Source: http://ecmweb.com/nec/sizing-circuit-protection-and-conductors-part-1?page=2
Timestamp: 2013-05-23 02:19:41
Document Index: 799831302

Matched Legal Cases: ['art 1', 'art 1', 'Art. 310', 'Art. 310', 'art 1', 'art 7']

Sizing Circuit Protection and Conductors — Part 1 | NEC content from Electrical Construction & Maintenance (EC&M) Magazine
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Article 310 provides several ampacity tables: Table 310.15(B)(16) through Table 310.15(B)(21). On top of this, Annex B also provides several ampacity tables: Table 310.15(B)(2)(1) through Table 310.15(B)(2)(10). Those Annex B tables provide more options for looking up your ampacity, but they aren’t required for that purpose. Always start with the Art. 310 tables, and carefully read the headings. If you don’t find an exact match, you might find it in Annex B. If you don’t find an exact match in either place, then pick the closest match in Art. 310. You will need to modify it per 310.15(B)(1) through 310.15(B)(7). For most applications, the only modifications you’ll need to make will be the adjustment factors detailed in 310.15(B)(3) and the ambient temperature correction factors [310.15(B)(2)]. This ambient temperature correction requirement exists because you need a larger ampacity to allow for the higher temperature. Choose either the 86°F table [Table 310.5(b)(2)(a)] or the 104°F table [Table 310.5(b)(2)(b)] to find your temperature correction factor. Then, you can either multiply the table ampacity by the correction factor or divide your calculated amps by the correction factor prior to looking up the ampacity. When you look at the ampacity table, you now have a choice of conductor temperature rating columns. Use the column that matches the rating of your lowest rated termination, device, or conductor [110.14(C)]. Terminations are normally rated for 60°C; thus, you normally use the 60°C column, regardless of the conductor’s temperature rating (unless you have special terminations rated for the higher temperature). You might put a portion of a circuit in a higher temperature environment, and thus have occasion to use the higher temperature column for that portion if not including terminations in that space. Example D3(a) in Annex D provides an excellent illustration of such a situation. « Sizing Circuit Protection and Conductors — Part 1 Prev12 Print
Anonymous (not verified) on Feb 16, 2013 I question your calculation of the ambient temp correction.
I belive you should use Table 310.15B2a based on 30 deg C coordinated with 30 deg C in Table 310.15B(16).
THHN is rated at 90 deg C, so the 90 deg C column can be used for ambient correction. At 125 deg F, the factor is 0.76 (90 C column table 310.15B2a).
25 A (90 C column table 310.15B(16)) x 0.76 = 19A.
Final result is 14AWG, but with the correct method of ambient temp correction calculation.
Mark Lamendola (not verified) on Feb 16, 2013 The NEC is a bit confusing on the issue of temperature correction. I personally can't decipher the answer just from reading 310.15. But the NEC does provide a way to find the answer.
First, here is why I used the 60 DegrC column. We're using the 60DegrF column of the table, not the 90DegrF column. So it seems logical you would also use the 60DegrF temperature adjustment factors. Even though THHN is rated at 90 DegrC, for purposes of ampacity for this circuit it's rated at 60 DegrC because its terminations are rated at 60 DegrC [110.14(C)].
That said, I see why you might use the 90 DegrC column; we are derating the conductor and not the terminations. Or are we? That approach assumes the terminations are not also in that elevated temperature space. Example D3(a) in Annex D presents such a scenario.
If you look at Example D3(a), you will find clarification on this question. The question being do we use the conductor's maximum temperature rating or do we use the temperature rating of the circuit when selecting the temperature correction factor column?
The answer is we use the temperature rating of the circuit. One place you can see this illustrated in Example D3(a) is under the heading "Ungrounded Feeder Conductors" and please note what it says in the surrounding explanations.
Also, read 310.15(A)(2) and 310.15(A)(3).
Jacobo (not verified) on Mar 25, 2013 I'd like to draw your attention to the fact that the fundament for load calculations is based on Article 220. In the case of branch circuits please refer to 220.10 and in the particular case of general-use lighting circuits, please refer to Table 220.12
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