Forums » Electrical Engineering

Sizing Branch Circuit Conductors

    • 85 posts
    September 8, 2019 8:09 PM PDT

    I have been tasked with explaining to some new engineers why the wiring size charts have been changed.

    The old chart from 1981 was using the wire ampacities at full insulation temperature of 90 degrees C in a 50 degree C ambient.  For instance: 

    14 AWG 20A

    12 AWG 24A

    10 AWG 32A

    1/0 AWG 178A (130 degree C insulation) 

    2/0 AWG 204A (130 degree C insulation)

    They were not sized for operation at 75 degrees C for terminating on a breaker.

    I figure the easiest way to really drive this home would be to go through the wiring sizing process.  I would like your input & any corrections or mistakes or glaring errors you may detect.

    Here is the main body of my explanation:

    SIZING CONDUCTORS FOR BRANCH CIRCUITS

    CONDUCTORS DO NOT HAVE A CURRENT RATING THEY HAVE AMPACITY

    AMPACITY: The maximum current, in amperes, that a conductor can carry continuously under the conditions of use without exceeding its temperature rating. (NEC ARTICLE 100)

    Conditions of use include:

    INSULATION TEMBERATURE RATING

    AMBIENT TEMPERATURE

    MORE THAN 3 CURRENT CARRING CONDUCTORS (CCC) BUNDLED OR IN A RACEWAY

    TERMINAL TEMPERATURE ALLOWANCE

    HISTORY

    In 1996 the NEC changed to require that continuous loads be calculated at 125%.  This change was made at the behest of the breaker manufactures.  This was to ensure that the breaker would not be loaded continuously above 80%.  Breakers have terminals listings of 75 degree C & use the connected conductors for cooling.  The conductors must be sized with extra capacity for cooling the connected breaker.

    THE REQUIREMENTS

    NEC article 240 covers Overcurrent Protection.

    NEC article 210 covers Branch Circuits.

    NEC Tables found at 310.15(B)(16) provides the table used for conductor sizing and references to the tables for derating.

    SIZING A CONDUCTOR FOR A BRANCH CIRCUIT

    1. Determine Load
    2. Select the OCPD
    3. Select conductors
    • The terminal temperature must not exceed 75 degrees C.
    • Derating if required:
    • Ambient Temperature
    • More than three Current Carrying Conductors (CCC) in bundle or raceway.
    • Will OCPD protect the conductor?

    EXAMPLE

    Circuits to be designed:

    4000 (5000) VA CONTINUOUS LOAD

    240V Single PHASE

    90 DEGREE C CONDUCTOR

    50 DEGREE C AMBIENT TEMPERATURE

    SELECT BREAKER SIZE

     

    4000 (5000) VA / 240V = 16.67A or 17A (20.8A or 21A)

    16.67A (20.8A) X 1.25 (CONTINUOUS LOAD) = 21.25A or 21A (26.04A or 26A)

    Standard size breakers are 15,20,30,40,50…

    A 30A BREAKER IS THE LOWEST STANDARD SIZE BREAKER THAT IS CAPABLE OF CARRING 21A (26A) EITHER CONTINOUSLY OR NON-CONTINOUSLY.

    SELECT CONDUCTOR SIZE

    (A)

    REFER TO NEC TABLE 310.15(b)(16) TEMPERATURE RATINGS OF CONDUCTORS AT 30 DEGREES C

    Under the 75 degree C column the first conductor listed greater than or equal to 30A is a 10 GAUGE.  Note the conductor will operate at 75 degrees with 35A of load in a 30 degrees C ambient temperature.

    (B)

    Is derating required?

    Is there more than three CCC in the bundle or raceway ?   No

    Is ambient temperature above 30 degrees C 86 degrees F?  YES

    The ambient temperature inside the gear is 50 degrees C.

    REFER TO TABLE 310.15(B)(2) AMBIENT TEMPERATURE CORRECTION FACTORS BASED ON 30 DEGREES C

    A correction factor of .75 is used for temperature 46-50 degrees C under the 75 degree C column.

    35A X .75 = 26.25A 

    We now see our conductor will operate at 75 degrees C at 26.3A not 35A.

    The load is 21A (26A), so the 10 Gauge conductor selection is good.

    (C)

    DOES THE BREAKER PROTECT THE CONDUCTOR?

    THIS CALCULATION IS NOT AN NEC REQUIREMENT

    NOTE THAT THE LOAD DOES NOT MATTER FOR THIS CALCULATION

     

    30A BREAKER

    10 AWG CONDUCTOR

    50 DEGREE C AMBIENT

    90 DEGREE C CONDUCTOR INSULATION

    REFER TO TABLE 310.15(B)(2) AMBIENT TEMPERATURE CORRECTION FACTORS

    Use the 90 degree C column for derating.

    35A X .82 = 28.7A

    The 10 AWG conductor operating temperature is 90 degrees C at 29A.

    The breaker does not completely protect the conductor.

    Can we use a 25A breaker?

    21A – YES, because the 80% is already built into this value.  We can go up to 25A.

    Conductor is Protected because the value is under 29A.

    (26A) –NO

    UPSIZE CONDUCTOR TO 8 AWG & RECALCULATE

    50A X .82 = 41A AT 90 DEGREES C

    CONDUCTOR IS PROTECTED

     

    Our SIS is 105 degrees C & the 10 AWG would be protected.  The 2017 NEC table does not include a 105 degree C column.

     

     

     

     

    ABOVE IS A THERMAL IMAGE OF A BREAKER COOLING ITSELF THROUGH THE CONNECTED CONDUCTORS.  NOTE THE SLIGHT RED IN THE CONDUCTORS NEAR THE BREAKER.  THE CONDUCTOR IS COOLER THE FARTHER IT IS FROM THE BREAKER AS THE COLOUR TURNS FROM YELLOW TOWARD GREEN.   

    SIZING CONDUCTORS FOR BRANCH CIRCUITS

    CONDUCTORS DO NOT HAVE A CURRENT RATING THEY HAVE AMPACITY

    AMPACITY: The maximum current, in amperes, that a conductor can carry continuously under the conditions of use without exceeding its temperature rating. (NEC ARTICLE 100)

    Conditions of use include:

    INSULATION TEMBERATURE RATING

    AMBIENT TEMPERATURE

    MORE THAN 3 CURRENT CARRING CONDUCTORS (CCC) BUNDLED OR IN A RACEWAY

    TERMINAL TEMPERATURE ALLOWANCE

    HISTORY

    In 1996 the NEC changed to require that continuous loads be calculated at 125%.  This change was made at the behest of the breaker manufactures.  This was to ensure that the breaker would not be loaded continuously above 80%.  Breakers have terminals listings of 75 degree C & use the connected conductors for cooling.  The conductors must be sized with extra capacity for cooling the connected breaker.

    THE REQUIREMENTS

    NEC article 240 covers Overcurrent Protection.

    NEC article 210 covers Branch Circuits.

    NEC Tables found at 310.15(B)(16) provides the table used for conductor sizing and references to the tables for derating.

    SIZING A CONDUCTOR FOR A BRANCH CIRCUIT

    1. Determine Load
    2. Select the OCPD
    3. Select conductors
    • The terminal temperature must not exceed 75 degrees C.
    • Derating if required:
    • Ambient Temperature
    • More than three Current Carrying Conductors (CCC) in bundle or raceway.
    • Will OCPD protect the conductor?

    EXAMPLE

    Circuits to be designed:

    4000 (5000) VA CONTINUOUS LOAD

    240V Single PHASE

    90 DEGREE C CONDUCTOR

    50 DEGREE C AMBIENT TEMPERATURE

    SELECT BREAKER SIZE

     

    4000 (5000) VA / 240V = 16.67A or 17A (20.8A or 21A)

    16.67A (20.8A) X 1.25 (CONTINUOUS LOAD) = 21.25A or 21A (26.04A or 26A)

    Standard size breakers are 15,20,30,40,50…

    A 30A BREAKER IS THE LOWEST STANDARD SIZE BREAKER THAT IS CAPABLE OF CARRING 21A (26A) EITHER CONTINOUSLY OR NON-CONTINOUSLY.

    SELECT CONDUCTOR SIZE

    (A)

    REFER TO NEC TABLE 310.15(b)(16) TEMPERATURE RATINGS OF CONDUCTORS AT 30 DEGREES C

    Under the 75 degree C column the first conductor listed greater than or equal to 30A is a 10 GAUGE.  Note the conductor will operate at 75 degrees with 35A of load in a 30 degrees C ambient temperature.

    (B)

    Is derating required?

    Is there more than three CCC in the bundle or raceway ?   No

    Is ambient temperature above 30 degrees C 86 degrees F?  YES

    The ambient temperature inside the gear is 50 degrees C.

    REFER TO TABLE 310.15(B)(2) AMBIENT TEMPERATURE CORRECTION FACTORS BASED ON 30 DEGREES C

    A correction factor of .75 is used for temperature 46-50 degrees C under the 75 degree C column.

    35A X .75 = 26.25A 

    We now see our conductor will operate at 75 degrees C at 26.3A not 35A.

    The load is 21A (26A), so the 10 Gauge conductor selection is good.

    (C)

    DOES THE BREAKER PROTECT THE CONDUCTOR?

    THIS CALCULATION IS NOT AN NEC REQUIREMENT

    NOTE THAT THE LOAD DOES NOT MATTER FOR THIS CALCULATION

     

    30A BREAKER

    10 AWG CONDUCTOR

    50 DEGREE C AMBIENT

    90 DEGREE C CONDUCTOR INSULATION

    REFER TO TABLE 310.15(B)(2) AMBIENT TEMPERATURE CORRECTION FACTORS

    Use the 90 degree C column for derating.

    35A X .82 = 28.7A

    The 10 AWG conductor operating temperature is 90 degrees C at 29A.

    The breaker does not completely protect the conductor.

    Can we use a 25A breaker?

    21A – YES, because the 80% is already built into this value.  We can go up to 25A.

    Conductor is Protected because the value is under 29A.

    (26A) –NO

    UPSIZE CONDUCTOR TO 8 AWG & RECALCULATE

    50A X .82 = 41A AT 90 DEGREES C

    CONDUCTOR IS PROTECTED

     

    Our SIS is 105 degrees C & the 10 AWG would be protected.  The 2017 NEC table does not include a 105 degree C column.

     

     

     A thermal image showing a breaker cooling itself through the connected conductors.

     

    • 85 posts
    September 8, 2019 8:23 PM PDT

    Here is the old chart, it would not upload as a pdf so it was converted to jpeg. 

    Note the last column is supposed to be derated for 50 degrees C operation, but the engineer is using the conductor at its full temperature rating. 


    This post was edited by Steve Ward at September 8, 2019 8:26 PM PDT
    • 85 posts
    September 10, 2019 9:34 AM PDT

    OK, looking back over the paper I have found some errors & have made adjusted. This paper is not going to be used as a guide for sizing wire, but just to keep me from going off track & getting stupid during the exercise.

     

     

    SIZING CONDUCTORS FOR BRANCH CIRCUITS

    CONDUCTORS DO NOT HAVE A CURRENT RATING THEY HAVE AMPACITY

    AMPACITY: The maximum current, in amperes, that a conductor can carry continuously under the conditions of use without exceeding its temperature rating. (NEC ARTICLE 100)

    Conditions of use include:

    INSULATION TEMPERATURE RATING

    AMBIENT TEMPERATURE

    MORE THAN 3 CURRENT CARRING CONDUCTORS (CCC) BUNDLED OR IN A RACEWAY

    TERMINAL TEMPERATURE ALLOWANCE

    HISTORY

    In 1996 the NEC changed to require that continuous loads be calculated at 125%.  This change was made at the behest of the breaker manufactures.  This was to ensure that the breaker would not be loaded continuously above 80%.  Breakers have terminals listings of 75 degree C & use the connected conductors for cooling.  The conductors must be sized with extra capacity for cooling the connected breaker.

    THE REQUIREMENTS

    NEC article 240 covers Overcurrent Protection.

    NEC article 210 covers Branch Circuits.

    NEC Tables found at 310.15(B)(16) provides the table used for conductor sizing and references to the tables for derating.

    SIZING A CONDUCTOR FOR A BRANCH CIRCUIT

    1. Determine Load
    2. Select the OCPD
    3. Select conductors
    • The terminal temperature must not exceed 75 degrees C.
    • Derating if required:
    • Ambient Temperature
    • More than three Current Carrying Conductors (CCC) in bundle or raceway.
    • Will OCPD protect the conductor?

    EXAMPLE

    Circuits to be designed:

    4000 (5000) VA CONTINUOUS LOAD

    240V Single PHASE

    90 DEGREE C CONDUCTOR

    50 DEGREE C AMBIENT TEMPERATURE

    SELECT BREAKER SIZE

     

    4000 (5000) VA / 240V = 16.67A or 17A (20.8A or 21A)

    16.67A (20.8A) X 1.25 (CONTINUOUS LOAD) = 21.25A or 21A (26.04A or 26A)

    Standard size breakers are 15,20,25,30,35,40,45,50…

    A 25A (30A) BREAKER IS THE LOWEST STANDARD SIZE BREAKER THAT IS CAPABLE OF CARRING 21A (26A) EITHER CONTINOUSLY OR NON-CONTINOUSLY.

    SELECT CONDUCTOR SIZE

    (A)

    REFER TO NEC TABLE 310.15(b)(16) TEMPERATURE RATINGS OF CONDUCTORS AT 30 DEGREES C

    Under the 75 degree C column the first conductor listed greater than or equal to 30A is a 10 GAUGE.  Note the conductor will operate at 75 degrees with 35A of load in a 30 degrees C ambient temperature.

    (B)

    Is derating required?

    Is there more than three CCC in the bundle or raceway ?   No

    Is ambient temperature above 30 degrees C (86 degrees F)?  YES

    The ambient temperature inside the gear is 50 degrees C.

    REFER TO TABLE 310.15(B)(2) AMBIENT TEMPERATURE CORRECTION FACTORS BASED ON 30 DEGREES C

    A correction factor of .75 is used for temperature 46-50 degrees C under the 75 degree C column.

    35A X .75 = 26.25A 

    We now see our conductor will operate at 75 degrees C at 26.3A not 35A.

    The load is 21A (26A), so the 10 Gauge conductor selection is good.

    (C)

    DOES THE BREAKER PROTECT THE CONDUCTOR?

    THIS CALCULATION IS NOT AN NEC REQUIREMENT

    NOTE THAT THE LOAD DOES NOT MATTER FOR THIS CALCULATION

     

    25A (30A) BREAKER

    10 AWG CONDUCTOR

    50 DEGREE C AMBIENT

    90 DEGREE C CONDUCTOR INSULATION

    REFER TO TABLE 310.15(B)(2) AMBIENT TEMPERATURE CORRECTION FACTORS

    Use the 90 degree C column for derating.

    35A X .82 = 28.7A

    The 10 AWG conductor operating temperature is 90 degrees C at 29A.

     

    25A Breaker

    Conductor is Protected because the breaker rating is under 29A.

     

    30A Breaker

    The breaker does not completely protect the conductor, because the breaker rating is over 29A

    UPSIZE CONDUCTOR TO 8 AWG & RECALCULATE

    50A X .82 = 41A AT 90 DEGREES C

    CONDUCTOR IS PROTECTED 41A is greater than the breaker rating (30A).

     

    Our SIS is 105 degrees C & the 10 AWG would be protected.  The 2017 NEC table does not include a 105 degree C column.


    This post was edited by Steve Ward at September 10, 2019 9:38 AM PDT
    • 33 posts
    September 10, 2019 4:27 PM PDT
    Steve Ward said:

    I have been tasked with explaining to some new engineers why the wiring size charts have been changed.

     

    Steve - 

    Good choice in not using your attached chart.  I can't tell where the data came from.  It's not NEC 1981, T310.16.  

    Here is a copy of NEC 1981, T310.16:

     

    As you can see the numbers are about the same as current NEC ampacity.

     

    • 85 posts
    September 10, 2019 6:34 PM PDT

    Carl, my understanding is that the "chart" was derived from the 1981 ampacity table you posted.  Those values may have been valid under that code cycle.  I have not run the numbers. According to EC&M, 1996 was the year everything changed.

    Thanks for table, I am curious if any of the values are different from today, especially the small conductors.

    • 33 posts
    September 14, 2019 6:31 PM PDT

    I wasn't ignoring you.  One of my clients insisted I pay attention to them for a few days.  AArggg. those pesky people with checkbooks think they can direct my time - Oh wait, they can money-mouth

    "my understanding is that the "chart" was derived from the 1981 ampacity table "

    The chart looks like it was for a special project.  There were several oddities:

    Fine strand wire for some sizes.

    Table 310.16 for 5KV and 7.5 KV.

    Corrected for 50C ambient.

    Some sizes corrected to 130C

    I don't know what that table was for.

     

    As for the termination temperatures that's in 110.14.  1981 NEC did not address it.  One had to look at the listing for the circuit breakers.

    The big change to 110.14 was in the 1993 code.  1987 was the same as previous.  There were a few minor changes to 110.14 in the 1996 - but not much.

    I don't put too much stock in EC&M articles, without checking their references.  I don't think they peer review their articles.

     

    As for ampacities, those are the same as the first book I saw maybe 1978 - maybe some small changes to the 90C column.

    Compare the 1981 table 310.16 I posted, to a current 2017 310.15.B.16.  There is not much difference.

     

    carl

     

    • 33 posts
    September 14, 2019 6:57 PM PDT

    A couple of minor suggestions for your example:

    1.  Pick a more normal building wire type, like thhn or xhhw.

    There are no 105C tables for 600V, because there are no devices rated for 105C (not exactly true but pretty close)

    For example, molded case circuit breakers will be rated for:

    60C (haven't seen any of them in the last 20 years)

    You can use any temperature rated insulation you want, but the wire has to be sized using the 60C column

    60/75C

    If you use 60C insulation, the wire has to be sized using the 60C column

    If you use 75C insulation (or any higher temp rating) then the wire can be sized using the 75C column

    75C  -  Have to use 75C insulation and size the wire using the 75C column

    The only places 90C insulation (or highere) helps is for the ambient derate, or bundling derate.  One can apply the derate percentages to the 90C rating.

    2.  Most often one picks the conductor ampacity to fit the load, and then picks the CB to protect the wire - often next size up.

    3.  Sometimes one picks the CB.  An example is 250%FLA for a transformer feeder.  The conductor has to be selected to be protected by the CB, not the transformer load.

     

    4.  And then there are motor loads, where the CB can be 250% or even more, and the conductors will b e sized for 125% FLA.

     

    I'm thinking you knew all this.  I think I am just phrasing it differently.  Hope this helped

     

    carl

    • 85 posts
    September 14, 2019 8:13 PM PDT

    Carl thanks for taking the time to help. 

    That "chart" was used until a few years ago as our standard.  The internal temperature of our gear is considered to be 50 Degrees C.  I managed to get it corrected several years ago. 

    The SIS was selected because that is the main wire we use in our assemblies. 

    I have found & corrected some other errors in my paper. 

    I am steering clear of transformers & motors for now.

    I have been running thermal tests here in the mini-lab with a 15A breaker. 

    10 AWG Conductors

    14 AWG Conductors 

    16 AWG Conductors

    I will probably post a album with the thermal images in a few weeks.

    Agree there are few devices with more than 75 degreeC terminals.

    Here is the latest version:

    SIZING CONDUCTORS FOR BRANCH CIRCUITS

    CONDUCTORS DO NOT HAVE A CURRENT RATING THEY HAVE AMPACITY

    AMPACITY: The maximum current, in amperes, that a conductor can carry continuously under the conditions of use without exceeding its temperature rating. (NEC ARTICLE 100)

    Conditions of use include:

    INSULATION TEMPERATURE RATING

    AMBIENT TEMPERATURE

    MORE THAN 3 CURRENT CARRING CONDUCTORS (CCC) BUNDLED OR IN A RACEWAY

    TERMINAL TEMPERATURE ALLOWANCE

    HISTORY

    In 1996 the NEC changed to require that continuous loads be calculated at 125%.  This change was made at the behest of the breaker manufactures.  This was to ensure that the breaker would not be loaded continuously above 80%.  Breakers have terminals listings of 75⁰ C & use the connected conductors for cooling.  The conductors must be sized with extra capacity for cooling the connected breaker.

    UL489 is the MCCB Standard.

    THE REQUIREMENTS

    NEC article 240 covers Overcurrent Protection.

    NEC article 210 covers Branch Circuits.

    NEC Tables found at 310.15(B)(16) provides the table used for conductor sizing and references to the tables for derating.

    SIZING A CONDUCTOR FOR A BRANCH CIRCUIT

    1. Determine Load
    2. Select the OCPD
    3. Select conductors
    • The terminal temperature must not exceed 75⁰
    • Derating if required:
    • Ambient Temperature
    • More than three Current Carrying Conductors (CCC) in bundle or raceway.
    • Will OCPD protect the conductor?

    EXAMPLE

    Circuits to be designed:

    4000 (5000) VA CONTINUOUS LOAD

    240V Single PHASE

    90⁰ C CONDUCTOR

    50⁰ C AMBIENT TEMPERATURE

    SELECT BREAKER SIZE

     

    4000 (5000) VA / 240V = 16.67A or 17A (20.8A or 21A)

    16.67A (20.8A) X 1.25 (CONTINUOUS LOAD) = 21.25A or 21A (26.04A or 26A)

    Standard size breakers are 15,20,25,30,35,40,45,50…  NEC Table 240.6(A)

    A 25A (30A) BREAKER IS THE LOWEST STANDARD SIZE BREAKER THAT IS CAPABLE OF CARRING 21A (26A).

    SELECT CONDUCTOR SIZE

    (A)

    REFER TO NEC TABLE 310.15(b)(16) TEMPERATURE RATINGS OF CONDUCTORS AT 30 DEGREES C

    Under the 75⁰ C column the first conductor listed greater than or equal to 25A (30A) is a 12 (10)  GAUGE.  Note the conductor will operate at 75⁰ C with 25A (35A) of load in a 30⁰ C ambient temperature.

    (B)

    Is derating required?

    Are there more than three CCC in the bundle or raceway ?   No

    Is ambient temperature above 30⁰ C (86 degrees F)?  YES

    The ambient temperature inside the gear is 50⁰ C.

    REFER TO TABLE 310.15(B)(2) AMBIENT TEMPERATURE CORRECTION FACTORS BASED ON 30⁰ C

    A correction factor of .75 is used for temperature 46-50⁰ C under the 75⁰ C column.

    25A X .75 = 18.8A or 19A

    (35A X .75 = 26.3A) 

    We now see our conductor will operate at 75⁰ C at 18.8A not 25A (26.3A not 35A). 

    The load for the 12 AWG conductor for the 4000KVA load is 21A , this exceeds 18.8A so the 12 AWG conductor  is insufficient. Upsize to 10 AWG.

    The load on the 10 AWG conductor for the 5KVA load is (26A), this is smaller than 26.3, so the 10 Gauge conductor selection is good.

    (C)

    DOES THE BREAKER PROTECT THE CONDUCTOR?

    THIS CALCULATION IS NOT AN NEC REQUIREMENT

    Will the breaker trip before the conductor exceeds its insulation temperature rating?

    NOTE THAT THE LOAD DOES NOT MATTER FOR THIS CALCULATION

     

    25A (30A) BREAKER

    10 AWG CONDUCTOR

    50⁰ C AMBIENT

    90⁰ C CONDUCTOR INSULATION

    REFER TO TABLE 310.15(B)(16) FOR AMPACITY AT 90⁰ C

    REFER TO TABLE 310.15(B)(2) AMBIENT TEMPERATURE CORRECTION FACTORS

    Use the 90⁰ C column for derating.

    40A X .82 = 32.8A or 33A

    The 10 AWG conductor operating temperature is 90⁰ C at 33A.

     

    25A Breaker:

    Conductor is Protected because the breaker rating is under 33A.

     

    (30A) Breaker:

    Conductor is protected because the breaker rating is under 33A

     

    NEC 240.4(D) Small Conductors & Maximum OCPD Size

    NEC240.4(D)(7) Maximum size OCPD is 30A for a 10AWG conductor