residential circuiting

[russellroberts]

Re: residential circuiting

At first glance,it seemed far-fetched,but after doing the math,Tony is correct.

Now,a lot of homes in my area have blown-in insulation in the attic,covering the wire with 6 to 12" of insulation,and sometimes the way batt insulation spreads out,the cable winds up not exactly under the batt,but covered by it so to speak,would this be a migitating factor?

 

[tonyi]

Re: residential circuiting

Originally posted by roger:
The average residential circuit would never be loaded enough that attic temps would need to be considered in calculating an adjustment into the conductor.

The problem here is in thinking in terms of averages. while this may be fine for sizing services and such, I'd be willing to bet that in most places wired to bare minimums there's a branch or two that frequently come close to breaker trip with everything else doing very little. The "average" in this situation is of course still low.

The stock configuration of places in my mom's development seems to be having bedrooms and living room on the same branches. In this particular instance, 3BR's (recepticals and lights), smokes, large living room, kitchen lights, both bathroom lights, a hall light, and an outside light, and range hood were all on two 14-3's (not anymore, I split things up a lot better) The BR branches serve all the living room recepticals. All it takes is certain lights on, an entertaiment center a couple of computers in the right recepticals and one side of one of the 14-3's will be a guaranteed overload trip at some point. In any case, that wire will be running quite hot before it trips if you work up to it a bit at a time.

People in the neighborhood running the "stock" setup have learned to train themselves by experience as to what they can run at the same time to avoid tripping breakers.

IMO, the code minimums are about 30-40 years out of date.

 

[tonyi]

Re: residential circuiting

Originally posted by russellroberts:
At first glance,it seemed far-fetched,but after doing the math,Tony is correct.

Now,a lot of homes in my area have blown-in insulation in the attic,covering the wire with 6 to 12" of insulation,and sometimes the way batt insulation spreads out,the cable winds up not exactly under the batt,but covered by it so to speak,would this be a migitating factor?

This is an interesting question Russ, and one not well addressed by the code. My suspicion is the answer would relate to the load on the branch and the insulation's ability to dissipate generated heat (something insulation isn't good at Mom asked me the next day what I'd done - she felt the A/C was functioning better. Less VD to the compressor, it probably was working more efficiently!

 

[russellroberts]

Re: residential circuiting

Tony, I suspect you being in S.Florida would be more concerned with hot attic temps than some other places.

We don't run too many wires in attics,being as the panels are in the basement usually with floortruss construction,if not basements,crawlspaces are the norm,and we pull a lot of our feeds underneath,as well as loops between receps,switches etc.

If it's a heavily loaded ckt stapled mid-space in the attic the temps could be a concern,but would you go with the worst it could be for a couple months ayear(here)or would you take the "average temp" with a general purpose ckt as Roger suggests?

Russell

 

[tonyi]

Re: residential circuiting

Originally posted by russellroberts:
Tony, I suspect you being in S.Florida would be more concerned with hot attic temps than some other places.

We don't run too many wires in attics,being as the panels are in the basement usually with floortruss construction,if not basements,crawlspaces are the norm,and we pull a lot of our feeds underneath,as well as loops between receps,switches etc.

If it's a heavily loaded ckt stapled mid-space in the attic the temps could be a concern,but would you go with the worst it could be for a couple months ayear(here)or would you take the "average temp" with a general purpose ckt as Roger suggests?

Russell

I know what you're saying, I've worked on stuff mostly up north too. Lotta switch loops and just lights in attics - low fixed load type stuff. Even so, IMO a #14 is a marginal case once it makes it way up to the attic and wherever its going in most cases strictly based on voltage drop considerations unless its target area is fairly close to the panel. I measured over 10% drop (@ 12A load) on one branch in mom's place at the far points of one #14 run that probably came in around 80'. Upping the bulk of that run to #10 (I kept the #14 in the walls in place, tapped on to the #10, and kept the 15A breaker) got that down to just under 3%. #12 probably would have been acceptable too...but I had this coil of 10-2 MC on hand, so I used it up

 

[brejay]

Re: residential circuiting

Here is an approach for those of you in a climate so warm. We don't have those extremes in Minnesota.

#12 THHN 2 90* C = 30 AMPS
4 CONDUCTORS @ 80% = 24 AMPS
130* F AT 76% = 18.24 AMPS
240.4 Allows us to round up to the nearest standard breaker or 20 amps.

 

[tonyi]

Re: residential circuiting

Originally posted by brejay:
Here is an approach for those of you in a climate so warm. We don't have those extremes in Minnesota.

#12 THHN 2 90* C = 30 AMPS
4 CONDUCTORS @ 80% = 24 AMPS
130* F AT 76% = 18.24 AMPS
240.4 Allows us to round up to the nearest standard breaker or 20 amps.

240.4(B)(1) can be a killer for a lot of residential type stuff though just due to the multiple outlets, portable loads provision. About the only thing I can think of where it might be useful is a dedicated fridge branch. For general purpose 20A branches with recepticals/lights (like bedrooms), the cord/plug portable loads language makes it a no-go.

 

[brejay]

Re: residential circuiting

Good point on the multi outlet provision. so put it on a 15 amp breaker.

 

[harley]

Re: residential circuiting

I don't think you can say THHN #12 is rated for 30 amps please ref. Table 210.24 Summary of Branch-Circuit Requirements 20 amp for #12 is the starting factor.

 

[iwire]

Re: residential circuiting

Originally posted by harley:
I don't think you can say THHN #12 is rated for 30 amps please ref. Table 210.24 Summary of Branch-Circuit Requirements 20 amp for #12 is the starting factor.

Harley, luckily for us that is exactly what we can do.

For derating you can start with the 90C column for THHN and do the derating for more than three conductors and if needed the derating for higher temperatures.

If when you are done with the derating and you come out with say 22 amps you still have to use a 20 amp breaker as indicated by 240.4(D).

We must remember that the figures in 310.16 are what the conductors are rated for. 240.4(D) modifies the overcurrent protection not the ampacity of the wire.

Also there are loads (like motors) that you are allowed to use 12 awg on breakers larger than 20 amps, take a look at 240.4(G)

 

[izak]

Re: residential circuiting

tonyi
Member
Member # 10066

posted October 02, 2003 01:15 PM
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quote:
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Originally posted by brejay:
...try 14-2-2 or 12-2-2. It is meant to address the issue of two circuits one pull for AFCI's and GFCI'c.Jay
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Cutler Hammer (and recently GE) make two pole AFCI breakers. Last I looked, the Cutlers were a bit pricy, but I bought some GE's for $70/ea at GE Supply in Miami. Still a bit more than two singles, but you can use a 14-3 or 12-3 and not have to contend with an up front 80% derating if its going through a hot attic. Southwire says the 14-2-2/12-2-2 stuff is supposed to be derated as if its run in pipe.



What Do 2 pole AFCI breakers have to do with not having to de-rate conductors in a cable?


Its a Cable
it has Four current carrying conductors
there's a de-rating factor for more than three in a raceway or cable
PLUS
in an ambient temp of over 86F ou have to De Rate AGAIN...

what does this have to do with 2 pole breakers??

 

[tonyi]

Re: residential circuiting

Originally posted by izak:
what does this have to do with 2 pole breakers??

14-2-2 in higher ambient temps may require a combined derating that makes it unusable.

12-2-2 in higher ambient temps may require a combined derating that makes suitable only for 15A.

Both of these situations might be avoided by using a 12/14-3 and a 2-pole AFCI since the 80% derating of the 12/14-2-2 doesn't happen with the 12/14-3.

 

[jpd24]

Re: residential circuiting

What seems to be the norm on AFCIs. I'm thinking receptacles on one circuit and lighting on another.This is a small 2 BR 1500 sq. ft. house.Do ceiling fans bother them? First time working with AFCIs.

 

[tonyi]

Re: residential circuiting

Ceiling fans are OK. Be aware that there are some genuinely defective units out there though. SQ-D was getting some "false trip" reports and the units turned out to be bum. This was some of the cheapest of the cheap Chinese stuff, not Hunters or something like that.

My own preference, not required by code, is to put BR lights/smokes on one branch and keep the recepticals on others. That way an ordinary overload trip from plugged in junk doesn't put you in the dark.