> They don't have to trip at the same time, they have to be able to be disconnected at the same time. To trip at the same time you need to have a multi-pole breaker with common trip. To disconnect at the same time you can used handle ties and single pole breakers. That may or may not result in more than one breaker tripping for a fault on a single pole breaker. In most cases, even with the handle ties, only the faulted breaker trips.


Thank you - I misspoke. you clarified things. The integral mechanism of a breaker would trip with ties, even if the other poles did not trip. Much like a fused disconnect where possibly only 1 leg trips, but all 2/3 poles have a common disconnecting means.


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## SdCountySparky (Aug 6, 2014)

Install #10's instead of 12's. Order some breakers ties and boat up your pulls unless the plans spec out dedicated neutrals.


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## Achilles (Sep 11, 2014)

I thought about 10's but being capped at 10#10 3/4" EMT doesn't help. Suppose it's either roll the dice or more conduit.

Most of my kitchen equipment requires GFI Breakers (per spec) so dedicated neutrals.

I was hoping code was lax on count neutrals as CCC


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## Achilles (Sep 11, 2014)

Interestingly I did a cost analysis comparing
50' - 1" EMT 16#10 vs
2*50' - 1/2" EMT 8#12

at $50 per hour +10% Materials the costs were nearly identical
1" EMT 16#10 = $700 (9H + $250)
2*1/2 8#12 = $685 (10.4H + $165)


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## KGN742003 (Apr 23, 2012)

The only time I use 1/2" is if I am extending existing conduit. The cost difference isnt that much and 3/4 is much easier to work with imo.


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## zac (May 11, 2009)

Yeah but you don't need a Bender with 1/2 conduit!


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## zen (Jun 15, 2009)

On a couple jobs in the last year maybe ive seen specs. 3 circuits per conduit, minimum cinduit size 3/4.. others have said they run #10 s out of the panel to a gutter above the ceiling that feeds #12 mwbc. Dont know if its right or not.

no matter what we theorize as electrics. It has always been .


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## Achilles (Sep 11, 2014)

KGN742003 said:


> The only time I use 1/2" is if I am extending existing conduit. The cost difference isnt that much and 3/4 is much easier to work with imo.


True enough - but with 1/2" @ $25, 3/4" @ $40. Once you drop below 10#12 it really pays to use 1/2"

(assuming you lay out the job well enough)

I typically run 3/4" HR and 1/2" or MC everywhere else, but in this case being limited buy GFCI breakers to 8 CCC I should have installed 1/2" now I have a lot of empty space in my 3/4"


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## Pharon (Jan 20, 2014)

Achilles said:


> Thoughts?


Move to Massachusetts. :thumbsup:

http://www.mass.gov/eopss/docs/dfs/osfm/cmr/cmr-secured/527012.pdf


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## Achilles (Sep 11, 2014)

Pharon said:


> Move to Massachusetts. :thumbsup:
> 
> http://www.mass.gov/eopss/docs/dfs/osfm/cmr/cmr-secured/527012.pdf


Nice


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## Carultch (May 14, 2013)

Pharon said:


> Move to Massachusetts. :thumbsup:
> 
> http://www.mass.gov/eopss/docs/dfs/osfm/cmr/cmr-secured/527012.pdf


Why does Massachusetts have a special table for ampacity adjustment factors that is different than the rest of the nation?

The last time I checked, the laws of physics are the same in every state. If 10 wires in a conduit will melt in New York, those same 10 wires in that same conduit/same air temp/same current in Massachusetts will also melt. Similarly, if those 10 wires are safe in Massachusetts, then they will also be safe in New York.

What does the Massachusetts code panel know that the rest of the nation doesn't know?


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## Carultch (May 14, 2013)

Pharon said:


> Move to Massachusetts. :thumbsup:
> 
> http://www.mass.gov/eopss/docs/dfs/osfm/cmr/cmr-secured/527012.pdf


I'd also like to know, what do they mean by "fully loaded conductor diversity" as a quantity?

Does this mean that if all wires are carrying all of their current, all the time, that I probably shouldn't be using the table form the MEC, but instead the NEC?

What would be some examples of some of these diversity percentages? 0%, 25%, 50%, 75%, and 100%.


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