Ampacities of Table 310.15(B)(16) for up to two current-carrying conductors?

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MarineTech

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Location
Camarillo, CA
I find Table 310.15(B)(16), as a general reference, with "Not more than three current-carrying conductors" fascinating.

Given that the majority of single phase cabling has two current carrying conductors. Why not have a two current-carrying conductor table and have the derate start at three current carrying conductors. Does anybody know?

For balanced three phase, "Not more than three" makes sense.

If we had a two current carrying conductor table, how would it compare to the "Not more than three" Table?

Thanks,

MarineTech
 
kwired

kwired

Electron manager
Location
NE Nebraska
MarineTech said:
I find Table 310.15(B)(16), as a general reference, with "Not more than three current-carrying conductors" fascinating.

Given that the majority of single phase cabling has two current carrying conductors. Why not have a two current-carrying conductor table and have the derate start at three current carrying conductors. Does anybody know?

For balanced three phase, "Not more than three" makes sense.

If we had a two current carrying conductor table, how would it compare to the "Not more than three" Table?

Thanks,

MarineTech
Click to expand...

We also have a deration value for 4-6 and 7-9, so why not have a separate table for just 4, or 5 or just 7 or 8?

As far as balanced three phase vs unbalanced three phase - you still produce no more heat than will be produced in any three conductors no matter what the balance is.

Same for unbalanced single phase three wire. If you have 5 amps on line 1, 10 amps on line 2, neutral carries 5 - same heat will be developed as would be if you had 10 amps on each line and none on the neutral, or if you had 1 amp on L1, 10 amps on L2 you would then have 9 on N and still same amount of heat production in the raceway.
 
ceb58

ceb58

Senior Member
Location
Raeford, NC
MarineTech said:
I find Table 310.15(B)(16), as a general reference, with "Not more than three current-carrying conductors" fascinating.

Given that the majority of single phase cabling has two current carrying conductors. Why not have a two current-carrying conductor table and have the derate start at three current carrying conductors. Does anybody know?

For balanced three phase, "Not more than three" makes sense.

If we had a two current carrying conductor table, how would it compare to the "Not more than three" Table?

Thanks,

MarineTech
Click to expand...
Look dude, the derating thing is confusing enough don't screw with it any more. :slaphead:
 
kwired

kwired

Electron manager
Location
NE Nebraska
Something tells me if we had a value for two and for three conductors, that most of the time you would still end up running same size conductors in either case, same if you had 5 instead of 6 conductors.
 
Dennis Alwon

Dennis Alwon

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Location
Chapel Hill, NC
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I don't understand what the OP wants. If we start derating at 3 current carrying conductors then why would we need a different table. Do you think we should have a higher ampacity for 2 wires? T. 310.15(B)(16) gives the ampacity for conductors before any derating. The number of conductors is not the only way to derate.
 
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MarineTech

Member
Location
Camarillo, CA
Dennis Alwon said:
I don't understand what the OP wants. If we start derating at 3 current carrying conductors then why would we need a different table. Do you think we should have a higher ampacity for 2 wires? T. 310.15(B)(16) gives the ampacity for conductors before any derating. The number of conductors is not the only way to derate.
Click to expand...

I was just trying to see, for historical reasons, if anybody knows why we don't have a table for two conductors?

Perhaps I'm wrong in thinking that a two conductor table would allow higher ampacities than the three? There is a lot of two current carrying conductor cable in residential (NM-B, etc.). Why use a table for three?

So, just looking for some background. Not trying to confuse anybody. Not asking for an NEC change. Just looking for a little history on this.

Thanks,

MarineTech
 
Dennis Alwon

Dennis Alwon

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Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
With the use of 90C wire the more than 3 conductor rule comes into play but it really does not affect the ampacity, in many cases, until you have more than 9 conductors. For instance #12 wire....

I also don't think that 2 current carrying conductors can carry more current, it is just that it gets affected when there are more than 3.
 
iwire

iwire

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Staff member
Location
Massachusetts
MarineTech said:
I find Table 310.15(B)(16), as a general reference, with "Not more than three current-carrying conductors" fascinating.

Given that the majority of single phase cabling has two current carrying conductors. Why not have a two current-carrying conductor table and have the derate start at three current carrying conductors. Does anybody know?

For balanced three phase, "Not more than three" makes sense.

If we had a two current carrying conductor table, how would it compare to the "Not more than three" Table?

Thanks,

MarineTech
Click to expand...

I will take a stab at it and say it may have to do with how much of each conductor is exposed to ambient temp vs being against another warm conductor. With three conductors in a tight bundle more than 50% of each conductor is still exposed to the ambient temp. On the other hand once you get to four conductors only about 25% of each conductor can be expected to be exposed to ambient temp with about 75% of each conductor in contact with other warm conductors.

Also as a side note a neutral is not always considered a current carrying conductor.
 
kwired

kwired

Electron manager
Location
NE Nebraska
Looking at it from the perspective of the OP's question and from some of the attempts to explain, two conductors should have higher ampacity than three. And like I said in earlier posts four conductors should have a higher ampacity than five. The differences maybe is not that great and whoever came up with what it in the adjustment table probably decided to come up with ranges that are on the safe side to use in the table instead of making the table more complicated.

The table as is is somewhat simple with three phase circuits.

One circuit - requires no adjustment

Two three wire circuits - 80%

Three three wire circuits - 70%

four or more - 50% or more
 
M

MarineTech

Member
Location
Camarillo, CA
Thanks to everybody, escpecially kwired and Hv&Lv, in responding to this rather academic discussion.

It is interesting, perhaps a two conductor table would show a 20% increase in amperage? Maybe stated another way as a two conductor circuit includes an additional safety factor when using an up to three table? Anyway just a guess.

Best regards,

MarineTech
 
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