310.15(A)(2) - Continuous Load factor and Wire Fill Derating - mutually exclusive?

[nateholt]

Does section 310.15(A)(2) relate to the following statement? If not, is the following statement still accurate?

"If the correction factor for conduit fill is more restrictive, producing a lower corrected ampacity than would result from a continuous load correction, then the correction for fill is made and the correction for continuous load is not made."

Thanks!
Nate Holt.

 

[infinity]

It depends on how many CCC's are in the raceway.

 

[nateholt]

CCC = ?

 

[Johnmcca]

Current Carrying Conductors

 

[Smart $]

Does section 310.15(A)(2) relate to the following statement? If not, is the following statement still accurate?

"If the correction factor for conduit fill is more restrictive, producing a lower corrected ampacity than would result from a continuous load correction, then the correction for fill is made and the correction for continuous load is not made."

Thanks!
Nate Holt.

Where did you get that statement?

Sounds like somebody's "in other words" statement. Regardless, it is technically in error. Setting a conductor type's size using non-continuous plus 125% continuous is always implemented. Adjusting for fill can compel using a larger conductor, never smaller.

 

[nateholt]

The context is this... let's say the 125% for continuous load is essentially equivalent to reducing conductor ampacity by factor or 0.8. And we have 9 CCCs in the raceway for fill derating of 0.7.

The way the comment was explained to me is that this condition meets the exception outlined in 310.15(A)(2). The ampacity would be calculated based upon just the smaller of the continuous load factor (0.8) or the fill factor (0.7). It would not be calculated based upon the product of these two factors (i.e. 0.56). In this example, it would be simply calculated based upon the 0.7 factor.

My question is this... is the above an accepted / accurate interpretation what is laid out in the NEC?

 

[Smart $]

The context is this... let's say the 125% for continuous load is essentially equivalent to reducing conductor ampacity by factor or 0.8. And we have 9 CCCs in the raceway for fill derating of 0.7.

The way the comment was explained to me is that this condition meets the exception outlined in 310.15(A)(2). The ampacity would be calculated based upon just the smaller of the continuous load factor (0.8) or the fill factor (0.7). It would not be calculated based upon the product of these two factors (i.e. 0.56). In this example, it would be simply calculated based upon the 0.7 factor.

My question is this... is the above an accepted / accurate interpretation what is laid out in the NEC?

No, it is not, IMO. The non-continuous plus 125% continuous stipulation of 210.19(A)(1) or 215.2(A)(1) does not reduce the allowable ampacity of the conductor. Therefore it plays no role in the application of 310.15(A)(2).

I believe you have a misconception of 310.15(A)(2)'s intent. What it means is if you have a different ampacity for two or more sections of a circuit, the lowest ampacity shall be used. Say you have 9 ccc's in a conduit leaving a panelboard. Twenty feet away there is a junction box after which 3 ccc's each go their separate ways in three conduits. Each set of 3 ccc's will have a higher ampacity than when all were in the same conduit. Because the ampacity when they were all in the same conduit is lower, that is the ampacity of the circuit conductors.

 

[guschash]

I think Smart$ hit it right on the head.

 

[don_resqcapt19]

The 125% for continuous loads is not a derating factor. It is the required minimum ampacity of the conductor. You must have a conductor with at least that minimum ampacity after you apply the adjustment and or correction factors for the conductor.

 

[nateholt]

No, it is not, IMO. The non-continuous plus 125% continuous stipulation of 210.19(A)(1) or 215.2(A)(1) does not reduce the allowable ampacity of the conductor. Therefore it plays no role in the application of 310.15(A)(2).

I believe you have a misconception of 310.15(A)(2)'s intent. What it means is if you have a different ampacity for two or more sections of a circuit, the lowest ampacity shall be used. Say you have 9 ccc's in a conduit leaving a panelboard. Twenty feet away there is a junction box after which 3 ccc's each go their separate ways in three conduits. Each set of 3 ccc's will have a higher ampacity than when all were in the same conduit. Because the ampacity when they were all in the same conduit is lower, that is the ampacity of the circuit conductors.

Thank you. Your example seems to fit with the wording in the NEC "... where more than one calculated or tabulated ampacity could apply for a given circuit length, the lowest value shall be used". And the exception that is listed immediately following talking about allowing the higher ampacity to kick back in 10 feet beyond the "transitiion" point also makes sense in light of your example.

 

[Smart $]

The 125% for continuous loads is not a derating factor. It is the required minimum ampacity of the conductor. You must have a conductor with at least that minimum ampacity after you apply the adjustment and or correction factors for the conductor.

I don't believe your latter statement is correct.

The ampacity after applying adjustment and correction factors must equal or exceed the calculated load current, i.e. no extra 25%. But it cannot be smaller than the preadjusted, precorrected size determined with the extra 25%. Refer to 210.19(A) and 215.2(A). Note Table 210.2 lists Specific-Purpose Branch Circuits which may be or are subject to different requirements.

 

[Smart $]

Example: Continuous load of 36A. Conductor type used is THHN Cu. 125% of 36A is 40A. We can use #10THHN Cu or larger. The lowest ampacity of this circuit is when run in a conduit having 4-6 conductors and derated to 80%. No correction for ambient. 40A times 80% is 36A. Our calculated load is 36A so a #10THHN Cu is allowed. If there were a correction for ambient over 30?C, we would have to use a #8 (or larger).

 

[don_resqcapt19]

Smart,
You are correct about the ampacity of the conductor, but we also have to look at the overcurrent protection of the conductor.

 

[Dennis Alwon]

I agree with Don. In that example #10 may only be allowed to have a 30 am ocpd based on 240.4(D)(7)

 

[augie47]

So as not to lead anyone astray and have them wire a 40 amp circuit with #10, would 110.14 not preclude #10 unless we happend to have a 90? terminations ?

 

[Smart $]

Smart,
You are correct about the ampacity of the conductor, but we also have to look at the overcurrent protection of the conductor.

Also correct. I was only discussing the process pertinent to determination of conductor ampacity rating. Using a load current in the range of #10 as an example was a poor choice on my part.

 

[Smart $]

I agree with Don. In that example #10 may only be allowed to have a 30 am ocpd based on 240.4(D)(7)

Correct. Not the topic of discussion, but that'd be an additional requirement that affects the circuit.

 

[Smart $]

So as not to lead anyone astray and have them wire a 40 amp circuit with #10, would 110.14 not preclude #10 unless we happend to have a 90? terminations ?

Correct. Not the topic of discussion, but that'd be an additional requirement that affects the circuit.

 

[acrwc10]

So as not to lead anyone astray and have them wire a 40 amp circuit with #10, would 110.14 not preclude #10 unless we happend to have a 90? terminations ?

Even with 90degree terminations, wouldn't we still be limited to 30amp OCP on a #10?