Multiple Conductors

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Dale Hayes

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Question: NEC Tables 310.18 and 310.19 (250C Conductors).

Table 310.19 lists the ampacity for a #2Ga. (250C) wire at 381 Amps. This amperage is based upon a single insulated conductor in free air.

Table 310.18 lists the ampacity (for not more than three current carrying conductors in a raceway) of #2Ga. (250C) wire at 191 Amps.

If I do a correction factor for having (24) #2Ga. wires in a single conduit, do I calculate the new amperage from Table 310.19 (381 Amps) or from Table 310.18 (191 Amps)?

The correction factor for having 24 conductors in a conduit is a .45 multiplier.

"If" I am correct to use the .45 factor calculation with Table 310.19, then my allowable ampacity is 172 Amps.

"If" I am correct to use the .45 factor calculation with Table 310.18, then my allowable ampacity is 86 Amps.

Dale
 
Correction

Correction

I forgot to mention that this calculation is based upon the 40C (104F) ambient as given in Tables 310.18 and 310.19.
 
You are not installing the conductors "in free air," so 310.19 would not apply. Sorry, but I think you are limited to 86 amps per conductor.
 
Dale Hayes said:
If I do a correction factor for having (24) #2Ga. wires in a single conduit, do I calculate the new amperage from Table 310.19 (381 Amps) or from Table 310.18 (191 Amps)?

Dale
Click to expand...

If the wires are in conduit you must use table 310.18, if they are in free air then you would use T310.19.

I am sorry if there is something I am overlooking.
 
"Question: NEC Tables 310.18 and 310.19 (250C Conductors).

Table 310.19 lists the ampacity for a #2Ga. (250C) wire at 381 Amps. This amperage is based upon a single insulated conductor in free air.

Table 310.18 lists the ampacity (for not more than three current carrying conductors in a raceway) of #2Ga. (250C) wire at 191 Amps.

If I do a correction factor for having (24) #2Ga. wires in a single conduit, do I calculate the new amperage from Table 310.19 (381 Amps) or from Table 310.18 (191 Amps)?

The correction factor for having 24 conductors in a conduit is a .45 multiplier.

"If" I am correct to use the .45 factor calculation with Table 310.19, then my allowable ampacity is 172 Amps.

"If" I am correct to use the .45 factor calculation with Table 310.18, then my allowable ampacity is 86 Amps.

Dale"


single insulated conductor in free air
24 conductors in a conduit

Those 2 statements are not compatible.
T310.15(B)(2)(a) isn't usable with a single conductor in free air table.

David
 
Free Air and Conduit

Free Air and Conduit

Thanks for the quick responses!

I guess it boils down to this " Air is Air" and "Conduit is "Conduit"!!!

Dale
 
Dale do you mind if I ask what the installation was?

In general electrical equipment below 600 volts will have a maximum 75 C rating regardless of what conductor you run.
 
Multiple Conductors

Bob,

Fortunately, the conductors were never installed. When the derating calculation for this particular wire was made, the calculation (for more than 3 conductors in a raceway or cable), was made based on the amperage of a single wire in free air (Table 310.19) rather than (Table 310.18).

Dale
 
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