Post by Electrical Code Academy Staff on Jul 28, 2016 13:18:11 GMT -6
Here is an interesting story....the individual installed 500 kcmil CU on an installation where the load was 309 amps of non-motor continuous load. Let's see it that was the correct size....
First - For simplicity we will use the rules under 210 for branch circuits. However, the rules also apply for feeders under 215.2 as well. The ambient is 105° F and the system is a 3-phase -4 wire system with nonlinear loads over 50 %. Yes, the neutral is countered as a current carrying conductor.
STEP 1 - 210.19(A)(1)(a) - The continuous load at 125% and the noncontinous load at 100%. The load is 309 Amps of continuous load only.
NEC Section 210.19(A)(1)(a) and/or 215.2(A)(1)(a) Calculation - 309 Amps (Actual Load) x 1.25 = 386.25 Amperes
STEP 2 - 210.19(A)(1)(B) - States that the minimum branch circuit conductor size shall have an ampacity not less than the maximum load (309 amps) after the application of any adjustments (more than 3 current carrying conductors)(.80) or temperature corrections (.87) based on 310.15(B)(2)(a) and (B)(3)(a).
The National Electrical Code (NEC) permits that once that conductors ampacity is corrected and adjusted, the installer is now required to protect the conductor at it’s new ampacity value. This will be discussed in more detail later in this article/lesson.
THE CALCULATION FOR STEP 2 PREVIOUSLY DISCUSSED
Based on 310.15(B) and 310.15(B)(16) of the National Electrical Code, A 500 Kcmil CU, THHN is rated for 430 Amps @ 90°C and is permitted to be used to determine the minimum size of the conductor when doing adjustments and corrections for various environmental and physical conditions of use. As long as the final new ampacity value is adequate for the load.
430 Amps x .80 (4 Current Carrying Conductors) x .87 (105-113° F) = 299 amps
THE REAL CONCERN
The National Electrical Code (NEC) states that you have to choose the larger of the two (regardless of being a branch circuit or a feeder calculation) . 210.19(A)(1)(a) results are 386.25 amps at 75° C and 210.19(A)(1)(b) after adjustments and corrections (and using 90° C conductors, resulted in 299 amps.
As an important reminder, NEC Table 310.15(B)(16) is based on no more than 3 current carrying conductors and an ambient temperature of 30°C (86°F). Anything otherwise changes the actual ampacity value of the conductor.
TERMINAL LIMITATIONS, CONDUCTORS, and OVERCURRENT PROTECTION DEVICES
Based on NEC Section 110.14(C), a 500 Kcmil CU terminal ratings is 75°C; shown in NEC Table 310.15(B)(16) as 380 amps. However, Since this condition of use results in more than 3 current carrying conductors and being located in an ambient temperature environment that exceeds 30° C by upwards of 10-15°C. it is of little use except to ensure the user initially selects a conductor with enough initial ampacity to meet the terminal limitations at 75° C.
Traditionally, the electrical engineer will break down the calculation into the Overcurrent Protection Device (OCPD) and the Conductors to better understand what roll each component plays. As expressed in Step 1 earlier, The conductor must be sized no less than 125% of the continuous load, in our case it was 309 amps x 1.25 = 386.25 amps. A 500 Kcmil CU is listed at 75C for 380 amps maximum.
In step 2, since derating has to take place the National Electrical Code (NEC) permits the use of the 90° C ampacity values in Table 310.15(B)(16) for adjustments and corrections. The end result requires at least a conductor that can handle the actual load (309 amps) after the adjustments and corrections take place. In this analysis example, is was determined to be 299 amps.
The NEC requires the conductor be no less than the larger of the two above. In this analysis, the 309 amps conductor would not acceptable at the 105-113° F range.
Based solely on the above information an engineer could prematurely conclude that the conductor should have been no less than 600 kcmil CU. However, there is yet another potential concern, from a reliability standpoint, that your designer should consider.
PROTECTING THE ELECTRICAL INSULATION FROM BREAKDOWN
When engineers size the overcurrent protection for the branch circuit (or feeder) as described in 210.20(A) and 215.3 respectfully, they also have to choose the appropriate overcurrent protective device as expressed in 240.4(B) and 240.6(A). Let’s examine 210.20(A), which also applies to 215.3.
Section 210.20(A) states that the Overcurrent Protection Device “Shall Not” be less than 125% of the continuous load and 100% of noncontinuous loads. In this analysis , an actual load of 309 amps. Basically, it results in 309 amps x 1.25 = 386.25 amps. Based on 240.6 this would require a 400 amps overcurrent protection device due to the “next size up” rule found in 240.4(B).
The Problem - the newly adjusted ampacity of the 500 Kcmil is only 299 amps on the given ambient and the 4 current carrying conductors. The NEC demands in section 240.4, a conductor has to be protected against overcurrent (short circuit, ground-fault and overload) in accordance with their ampacities specified in 310.15(which includes all the adjustment and correction values).
The NEC will not permit a conductor with a new ampacity value of 299 amps to be protected by a 400 amp Overcurrent Protection Device (OCPD), ignoring the rare motor and HVAC situations expressed in 240.4(A)-(G), which is not the case in our lesson. In fact, it would be either a 300 or 350 amp OCPD at face value. However, that still will not work, the minimum sized Overcurrent Protection Device is 400 amps per 210.20(A) and/or 215.3, which will require a conductor that can legally be protected by that 400-amp device.
WHICH CONDUCTOR BASED ON THIS “CONDITION OF USE” CAN CORRECTLY TERMINATE ON A 400 AMP OCPD
Is a 600 Kcmil acceptable? - 475 amps @ 90°C x .80 x .87 = 330.6 amps which must be protected by a 350 amps OCPD - Answer is No; it can not in this case be protected by a 400 amp OCPD.
Is a 700 Kcmil acceptable? - 520 amps @ 90°C x .80 x .87 = 361.92 amps is acceptable to be protected by a 400 amp OCPD in accordance with the “next size up” rule in 240.4(B). Answer is Yes.
A 400 amp OCPD device on a conductor that has a newly corrected and adjusted (derated) ampacity value of 299 amps is not actually protecting the conductors as demanded in NEC Section 240.4 or 210.20(B) or 215.3 and will continue to remains subject to overload, short-circuit or ground-fault conditions with long term exposure.
CIRCUIT BREAKER ANALYSIS- HEAT SINK EFFECT
Circuit Breaker manufacturers rely on electrical wire that terminate to them to act as a heat sink. This practice aids in removing damaging heat from the terminal and device while distributing it evenly onto the conductive electrical wire that is attached. This is the reason that the NEC based the continuous load at 125% in order to limit the current on the device ensuring reliable operation. Due to the various conditions of use (number of current carrying conductors and elevated ambient temperatures above 30C) the 400 amp overcurrent protective device is trying to use the 500 Kcmil to effect this process. This could account for the increased temperature on the short sections of THHN closest to the terminations due to inadequate size of the heat sink needed..or in this case the 700 Kcmil.
CONCLUSION
if the continuous load is 309 amps, the OCPD has to be 125% of that load. This results in a minimum 400 amp OCPD. This OCPD has to protect the conductors and most notable the insulation on those conductors. When a conductors ampacity is changed due to conditions of use (# of current carrying conductors or elevated ambient temperaures) the newly calculated ampacity of a conductor has to be protected. If the new ampacity is 299 amps there is no way it can be protected by a 400 amps OCPD. Can you decrease the size of the OCPD....the easy answer to that is NO...because as per the NEC, based on the 309 amp continuous load present the OCPD has to be no less than 125% of that load.
This will force you to increase the conductor size even if a conductor can handle the ACTUAL LOAD......after adjustments and corrections. At the end of the day you have to follow 240.4!
Hope you all enjoyed this lesson........
First - For simplicity we will use the rules under 210 for branch circuits. However, the rules also apply for feeders under 215.2 as well. The ambient is 105° F and the system is a 3-phase -4 wire system with nonlinear loads over 50 %. Yes, the neutral is countered as a current carrying conductor.
STEP 1 - 210.19(A)(1)(a) - The continuous load at 125% and the noncontinous load at 100%. The load is 309 Amps of continuous load only.
NEC Section 210.19(A)(1)(a) and/or 215.2(A)(1)(a) Calculation - 309 Amps (Actual Load) x 1.25 = 386.25 Amperes
STEP 2 - 210.19(A)(1)(B) - States that the minimum branch circuit conductor size shall have an ampacity not less than the maximum load (309 amps) after the application of any adjustments (more than 3 current carrying conductors)(.80) or temperature corrections (.87) based on 310.15(B)(2)(a) and (B)(3)(a).
The National Electrical Code (NEC) permits that once that conductors ampacity is corrected and adjusted, the installer is now required to protect the conductor at it’s new ampacity value. This will be discussed in more detail later in this article/lesson.
THE CALCULATION FOR STEP 2 PREVIOUSLY DISCUSSED
Based on 310.15(B) and 310.15(B)(16) of the National Electrical Code, A 500 Kcmil CU, THHN is rated for 430 Amps @ 90°C and is permitted to be used to determine the minimum size of the conductor when doing adjustments and corrections for various environmental and physical conditions of use. As long as the final new ampacity value is adequate for the load.
430 Amps x .80 (4 Current Carrying Conductors) x .87 (105-113° F) = 299 amps
THE REAL CONCERN
The National Electrical Code (NEC) states that you have to choose the larger of the two (regardless of being a branch circuit or a feeder calculation) . 210.19(A)(1)(a) results are 386.25 amps at 75° C and 210.19(A)(1)(b) after adjustments and corrections (and using 90° C conductors, resulted in 299 amps.
As an important reminder, NEC Table 310.15(B)(16) is based on no more than 3 current carrying conductors and an ambient temperature of 30°C (86°F). Anything otherwise changes the actual ampacity value of the conductor.
TERMINAL LIMITATIONS, CONDUCTORS, and OVERCURRENT PROTECTION DEVICES
Based on NEC Section 110.14(C), a 500 Kcmil CU terminal ratings is 75°C; shown in NEC Table 310.15(B)(16) as 380 amps. However, Since this condition of use results in more than 3 current carrying conductors and being located in an ambient temperature environment that exceeds 30° C by upwards of 10-15°C. it is of little use except to ensure the user initially selects a conductor with enough initial ampacity to meet the terminal limitations at 75° C.
Traditionally, the electrical engineer will break down the calculation into the Overcurrent Protection Device (OCPD) and the Conductors to better understand what roll each component plays. As expressed in Step 1 earlier, The conductor must be sized no less than 125% of the continuous load, in our case it was 309 amps x 1.25 = 386.25 amps. A 500 Kcmil CU is listed at 75C for 380 amps maximum.
In step 2, since derating has to take place the National Electrical Code (NEC) permits the use of the 90° C ampacity values in Table 310.15(B)(16) for adjustments and corrections. The end result requires at least a conductor that can handle the actual load (309 amps) after the adjustments and corrections take place. In this analysis example, is was determined to be 299 amps.
The NEC requires the conductor be no less than the larger of the two above. In this analysis, the 309 amps conductor would not acceptable at the 105-113° F range.
Based solely on the above information an engineer could prematurely conclude that the conductor should have been no less than 600 kcmil CU. However, there is yet another potential concern, from a reliability standpoint, that your designer should consider.
PROTECTING THE ELECTRICAL INSULATION FROM BREAKDOWN
When engineers size the overcurrent protection for the branch circuit (or feeder) as described in 210.20(A) and 215.3 respectfully, they also have to choose the appropriate overcurrent protective device as expressed in 240.4(B) and 240.6(A). Let’s examine 210.20(A), which also applies to 215.3.
Section 210.20(A) states that the Overcurrent Protection Device “Shall Not” be less than 125% of the continuous load and 100% of noncontinuous loads. In this analysis , an actual load of 309 amps. Basically, it results in 309 amps x 1.25 = 386.25 amps. Based on 240.6 this would require a 400 amps overcurrent protection device due to the “next size up” rule found in 240.4(B).
The Problem - the newly adjusted ampacity of the 500 Kcmil is only 299 amps on the given ambient and the 4 current carrying conductors. The NEC demands in section 240.4, a conductor has to be protected against overcurrent (short circuit, ground-fault and overload) in accordance with their ampacities specified in 310.15(which includes all the adjustment and correction values).
The NEC will not permit a conductor with a new ampacity value of 299 amps to be protected by a 400 amp Overcurrent Protection Device (OCPD), ignoring the rare motor and HVAC situations expressed in 240.4(A)-(G), which is not the case in our lesson. In fact, it would be either a 300 or 350 amp OCPD at face value. However, that still will not work, the minimum sized Overcurrent Protection Device is 400 amps per 210.20(A) and/or 215.3, which will require a conductor that can legally be protected by that 400-amp device.
WHICH CONDUCTOR BASED ON THIS “CONDITION OF USE” CAN CORRECTLY TERMINATE ON A 400 AMP OCPD
Is a 600 Kcmil acceptable? - 475 amps @ 90°C x .80 x .87 = 330.6 amps which must be protected by a 350 amps OCPD - Answer is No; it can not in this case be protected by a 400 amp OCPD.
Is a 700 Kcmil acceptable? - 520 amps @ 90°C x .80 x .87 = 361.92 amps is acceptable to be protected by a 400 amp OCPD in accordance with the “next size up” rule in 240.4(B). Answer is Yes.
A 400 amp OCPD device on a conductor that has a newly corrected and adjusted (derated) ampacity value of 299 amps is not actually protecting the conductors as demanded in NEC Section 240.4 or 210.20(B) or 215.3 and will continue to remains subject to overload, short-circuit or ground-fault conditions with long term exposure.
CIRCUIT BREAKER ANALYSIS- HEAT SINK EFFECT
Circuit Breaker manufacturers rely on electrical wire that terminate to them to act as a heat sink. This practice aids in removing damaging heat from the terminal and device while distributing it evenly onto the conductive electrical wire that is attached. This is the reason that the NEC based the continuous load at 125% in order to limit the current on the device ensuring reliable operation. Due to the various conditions of use (number of current carrying conductors and elevated ambient temperatures above 30C) the 400 amp overcurrent protective device is trying to use the 500 Kcmil to effect this process. This could account for the increased temperature on the short sections of THHN closest to the terminations due to inadequate size of the heat sink needed..or in this case the 700 Kcmil.
CONCLUSION
if the continuous load is 309 amps, the OCPD has to be 125% of that load. This results in a minimum 400 amp OCPD. This OCPD has to protect the conductors and most notable the insulation on those conductors. When a conductors ampacity is changed due to conditions of use (# of current carrying conductors or elevated ambient temperaures) the newly calculated ampacity of a conductor has to be protected. If the new ampacity is 299 amps there is no way it can be protected by a 400 amps OCPD. Can you decrease the size of the OCPD....the easy answer to that is NO...because as per the NEC, based on the 309 amp continuous load present the OCPD has to be no less than 125% of that load.
This will force you to increase the conductor size even if a conductor can handle the ACTUAL LOAD......after adjustments and corrections. At the end of the day you have to follow 240.4!
Hope you all enjoyed this lesson........