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Calculations of Conductors' Ampacity (NEC 2002)

By Warren Goodrich
Calculations of Conductors' Ampacity (NEC 2002)

Conductor Ampacity

The total amp load 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.

NOTE: The breaker must be sized no larger in amp size than the ampacity rating of a conductor after de-ration calculations have been performed to find the maximum ampacity of that conductor.

Ambient Temperature

The normal temperature range that surrounds the area of 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 as long as that conductor is not a bare or green equipment grounding conductor.

NOTE: A white wire serving a single 120 volt circuit is not a neutral but instead of a neutral this conductor is a grounded leg only and is considered as a current carrying conductor that must be sized at the same ampacity rating as the ungrounded [hot conductor] that grounded leg serves as a return path back to the circuits source of origin.

NOTE: A white wire that is a neutral must be a conductor that serves two ungrounded conductors [hot conductors] that has a source from different phases. The voltage of those two ungrounded conductors being served by a neutral conductor must be at least 200 volts or more on a multi-wire feeder or branch circuit made up of two ungrounded [hot] conductors from different phases. These two ungrounded conductors must measure voltage between the two ungrounded conductors and use a double or triple pole breaker or set of fuses. No single 120 volt circuit is served by a neutral conductor.

Conductor Ampacity Correction Factors

This is the amp load maximum allowed by the NEC Chart 310.16 multiplied by the ambient temperature correction factor found in the NEC Chart 310.16found at the bottom of that chart’s page. The NEC requires you to reduce the ampacity of any conductor due to ambient temperature listed in the bottom of that chart. Then you must perform a second ampacity de-ration per NEC Article 310.15.B.2.A.

The conductor ampacity adjusted from the ambient temperature de-ration calculation results must then be multiplied by the percentage of ampacity de-ration that you are required by the NEC to reduce according to NEC Article 310.15.B.2.A. This de-ration calculation is due to the number of current carrying conductors installed in the same conduit or any bundled cables that is more than 24”long. This de-ration due to number of current conductors in a raceway only applies if you have a total number of more than three current carrying conductors in the conduit or raceway. This Article 310.15.B.2.A also applies to any bundled together nonmetallic sheathed cables and as well as applying to any current carrying conductors installed within a raceway or conduit.

This NEC Article 310.15.B.2.Adoes not apply to short sections bundled less than 24”or to conductors installed within a nipple that is less than 24”long.

Residential Service Entrance and Feeder Conductor Ampacity Ratings
Also this NEC Article 310.15.B.2.A and / or NEC Table 310.16 correction factor [ at bottom of the page ] does not apply to the residential service entrance and feeder conductor ampacity ratings allowed in NEC Table 310.15.B.6This service entrance and feeder conductor chart is exempt from both the ambient temperature and the number of conductors in a raceway requirement. NEC Table 310.15.B.6 naturally limits the number of conductors to no more than three when the conductors are a single phase residential service. There is also a rule pertaining to this chart that states that any feeder being served by a service entrance conductor supplying it allows the feeder also being served to be sized no larger than the service entrance conductor serving that feeder. There are times when you may find 4 feeders in a conduit being served by a service entrance that is sized by this NEC Table 310.15.B.6that is also controlled in size by this same chart NEC Table 310.15.B.6 by that feeder not being required to be larger than the service entrance conductors supplying it power. This feeder also would be exempt from the de-ration factors required by NEC Article 310.15.B.2.A and / or NEC Table 310.16 correction factor [ at bottom of the page ].

Special Note: Any conduit shorter than 24”requires no de-rating calculation in considering the number of conductors installed in a raceway. NEC Article 310.15.B.2. 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. NEC 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 dual 120 volt branch circuit that has been split into two 120 volt branch circuits from one 240 volt circuit that has two hot conductors, one insulated white neutral conductor and one equipment grounding conductor. Another example that would use a neutral conductor is a dryer or electric range that uses both 240 volts and 120 volts in the same multi-wire branch circuit. NEC Article310.15.B.4.A.A 120 VOLT BRANCH CIRCUIT SERVING RECEPTACLES DOES NOT APPLY TO THIS RULE FOR NEUTRALS. A 120 VOLT RECEPTACLE DOES NOT HAVE A NEUTRAL CONDUCTOR IN ITS BRACH CIRCUIT DESIGN.

Special Note: You must count the white grounded leg serving a single ungrounded [hot]conductor of a 120 volt branch 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 ungrounded [hot] conductors of a 240 two pole circuit {this conductor would be a true neutral conductor}. NEC Article 310.15.B.2and 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. NEC Article 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 3 conductors = adjusted Conductor ampacity recognized as the ampacity of that conductor by the NEC.

Example #1: Attic in Florida

1st Example:

[ Written Form ]

Conductor ampacity of an 8 awg TW conductor installed in a Florida attic, with an approximate average ambient temperature of 130 degrees F, with a total number of 26 current carrying conductors in that conduit, with no diversity =

NEC Table 310.16@ 60 Degree Column 8 awg copper is rated at 40 amps X NEC Table 310.16correction factor [ at bottom of the page ] for ambient temperature is 41 % equaling 16.4 amps of ampacity after the temperature de-ration calculation then this answer is multiplied X number of current carrying conductors in a raceway reduction required in NEC Chart 310.15.B.2.Aat 45 % = 7.38 amps = The resulted total reduction of ampacity after both the ambient temperature de-ration and the de-ration due to the 26 current carrying conductors in a raceway is adjusted down to 7 amps for the total ampacity of the above 8 awg TW conductor as described. Therefore the total adjusted ampacity of that 8 awg copper conductor as described in its installation design is now a 7 amp rated ampacity for an 8 awg copper conductor.

Finishing the Calculation
The explanation of this calculation would show that the requirement found in NEC Table 310.16 ambient temp de-ration is only a partial de-ration process. You must then again de-rate that conductor’s ampacity further concerning ampacity de-ration calculation due to the number of conductors contained in that raceway as required in NEC Table 310.15.B.2.A

40amps X 41% = 16.4 amps after ambient temperature de-ration then multiplied by 45% required due to the 26 current carrying conductors in that same raceway which would then have a final de-rated ampacity of that 8 awg copper conductor of 7.38 amps. This 7 amps of ampacity of this 8 awg copper conductor would then be considered the true adjusted ampacity rating of that 8 awg copper conductor. The NEC requires that you must have a minimum ampacity of 15 amps for any conductor used in a wiring system of 120 volts or more. NEC Table 210.3 That conductor must also be capable to carry the load apply requiring the ampacity to equal the load it carries NEC Table 210.19.A.1 and NEC Table 210.24 This 8 awg conductor as described above in its installation would not be allowed to be used because it not only is not rated at in ampacity of at least 15 amp but it also would not carry the load expected of it.

Same Example:

[ Numeric Form ]

8 awg 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 awg TW conductor after the correction factors are reduced.

Example Results:

Therefore that 8 awg 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. In NEC Table 310.16 column for 60 degree C the ampacity rating says that this conductor is rated at 40 amps for TW insulated conductors. The calculated answer is smaller the original ampacity rating found in NEC Table 310.16 column for 60 degree C , therefore you must use the smallest ampacity rating which would be your calculated de-rated ampacity rating.

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. NEC Article 210.3 and NEC Table 210.19.A.1 and NEC Table 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. NEC Article 210.11 Motors conductors must also consider ampacity de-ration calculations. NEC Table 310.15.B.2.A and NEC Table 310.16 and NEC Article 430.22 and NEC Article 430.24

Actually to my knowledge the table for residential service entrance type conductors as per NEC 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 because you have no de-ration to calculate and if you are using a higher temperature rated conductor such as THHN which is rated 90 degrees centigrade NEC Table 310.16 or THW which is rated at 75 degrees centigrade NEC Table 310.16, and the conductor is smaller than 1 awg, regardless, you must use the 60 degree column as the ampacity rating as required in NEC Article 110.14.C.1.A.1If the conductor is a 1 awg or larger conductor then you must use the 75 degree column as the ampacity rating NEC Article 110.14.C.2.

There is an exception that may apply if you are using no connectors except your breaker connection and if you breaker connection is rated at 75 degrees then you may use the 75 degree column if your conductor is also rated at 75 degrees such as THW OR THWN etc.

If you are required to calculate an ampacity de-ration calculation for either ambient temperature or more than 3 current carrying conductors in a raceway, you may start your calculation by using the ampacity rating found in NEC Table 310.16 column for 90 degree C if the conductor’s insulation is originally rated for 90 degrees. You may also start your de-rating calculations by using the ampacity rating found in NEC Table 310.16 column for 75 degree C if the conductor’insulation is originally rated for 75 degrees. You must then finish your de-rating calculation for the ampacity rating according to NEC Articles 110.14.C after you have completed your ampacity de-ration calculation, you must then compare that calculation de-rated ampacity answer that you calculated using the original temperature degree rated ampacity in your calculation, with the either the 60 degree column ampacity rating, if the conductor is smaller than 1 awg or with the 75 degree column ampacity rating. If the conductor is 1 awg or larger. NEC Article 110.14.C.1 & 2 You must use the worst scenario [lowest amps in the comparison, comparing your de-rated ampacity answer to the ampacity rating dictated by NEC Article 110.14.C.1 & 2 ] The lowest ampacity rating must be chosen concerning the ampacity rating of that conductor after comparing the de-rated ampacity you found after learning your de-rating calculation answer to the ampacity rating found in the column found in NEC Table 310.16 column for 60 degree C that NEC Article 110.14 requires, if smaller than 1 awg = 60 degree column or if 1 awg or larger = 75 degree column. NEC Article 110.14.C.1 & 2

This NEC Article 210.14.C.1 & 2 requirement of a lower ampacity rating than found in NEC Table 310.16 i s because the terminal ends and wire nuts that are listed and approved on the market in the electrical industry are only rated at 60 degrees centigrade if smaller than 1 awg or 75 degree column if larger than 1 awg conductors as per NEC Article 110.14.C.1 & 2.

There can be times when the ampacity de-ration calculation’s answer is higher in ampacity when starting with the original insulation temperature rating of the conductor than the ampacity of the column in NEC Table 310.16 dictated by NEC Article 110.14.C 1 & 2.

Example #2: Canadian Attic

[ Written Form ]

The conductor ampacity of an Awg.. 8 awg 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 = NEC Table 310.16 @ 60 Degree Column is 40 amps X NEC Table 310.16correction factor [ at bottom of the page ] for ambient temperature is 108 % X Reduction NEC Chart 310.15.B.2.Ais 45 % = 19.44 amps = Reduction to 19 amps for the total ampacity of the above 8 awg TW conductor. 40amps X 108% X 45% = 19.44 amps

2nd Same Example:

[ Numeric Form ]

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

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

2nd Example Results:

Therefore that 8 awg 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. Again we compare the requirements in NEC Table 310.16 in the 60 degree column as dictated by NEC Article 110.14.C.1 & 2 to the answer we have after the de-rating calculation above we should find that the de-rated calculation answer came up to 19.44 amps. The ampacity rating found in NEC Table 310.16 in the 60 degree columnis 40 amps. We must again take the smallest ampacity rating which would be the 19.44 amps. Therefore 19.44 amps must be our answer as to the maximum ampacity of this 8 awg TW copper conductor installed in an attic in Florida in a conduit with a total of 26 other conductors in that same conduit.

Now with no intent to muddy the waters we should find that if this conductor is serving a dedicated branch circuit that is not a part of a multi-outlet branch circuit and if this conductor is rated less than 800 amps and if this conductor’s ampacity does not correspond with an overcurrent device [breaker or fuse] then we may install the next higher amp rated breaker of fuse found listed in NEC Article 240.6

Final Thoughts on Conductors

Most commonly used of all above remember that when using smaller conductors, then if your conductor is sized 14 awg through and including 10 awg, you must size the ampacity of those conductors by using NEC Article 240.4.D which says that copper conductors in smaller gauges listed must be sized by the following ampacity; 14 awg cu. shall not exceed 15 amp / 12 awg cu. shall not exceed 20 amps / 10 awg cu. shall not exceed 30 amps over riding the ampacity ratings found in NEC Table 310.16but only after any de-rating calculations required due to ambient temperature or more than 3 current carrying conductors in a raceway has been calculated. When calculating the ampacity of 14, 12, or 10 awg conductors you must first perform the ampacity de-ration calculation as per ambient temperature found at the bottom of the Chart found in NEC Table 310.16 and then as per more than 3 conductors in a raceway found in NEC Table 310.15.B.2.A.

This document is based on the 2002 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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