Does anyone know if there was ever an exception or different derating percents in the past when using larger conduits? For example, you have 9 #12s in 1/2" conduit (70% derate) vs having 9 #12s (still have to use 70% derate) in 2" conduit? If it is for heat wouldn't larger conduit be more accommodating of this?
Derating in larger conduit
- Thread starter electricg
- Start date
- Status
- Location
- Massachusetts
There is no exception based on conduit size.
Ten number 14s gets the same derate in 3/4" or 6".
Ten number 14s gets the same derate in 3/4" or 6".
- Location
- Massachusetts
How about changing that.
Sure, I just have to open my NEC editing app, make the change and by morning everyone's copy of the NEC will be updated.
Kidding aside I think you are correct that it could be changed I just think it would be difficult to write the section and enforce it.
- Location
- Illinois
- Occupation
- retired electrician
You have until November 3, 2017 to work up a public input (formerly known as a proposal) for a change to the 2020 code.How about changing that.
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
The fact is no matter how large the conduit is the conductors will still be bundled on the bottom. Take a 5' straight piece of 2" conduit. Install 10 #12 conductors in it and they are all bundled atop each other. IMO, a proposal will not get accepted.
- Location
- Massachusetts
Standard derating does not apply to cable tray, they have there own much different requirements. I think the same could be applied to lighting filled conduits.
However in the big picture I think it would add much confusion to all conduit installations to fix a rarely occurring problem.
However in the big picture I think it would add much confusion to all conduit installations to fix a rarely occurring problem.
Carultch
Senior Member
- Location
- Massachusetts
Standard derating does not apply to cable tray, they have there own much different requirements. I think the same could be applied to lighting filled conduits.
However in the big picture I think it would add much confusion to all conduit installations to fix a rarely occurring problem.
What about wire troughs and wire trays?
There is a rule that allows you to neglect derates up to 30 wires. Once you put the 31'st wire in it, does that all of a sudden get derated to 40%?
Yes for wireways, but only in the section where the number of conductors exceed 30, not necessarily the entire wireway.
Wire tray falls under cable tray requirements, which differ from 310.15(B)(3)(a) derating.
Carultch
Senior Member
- Location
- Massachusetts
I think I meant to say, "wire troughs and wireways".
Essentially what I gather is that the 31st wire in an enclosee wiring structure, and all the others with it are suddenly derated from 100% at 30 down to 40% at 31. Except in Massachusetts.
So what is it in Massachusetts?
- Location
- Massachusetts
No that is incorrect. It is the 31 st wire in one cross section that triggers the requirement.
Carultch
Senior Member
- Location
- Massachusetts
Massachusetts has amendments to the NEC, one of which includes an override table for the bundling derate factors.
http://www.mass.gov/eopss/docs/dfs/osfm/cmr/527cmr12-00.pdf
1-3 = still the same
4-6 = still the same
7-24 = 70%
25-42 = 60%
43 and more = 50%
Hell if I know what the heck they did to justify this table. The last time I checked, the laws of electricity and heat transfer work exactly the same everywhere in the entire world.
I wonder if ampacities would need a major adjustment for use in an airless environment, and evacuated conduit. Because the wire looses its convection for dissipating heat.
...in the section(s) where the condition exists, not in the remainder.
Weather???
Carultch
Senior Member
- Location
- Massachusetts
Specifically, sections exceeding 2 ft in length, and not meeting the 10%/10ft "stub rule" (310-15(a)(2), exception).
Yes, that requirement would kick in, and the exception could apply.
- Location
- Bremerton, Washington
- Occupation
- Master Electrician
How about changing that.
You certainly could make a proposal, as its your idea. You have about 2 1/2 years to get it in. Often a proposal will tell you the intent of the code panel. We have a topic for code changes, do your research first. if you make the proposal with out technical substantiation, it will be rejected.
- Location
- Massachusetts
Better question would be how did the NEC justify changing the table?
Massachessetes still uses the original NEC derating table, when the NEC made the derating more restrictive MA said no, we are not doing that.
You can still find the table MA uses in the annex B of the NEC.
Take a look at Table B.31O.15(B)(2)(11) Adjustment Factors for More Than Three Current-Carrying Conductors in a Raceway or Cable with Load Diversity
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
Remember ampacity adjustment is to protect conductor insulation not the conductor itself. A 12 AWG copper conductor can carry 100 amps of current just fine - but will see an increase in temperature that will damage common thermoplastic insulation types.
Put the conductor in a vacuum, you still possibly have thermal melt down effects on that insulation, and maybe at a different temperature then when not in a vacuum but you will not have any oxidation related break downs.
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