Why De-Rated

[Toros]

Hi
Can You explain me
why do we de rate when we have more than 3 or 4 conductors in a reaceway
what is the science behind that???

tanx

 

[GoldDigger]

Just as the current rating of a wire is lower in a raceway than in free air, the heat from multiple conductors will build up in the raceway and so the current limit for a given insulation temperature will be loser.
The difference between free air and raceway is calculated based on up to three wires. (Using three wires as the worst case.)

 

[jumper]

Heat build up.

 

[Carultch]

Hi
Can You explain me
why do we de rate when we have more than 3 or 4 conductors in a reaceway
what is the science behind that???

tanx

For 3 active conductors and less, each conductor has a path for heat to escape, without crossing through another active conductor.

For 4 or more active conductors, one of the conductors has to pass its heat through another.

 

[GoldDigger]

That does not make any sense to me.
With four conductors in a square or diamond pattern each conductor has an unobstructed path to the wall of the raceway.

 

[Carultch]

That does not make any sense to me.
With four conductors in a square or diamond pattern each conductor has an unobstructed path to the wall of the raceway.

I think it is due to the fact that there is a chance that it forms a Y-shape, where the center conductor doesn't have an unobstructed path.

Or it could ultimately be from laboratory testing, where the temperature rise is measured significantly higher after the 4th conductor.

 

[GoldDigger]

I think it is due to the fact that there is a chance that it forms a Y-shape, where the center conductor doesn't have an unobstructed path.

Or it could ultimately be from laboratory testing, where the temperature rise is measured significantly higher after the 4th conductor.

I, on the other hand, think that it is more likely that the basic physical principle involved is that four is greater than three so more total heat is being generated.

 

[Toros]

more about de rating

more about de rating

Hi
Can You explain me
why do we de rate when we have more than 3 or 4 conductors in a reaceway
what is the science behind that???

tanx

if the scaping of heat is the issue, why can not we just increase the size of racway?????

Example: (8) #10 thhn, cu in a a 2" EMT, do we still have to de rate?????

tanx

 

[GoldDigger]

if the scaping of heat is the issue, why can not we just increase the size of racway?????

Example: (8) #10 thhn, cu in a a 2" EMT, do we still have to de rate?????

tanx

My guess is that the NFPA does not want to have too many variables in the calculation, since that would take the determination of ampacity away from a simple combination of tables and percentage adjustments to something closer to the full Neher-McGrath calculation for every situation.

 

[Carultch]

My guess is that the NFPA does not want to have too many variables in the calculation, since that would take the determination of ampacity away from a simple combination of tables and percentage adjustments to something closer to the full Neher-McGrath calculation for every situation.

So if you did a Neher-McGrath calculation (only permissible under engineering supervision, I know), would you find that you can take credit for an oversized raceway when calculating the ampacity of the wires inside? It surprised me that ampacity is not a function of raceway size by the conventional calculation.

 

[iwire]

I think it is due to the fact that there is a chance that it forms a Y-shape, where the center conductor doesn't have an unobstructed path.

I agree.

I, on the other hand, think that it is more likely that the basic physical principle involved is that four is greater than three so more total heat is being generated.

I do not agree.

Two is greater than one, three is greater than two etc.

 

[ActionDave]

For 3 active conductors and less, each conductor has a path for heat to escape, without crossing through another active conductor.

For 4 or more active conductors, one of the conductors has to pass its heat through another.

Interesting. I've never thought about it that way before and it kind of makes sense. The only reason I doubt it's true is because I can't see that much thought going into the process that the NEC used to come up with the tables.

I don't see that they put a whole lot of thought into wire sizing at all. It seems like everything to do with ampacity they said, "That looks like it will work, can't see where there will be a problem. Let's go with it."

 

[GoldDigger]

So if you did a Neher-McGrath calculation (only permissible under engineering supervision, I know), would you find that you can take credit for an oversized raceway when calculating the ampacity of the wires inside? It surprised me that ampacity is not a function of raceway size by the conventional calculation.

Yes.

 

[Ponchik]

if the scaping of heat is the issue, why can not we just increase the size of racway?????

Example: (8) #10 thhn, cu in a a 2" EMT, do we still have to de rate?????

tanx

yes you still derate.

The size of the conduit does not matter because when you install conductors in a race way they will lay on each other and the heat gets trapped between the conductors. So even if you have a 6" conduit for your above example the heat still gets trapped between the conductors.

 

[GoldDigger]

I do not agree.

Two is greater than one, three is greater than two etc.

That is your right.
My deep feeling (which is of course always right, since I am a decider) tells me that they simply figured that one will be worse than free air, two will be still worse, as will three. Rather than have three tables we will just calculate for three and add derating from there.
Do you think that there is some similar physical/geometrical reason for the next divisions at 7, 10, 21 and 31 CCCs?

 

[iwire]

That is your right.
My deep feeling (which is of course always right, since I am a decider) tells me that they simply figured that one will be worse than free air, two will be still worse, as will three. Rather than have three tables we will just calculate for three and add derating from there.

I don't see the logic there. My right.

I do see logic in Carlutch's view.

But it really makes no difference, it is what it is.

 

[ggunn]

But it really makes no difference, it is what it is.

That really is the bottom line. An installation is either code compliant or it's not. It makes no difference whether it makes sense to us or not; we don't get to second guess the code as long as it says what it says. There are procedures in place to give your input for upcoming code cycles; good luck with that.

 

[Carultch]

Do you think that there is some similar physical/geometrical reason for the next divisions at 7, 10, 21 and 31 CCCs?

I don't expect that there is. But if you think there might be, I recommend looking up "circle packing in a circle", and see if you notice these patterns.

These divisions have changed over the years, and the 0.7 derate category used to extend from 7 all the way through 24. In Massachusetts, there is an override table that declined this change, and kept the original table.

 

[don_resqcapt19]

...
These divisions have changed over the years, and the 0.7 derate category used to extend from 7 all the way through 24. In Massachusetts, there is an override table that declined this change, and kept the original table.

Not exactly. The .7 only applied to conductors 10-24 if you had a 50% load diversity. The reason the code was changed is because no one could come up with a definition of "50% load diversity" that was acceptable to the CMP.

 

[Fulthrotl]

My guess is that the NFPA does not want to have too many variables in the calculation, since that would take the determination of ampacity away from a simple combination of tables and percentage adjustments to something closer to the full Neher-McGrath calculation for every situation.

and, truth be told, conductor fill has nothing to do with heat in a conduit...

current flow does.... so, the connected load to the conductors in a conduit
would make a lot more sense, but be harder to implement in the field...