Table 310.16 How To Read

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electech77

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Partial quote--"Table 310.16 Allowable ampacities of insulated conductors..., Not More Than Three Current Carrying Conductors in Raceway..."

We need to know the ampacity of #10 stranded THHN...then we will be pulling 6 circuits for general power receps. But, the table says "not more than three current carrying conductors..." what does this mean? I am not understanding the way the table is titled (above quote)...what am I missing?

Electech77
 
Those are the ampacities of conductors when the raceway has only 3 CCC's in an ambient temperature of 86 degree F. For different ambient temps you would use the correction factors at the bottom of the table. For more than 3 CCC's within a raceway you would need to apply the derating factors listed in 310.15(B)(2)(a). What are the specifics of your installation?
 
Ahh, yes. Now it is (slowly) coming back. It has been 10+ years since I have was in the field. As I suspected, it is time for another code class--I seem to have simply forgotten (just about everything).
The installation is in a commercial building (mfg type plant) and we are setting up a "Test Lab" for the research and development of a product. Thus, we need some general power and, do not want to spend a ton of money until we are sure of the prototype.
The question was more about how to understand and apply the code...as mentioned above, I have forgotten--or did not do it enough to remember!

Thank you for the reply...
 
6 outlets for general power for what? if you know what the load is or will be then size the wire accordingly. Is this six individual outlets or six string of outlets? Residential or commercial? How long is the conduit or cable run?
what type of outlet 15a, 20a, 30a?

the more info you supply, the better the answer....:)
 
It is also worth noting that when you calculate the new ampacity, based on having to derate for temperature or number of conductors (presuming one or the other or both apply to your installation), you are allowed to start with the ampacity value in the 90C column (here, I am presuming you are using 90C rated wire). But when you do, you can't use a final answer that is higher than the value shown in the 75C column.

As as example, suppose you have six current carrying conductors of size 10 and of type THHN, all in the same conduit. Suppose further that the ambient temperature is 32C. From Table 310.15(B)(2)(a), we see that we need to apply a derating of 80%. From the bottom of Table 310.16, we see that we need to also apply a temperature derating of 96%. But we get to start with a "base ampacity" of 40 amps. Thus, 40 times 80% times 96% is 30.7. That is above 30. So we can still use #10 conductors, and we can still protect them with a 30 amp breaker.
 
i am not sure if this applies to thes 6 circuits, but thought i would mention MWBCs, in case you need to reduce conduits or have a derating issue because of the number of conductors or the distance. JMSO :)
 
electech77 said:
Partial quote--"Table 310.16 Allowable ampacities of insulated conductors..., Not More Than Three Current Carrying Conductors in Raceway..."

We need to know the ampacity of #10 stranded THHN...then we will be pulling 6 circuits for general power receps. But, the table says "not more than three current carrying conductors..." what does this mean? I am not understanding the way the table is titled (above quote)...what am I missing?

Electech77
Click to expand...

It would be nice to have a quick reference table for 3 phase, 4 wire runs along with the bulk of available quick ref. tables being based on 1 or 2 phase, 3 wire runs.
 
Krim said:
It would be nice to have a quick reference table for 3 phase, 4 wire runs along with the bulk of available quick ref. tables being based on 1 or 2 phase, 3 wire runs.
Click to expand...


I think you mean single phase 120v and single phase 120v multi wire circuits.
Maybe single phase 240v.
Not overly important, just clarifying.

The ungrounded conductors are easy enough to figure as current carrying, but the grounded conductors require an understanding of 310.15(4) to know when their considered current carrying.
 
infinity said:
Those are the ampacities of conductors when the raceway has only 3 CCC's in an ambient temperature of 86 degree F. For different ambient temps you would use the correction factors at the bottom of the table. For more than 3 CCC's within a raceway you would need to apply the derating factors listed in 310.15(B)(2)(a). What are the specifics of your installation?
Click to expand...


Are there any exceptions to 310.15(B)(2)(a) based upon the percent the raceway is filled?

With over 2 conductors you can have up to 40% of a raceway filled.

If, for example, only 10% of the raceway was filled would that alter the derating process?
 
Mike Furlan said:
Are there any exceptions to 310.15(B)(2)(a) based upon the percent the raceway is filled?

With over 2 conductors you can have up to 40% of a raceway filled.

If, for example, only 10% of the raceway was filled would that alter the derating process?
Click to expand...
Table 310.15(B)(2)(a) has no exceptions based on fill except for raceways less than 24". You can have a 3" conduit with 10 CCC #12 wires and you still have to derate 50%
 
If I have a neutral that is a current carrying conductors together with 3 ungrounded phase conductorand an equipment grounding conductor in a conduit, then i will use Table 310.15(B)(2)(a) instead of Table 310.16.
Also use the adjustment factors and corrections factors stated in the FPN..
 
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