Conductor derate.

[GoldDigger]

The "small conductor rule" specifies a maximum OCPD size for small conductors, not a minimum

But you can also look at it as specifying a minimum size wire for that same OCPD. English is fun when it is not clear what word from a previous sentence is being modified.

 

[kwired]

The "small conductor rule" specifies a maximum OCPD size for small conductors, not a minimum.

If the #12 conductor has an adjusted ampacity of 15, it cannot be protected by a 20A OCPD (unless it falls under one of the specific conductor applications.)

Since under general rules a conductor must be protected at it's ampacity or next standard size up, then if the adjusted ampacity is 15 then the protection must be 15. If the adjusted ampacity were 16 then it could be protected at 20 amps as long as it were not loaded more than 16.

So my heater example is a bad example unless maybe we throw in a low ambient temperature to increase the conductor ampacity to more than 15 amps.

 

[Dugswork]

Derate for more than 3 current - conductors in a raceway or Cable

Derate for more than 3 current - conductors in a raceway or Cable

New to the site and first forum post here, I have a question that seems to fit under this topic so I didn't start a new one. I have the conductors from four 30amp 220v breakers traveling through a nipple less than 24inches to a set of contactors and then back in the same nipple to the large junction box they derive from for a total of 16 THHN conductors. I have a couple of questions.

• Does 310.15{3}{a} "or where single conductors or multiconductor cables are installed without .......longer than 600mm{24in.} mean I do not have to derate the conductors using table 310.15{3}{a} "more than 3 current-carrying conductors in a raceway or cable".
  [*]Also I remember in the past someone saying in a class that in a situation like this there would be no induced voltage because the magnetic fields would negate each other because the same wires & loads are "traveling in both directions".
  

Thanks for your reply.

 

[ActionDave]

New to the site and first forum post here, I have a question that seems to fit under this topic so I didn't start a new one.

Welcome. Go ahead and start a new thread. It is better that way.

I have the conductors from four 30amp 220v breakers traveling through a nipple less than 24inches to a set of contactors and then back in the same nipple to the large junction box they derive from for a total of 16 THHN conductors. I have a couple of questions.

• Does 310.15{3}{a} "or where single conductors or multiconductor cables are installed without .......longer than 600mm{24in.} mean I do not have to derate the conductors using table 310.15{3}{a} "more than 3 current-carrying conductors in a raceway or cable".
  

• Correct.

• Also I remember in the past someone saying in a class that in a situation like this there would be no induced voltage because the magnetic fields would negate each other because the same wires & loads are "traveling in both directions".

• Correct.

 

[GoldDigger]

...Also I remember in the past someone saying in a class that in a situation like this there would be no induced voltage because the magnetic fields would negate each other because the same wires & loads are "traveling in both directions".

The problem that you are likely thinking about is probably not induced voltage but simply the induced magnetic fields in the ferro-magnetic nippple and the box walls on each end.
When you have unbalanced (not summing to zero after allowing for direction) current inside a closed magnetic core (the nipple, etc.), the induced magnetic field will cause hysteresis losses in the metal, causing it to heat up. Fortunately in your case that will not happen.

For what it is worth, if the two travelers of a three-way circuit go through a steel conduit without the neutral being in the same conduit, you will also have unbalanced current. But for switch legs of this type the current is so low (compared to what would cause objectionable heating) that it is not a practical problem.

 

[kwired]

For what it is worth, if the two travelers of a three-way circuit go through a steel conduit without the neutral being in the same conduit, you will also have unbalanced current. But for switch legs of this type the current is so low (compared to what would cause objectionable heating) that it is not a practical problem.

To expand that a little you either need the neutral that is carrying the same current as the travelers or the common that is carrying the same current as the travelers in the same raceway. Either way you have same current flowing in one direction as you do in the opposite direction resulting in a zero net of magnetic effects.

 

[Dugswork]

Welcome. Go ahead and start a new thread. It is better that way.

• Correct.

• Correct.

I started a new thread as per suggestion {it must be waiting for moderator approval} thanks for the responses so far.

 

[Sierrasparky]

Reading this thread I really start to see things I did not know. I always knew about derating conductors in the same conduit because of heating. What I never realized is all the different rules and exceptions. I just stayed away from so many conductors in one conduit. ( other than switched loads from the same circuit)

 

[kwired]

Reading this thread I really start to see things I did not know. I always knew about derating conductors in the same conduit because of heating. What I never realized is all the different rules and exceptions. I just stayed away from so many conductors in one conduit. ( other than switched loads from the same circuit)

Even if you have just switched loads from same circuit, each conductor involved is a current carrying conductor and ampacity adjustment rules apply to it. But if all from same circuit only the "common" and neutral if there is a single neutral are the ones carrying the entire connected load, and possibly are the only ones that may need increased in size. It does depend on what is supplied and based on that what conductor ampacity is required.

General branch circuit rules in 210.19 says "..conductors shall have an ampacity not less than the maximum load to be served". The ampacity after adjustments must still be greater than the load served.

210.19(A)(2) however says "Conductors of branch circuits supplying more than one receptacle for cord-and-plug-connected portable loads shall have an ampacity of not less than the rating of the branch circuit." so in that case if on a 20 amp circuit the conductor must still have an ampacity of 20 amps after adjustment factors even if there is less load intended for this circuit.

 

[cvciii]

derate or ajustment factors, one and the same just a new name

derate or ajustment factors, one and the same just a new name

Two thing come into play, first is the conductor fill ratio of 40% which is never reached and the second and must misunderstood is the derating using the 90deg column of 310.15 which will reduce the allowed amps per conductor. Each conductor had heat placed on it that affects the one next to it and breaks down the insulation. Wire has memory and when it comes off a spool it falls into place which is one of man reason in trench ducts that can be very wide the NEC in 2008 made word changes to each article so it would not be missed as it has over the years. Two things have to happen to increase wires in raceways, one is to have free air space and heat reduction. There is only one patented and tested way this can be done and it is coolWIRE which has been vetted by all bodies from private to government.

 

[kwired]

Two thing come into play, first is the conductor fill ratio of 40% which is never reached and the second and must misunderstood is the derating using the 90deg column of 310.15 which will reduce the allowed amps per conductor. Each conductor had heat placed on it that affects the one next to it and breaks down the insulation. Wire has memory and when it comes off a spool it falls into place which is one of man reason in trench ducts that can be very wide the NEC in 2008 made word changes to each article so it would not be missed as it has over the years. Two things have to happen to increase wires in raceways, one is to have free air space and heat reduction. There is only one patented and tested way this can be done and it is coolWIRE which has been vetted by all bodies from private to government.

Raceway fill (40%) is about pulling conductors without damaging them much more so than it is about the heat those conductors will create within the raceway. I don't know why you claim 40% is never reached, some specifications may keep that from happening but so be it.

What does "wire memory" from being spooled up have to do with ampacity?

Though I agree that having free air space in a raceway probably helps with heat, NEC still sees ampacity of 6 12 AWG in a 1/2 raceway the same as it is in a 4 inch raceway (in general). Heat reduction - I guess depends on exactly what you are referring to. If you have a method of maintaining a specific ambient temperature then you can increase ampacity accordingly but number of current carrying conductors in raceway would still need the usual adjustments. Now a specific product like you mention may have been tested and listed for such use - follow the instructions for said product

Oh, welcome to the forum.