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Home > Home Wiring USA Archive: NEC 1999 > Definitions and Calculations > Calculations of Conductors' Ampacity (NEC 1999)

Calculations of Conductors' Ampacity (NEC 1999)

By Warren Goodrich
Calculations of Conductors' Ampacity (NEC 1999)

Conductor Ampacity = The total number of amps that the NEC allows a conductor to carry and be considered to meet the minimum safety standards after de-ration calculations are performed as required.

Ambient Temperature = The normal temperature range that surrounds an electrical conductor in which that conductor is to be used.

Current Carrying Conductors = Any conductor found in a conduit that is not a true neutral and is not a bare or green equipment grounding conductor.

Example: Attic in Florida, or Attic in Canada.

Conductor Ampacity Correction Factors = Load allowed by the NEC chart 310-16 multiplied by the ambient temperature correction factor found in the NEC chart 310-16 found at the bottom of the page multiplied by the percentage of ampacity you are ordered to reduce, due to the number of current carrying conductors installed in the same conduit or bundled cables that is more than 24” long and the total conductors installed is more than three current carrying conductors.

“Special Note” Any conduit shorter than 24” requires no de-rating calculation in considering number of conductors installed in a raceway. Article 310-15-B-2-A Exception 3

“Special Note” When counting current carrying conductors you do not need to count equipment grounding conductors in your de-ration of ampacity due to the number of conductors in a raceway at all, they are not current carrying conductors. Article 310-15-B-5

“Special Note” When counting current carrying conductors you do not need to count in your de-ration of the ampacity of a conductor, any white neutral conductors, that are carrying only the unbalanced load of two hot conductors of the same two pole circuit and installed in the same raceway. { example would be a { multi-wire circuit } that is two circuits using the same common white wire to a receptacle box when that receptacle is 240 volt rated like for an air conditioner, dryer, range, etc. Article 310-15-B-4-A

You must count the white grounded leg of a single hot conductor of a one pole circuit {this white conductor would not be a true neutral conductor} for your conduit fill calculations {example white conductor of a 120 volt receptacle} this is true only if the grounded leg is not used as a neutral designed to carry the unbalanced load while serving between two hot conductors of a 240 two pole circuit {this conductor would be a true neutral conductor}. 310-15-B-2 and 310-15-B-4-A

“Special Note” On 4 wire 3 phase wye-connected systems the neutral must be counted as a current carrying conductor. 310-15-B-4-B&C

Conductor Ampacity = ampacity x correction factor for ambient temperature x correction factor for number of conductors in a conduit over 4 conductors = adjusted Conductor ampacity recognized as the ampacity of that conductor by the NEC.

Example;

[Written Form]

Conductor ampacity of an Awg #8 TW installed in a Florida attic, with an approximate average ambient temperature of 130 degrees F, with 26 other current carrying conductors in a conduit, with no diversity =

Table 310-16 @ 60 Degree Column is 40 amps X Table 310-16 correction factor [at bottom of the page] for ambient temperature is 41 % X reduction Chart 310-15-B-2-A is 45 % = 7.38 amps = total reduction down to 7 amps for the total ampacity of the above # 8 TW conductor as described.

310-16-ambient temp de-ration 310-16-Partial de-ration-# conductors in raceway

40amps X 41% = 16.4 amps X 45% = 7.38 amps

True ampacity of conductor

Same Example;

[Numeric Form]

# 8 TW ampacity = 40 amps X 41 % {ambient temp} = 16.4 amps

16.4 amps X 45 % {26 conductors} = 7.38 amps capacity of a

#8 TW conductor after the correction factors are reduced.

Example Results;

Therefore that # 8 TW has a total ampacity of 7.38 amps found in a Florida attic with ambient temperature of 130 degrees F., and found with 26 other current carrying conductors, in the same conduit.

Remember that only 15 amp or larger rated conductors after de-ration calculation are allowed to serve 120 volts or more in any type of structure. Article 210-24 Also remember that 20 amp rated conductors after de-ration calculation are required serving any receptacle installed in a bath room, kitchen, dining, nook, pantry, or laundry. Article 210-11 Motors conductors must also consider ampacity de-ration calculations. Articles 310-15-B-2-A and 310-16 and 430-22 and 430-24 Actually to my knowledge the table for residential service entrance type conductors as per table 310-15-B-6 is the only ampacity that does not require ampacity de-ration calculations.

“Special Note” If you are not required to calculate for de-rations as required for temperature or number of conductors in a raceway and if you are using a higher temperature rated conductor such as THHN which is rated 90 degrees centigrade Table 310-16 or THW which is rated at 75 degrees centigrade Table 310-16, and the conductor is smaller than # 1, you must use the 60 degree column as the ampacity rating Article 110-14-C-1 or if the conductor is # 1 or larger then you must use the 75 degree column as the ampacity rating Article 110-14-C-2.

If you must calculate a de-ration for temperature or number of conductors in a raceway, you may start your calculation with the 90 degree rating if the conductor is originally rated for 90 degrees or 75 degree if the conductor is originally rated for 75 degrees temperature rating of your conductor and finish your de-rating calculation for the ampacity rating. Articles 110-14-C

You must then compare that de-rated answer that you calculated with the 60 degree column if the conductor is smaller than #1 or 75 degree column if the conductor is # 1 or larger. Article 110-14-C-1&2

You must use the worst scenario in the ampacity rating of that conductor after comparing the de-rated ampacity you found after your calculation to the ampacity found in the column that 110-14 required, if smaller than #1 = 60 degree column or #1 or larger = 75 degree column. Article 110-14-C-1&2

This requirement is because the terminal ends that are listed and approved on the market are only rated at 60 degrees centigrade if smaller than #1 or 75 degree column if larger than #1 conductors as per Article 110-14-C-1&2.

2nd Example;

[Written Form]

The conductor ampacity of an Awg.. # 8 TW installed in a Canadian attic with approximate average temperature [ambient temp] of 70 degrees F. with 26 other current carrying conductors in a conduit with no diversity = Table 310-16 @ 60 Degree Column is 40 amps X Table 310-16 correction factor [at bottom of the page] for ambient temperature is 108 % X Reduction Chart 310-15-B-2-A is 45 % = 19.44 amps = Reduction to 19 amps for the total ampacity of the above # 8 TW conductor. 40amps X 108% X 45% = 19.44 amps

2nd Same Example;

[Numeric Form]

# 8 TW ampacity = 40 amps X 108 % = 43.2 amps

43.2 amps X 45 % = 19.44 amps capacity of a #8 TW conductor after the correction factors are reduced.

2nd Example Results;

Therefore that # 8 TW has a total ampacity of 19.44 amps found in a Canadian attic, and found with 26 other current carrying conductors, in the same conduit.

This document is based on the 1999 national electrical code and is designed to give you an option, as a self-help, that should pass minimum code requirements. While extreme care has been implemented in the preparation of this self-help document, the author and/or providers of this document assumes no responsibility for errors or omissions, nor is any liability assumed from the use of the information, contained in this document, by the author and / or provider.

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