WIRES & CABLES: APPLICATIONS ON LOW VOLTAGE FEEDERS & BRANCH CIRCUITSII II: MYTHS & FACTS IN WIRES & CABLES APPLICATION 1 A TYPICAL LARGE CORPORATE OFFICE BUILDING MYTHS & FACTS IN WIRES & CABLES APPLICATION 2 THERMOPLASTIC WIRE & CABLES (TW, THW, THHN) 600 VOLTS, Stranded MM Size Approximate AWG Size 14 12 10 8 6 4 2 1 1/0 2/0 3/0 4/0 135 160 185 120 100 90 70 85 110 125 145 160 195 220 55 65 40 45 30 30 20 20 30 40 50 70 90 115 130 150 170 205 255 15 15 25 Ampacity TW 60 deg C Ampacity THW 75 deg C Ampacity THHN 90 deg C 2.0 mm2 3.5 mm2 5.5 mm2 8.0 mm2 14 mm2 22 mm2 30 mm2 38 mm2 50 mm2 60 mm2 80 mm2 100 mm2 AMPACITY TABLE (Based on Table on AVESCO pocket book) Not more than 3 conductors in a raceway or cable 3 Stranded MM Size Approximate AWG Size 250 MCM 300 MCM 400 MCM 500 MCM 650 MCM 750 MCM 800 MCM 1000 MCM 455 405 400 475 485 540 370 435 315 375 280 330 240 280 295 355 400 470 535 515 580 210 255 265 Ampacity TW 60 deg C Ampacity THW 75 deg C Ampacity THHN 90 deg C 125 mm2 150 mm2 200 mm2 250 mm2 325 mm2 400 mm2 500 mm2 AMPACITY TABLE (Based on Table on AVESCO pocket book) Not more than 3 conductors in a raceway or cable 4 . THHN) 600 VOLTS. THW.THERMOPLASTIC WIRE & CABLES (TW. 16 5 .AMPACITY TABLE (Lifted from AVESCO pocket book) Note: This Table is not quite identical with NEC Table 310. BE CAREFUL IN REFERENCING TABLES ON WIRE & CABLE AMPACITIES. BECAUSE THEY ARE NOT ACTUALLY IDENTICAL… See the differences on the next slide… 6 . PHILIPPINE & EUROPEAN MANUFACTURED CABLES 7 .COMPARISON: NORTH AMERICAN. .Just curious… WHICH IS THE MORE WIRE.? TW.? THHN...? EXPENSIVE WIRE..? 8 .? THW.. ? THW..? THHN ? THHN..? Why…? 9 .WHICH IS THE SUPERIOR WIRE? TW.. IN THE PHILIPPINES: THE MOST POPULAR BUILDING WIRES ARE THE THHN? WHY…? 10 .DO YOU AGREE THAT TODAY. It’s worthwhile revisiting the basics… How do we size a circuit…? 11 . It’s worthwhile revisiting the basics… Let’s Test Ourselves Let s with the following exercise… 12 . 16 Q: What size (TW. THW.NEC TABLE 3. THHN) conductor does the NEC require for a 50A circuit? a) # 10 b) # 8 c) # 6 d) Any of these? Note: NEC TABLE 3.16 doesn’t have mm sizes. 13 . 16 SOLUTION: SOLUTION: 1) Rating of Circuit: 50 A (Given) 2) Referring to the Table NEC 310.16: PROBABLE ANSWERS: TW: Use. Ampacity: 55A 14 . # 6 (14 mm2). Ampacity: 50A THHN: Use.0 mm2). # 8 (8. Ampacity: 55A THW: Use.0 mm2).TEST CASE NEC TABLE 3. # 8 (8. WHAT IS THE BEST CHOICE.? 15 .. If your answer is: #8 THW or #8 THHN… 16 . THAT IS A MYTH…! 17 .Sorry Guys. # 6 (14 mm2). # 6 (14 mm2). # 6 (14 mm2). Ampacity: Ampacity: 55A THW: Use. Ampacity: Ampacity: 55A @ 60 deg C Column THHN: Use.SOLUTION: SOLUTION: Rating of Circuit: 50 A (Given) THE CORRECT ANSWERS ARE: TW: Use. Ampacity: Ampacity: 55A @ 60 deg C Column 18 . .HOW COME.? LET US FIRST GO BACK TO BASICS… 19 . HOW DO WE DESIGN A NONNONMOTOR BRANCH CIRCUIT…? 20 . ) loaded i i A branch circuit is that part of a wiring system extending beyond the last or final protective device to the load it specifically serves. circuit. 21 .A safe electrical system starts from the very basic wiring fundamentals – the branch circuit. In general terms. a branch circuit could either be non-motor (as in lighting. non& computers) or motor l d d circuit. receptacle outlets. The branch circuit represents the last step in the transfer of power from the service or source of energy to utilization devices. A branch circuit will qualify as such when it has a protective device from the point of tapping. 22 . A branch circuit may have several lighting or receptacle outlets connected to it as a circuit or may serve a single load as in motor or heavy appliance. Therefore. the branch circuit is made up of : 1) ) The Over-Current Protective Device (OCPD) Over( ) 2) The Conductor (Wire or Cable) 3) The Load (Motor or Non-Motor Loads) Non- 23 . QUESTIONS: From where shall we base our loads? From where shall we base our sizing of conductors? From where shall we base our OCPD? 24 . 25 . 26 . 27 . (NEC 210Section 210-19a).BRANCH CIRCUITS – WIRING FUNDAMENTALS RELEVANT CODE REQUIREMENTS Th ampacity of b The it f branch circuit h i it conductors must not be less than the maximum load to be served. 210(NEC Section 210-3).BRANCH CIRCUITS – WIRING FUNDAMENTALS RELEVANT CODE REQUIREMENTS The rating of a branch circuit is established or defined by the rating or setting of its protective device. . BRANCH CIRCUITS – WIRING FUNDAMENTALS RELEVANT CODE REQUIREMENTS The ampacity of branch circuit conductors must not be less than the rating of the branch circuit. (NEC 210Section 210-19a). RELEVANT CODE REQUIREMENTS Circuit conductors shall be protected against overoverampacities, current in accordance to their ampacities, but where the ampacity of the conductor does not correspond with the standard ampere rating of a fuse or a circuit breaker, the next higher rating shall be p , g g permitted only if this rating does not exceed 800 amperes. (NEC amperes. Section 240-3). 240- THIS MEANS A MANDATORY MATCH BETWEEN THE CONDUCTOR AMPACITY WITH ITS OCPD (OVER(OVERCURRENT PROTECTIVE DEVICE). BRANCH CIRCUITS – WIRING FUNDAMENTALS RELEVANT CODE REQUIREMENTS The maximum continuous load that can be served by the branch circuit conductors must not be more than 80% of the ampacity of the 210conductors. (NEC Section 210-19a). 31 BRANCH CIRCUITS – WIRING FUNDAMENTALS RELEVANT CODE REQUIREMENTS The current permitted to be carried by the branch circuit conductors has to be 80% if the load is continuous. This rule refers to a limit of the load to be 210carried by the conductors. (NEC 21022c). 32 . BRANCH CIRCUITS – WIRING FUNDAMENTALS RELEVANT CODE REQUIREMENTS Continuous load refers to a load that operates for three (3) hours or more, such as store lighting, office lighting and similar loads. This rule limits the load on the circuit conductors; it does not change the ampacity of the circuit conductors or the rating or oversetting of the circuit over-current protective 210device. (NEC 210-22c). 33 BRANCH CIRCUITS – WIRING FUNDAMENTALS RELEVANT CODE REQUIREMENTS The total load on any over-current overdevice in a panelboard must not exceed 80% of the rating of the overdevice. over-current device. (NEC Section 384384-16c). 34 BRANCH CIRCUITS – WIRING FUNDAMENTALS RELEVANT CODE REQUIREMENTS SOME EXERCISES… 35 36 . 37 . 38 . 39 . 40 . 41 . ARE THE CIRCUITS DISCUSSED IN PRECEDING EXAMPLES ALREADY SAFE FROM FIRE HAZARDS…? 42 . NOT QUITE…! because… 43 . There is the so-called soDERATING & CORRECTION FACTORS 44 . Note that the Derating & Correction Factors Cause to CURRENTChange the CURRENTCARRYING CAPACITY OF THE CONDUCTORS! 45 . Correction Factors must be considered.BRANCH CIRCUITS – WIRING FUNDAMENTALS RELEVANT CODE REQUIREMENTS The normal maximum ampacities of conductors in cables or raceways are 310given in NEC Tables 310-16 (copper) on a 30 deg C ambient temperature. For ambient temperatures under or over 30 deg C. 46 . 47 . This means a change in ampacities of circuit conductors. 48 .BRANCH CIRCUITS – WIRING FUNDAMENTALS RELEVANT CODE REQUIREMENTS These normal ampacities may have to be derated where there are more than th three conductors i a cable or raceway d t in bl (Note 8 to Tables 310-16 through 31031031019). ambient 49 . IN A CONDUIT OR CABLE”. and… Correction to the conductor ampacities when installed or operated at temperatures over or under 30 deg C ambient.THUS THERE ARE TWO THINGS TO CONSIDER THAT REDUCE CONDUCTOR AMPACITIES… THESE ARE THE FOLLOWING: AMPACITY DERATING DUE TO “MORE THAN THREE (3) CURRENT CARRYING CONDUCTORS CABLE”. 0 mm2 3.5 mm2 8.5 mm2 5.THERMOPLASTIC WIRE & CABLES (TW. THHN) 600 VOLTS. THW. Stranded (NEC Table 310.0 mm2 14 mm2 22 mm2 30 mm2 38 mm2 50 mm2 60 mm2 80 mm2 100 mm2 50 .16) MM Size Approximate AWG Size # 14 # 12 # 10 #8 #6 #4 #2 #1 1/0 2/0 3/0 4/0 145 165 195 125 110 95 70 85 115 130 150 175 200 230 55 65 40 50 30 35 25 25 30 40 55 75 95 130 150 170 195 225 260 20 20 25 Ampacity TW 60 deg C Ampacity THW 75 deg C Ampacity THHN 90 deg C 2. 51 . Given: 6 # 12 THW in a conduit at ambient temperature of 40 deg C Q1: What is the effective ampacity of the conductor? Q2: What is the maximum permissible continuous load for the circuit? Q3: What is the maximum permissible nonnon-continuous load for the circuit? 52 . Load) = 14.80 or 11.264 A Max permissible load (Non-Cont.08 x 0. Load) = 14.08 x 1 or 14.88 = 14.08 A Notes: N t a) Derating Factor of 0.80 x 0.310Ampacity: # 12 THW = 20 A (NEC Table 310-16) Effective Ampacity: 20 A x 0.80 for six conductors in conduit b) Correction Factor for of 0.08 A (Non53 .88 for ambient temp of 40 deg C Max permissible load (Cont. 264 A? Q5: WHAT WILL BE THE SIZE OF THE BRANCH CIRCUIT OCPD? 54 .08 A? c) The Maximum Permissible Load which is11.Q4: WHERE SHALL WE BASE THE OCPD? a) Published Ampacity of #12 AWG which is 20 A? b) The ‘Effective’ Ampacity of #12 AWG which is14. 08 A only.WHAT WILL BE THE SIZE OF THE BRANCH CIRCUIT BREAKER OR FUSE? Answer is: 15 A Because of the mandatory match of the OCPD with respect to the ampacity of the conductor! Note that in this case. 55 . the effective ampacity of the # 12 conductor happens to be derated at 14. IF THE 20 A OCPD FOR A #12 AWG CONDUCTOR IN PREVIOUS EXERCISE IS TO BE RETAINED… THE DERATING & CORRECTION FACTORS WILL MAKE THE CIRCUIT CONDUCTORS BIGGER…! 56 . If you decide 20 A OCPD. then the size of the conductors shall be: # 10 THW Not # 12 THW! 57 . ASSUMING THAT THE DERATING & CORRECTION FACTORS HAVE ALREADY BEEN CONSIDERED. Q: IS SIZING OF CIRCUITS FINISHED…? 58 . NOT QUITE…! 59 . CONDUCTORS MUST MATCH THE TEMPERATURE RATINGS OF CIRCUIT DEVICES… 60 .BECAUSE. Properly sized conductors allow the circuit breaker thermal-sensing elements to match the conductor thermal protection requirements.g.Wiring Conductor Ampacity to Temperature Rating Molded-case circuit breakers are marked with bot the s e a d su at o te pe atu e at g both t e size and insulation temperature rating (e. #2 Cu. 61 . 60/75°C) of the conductors approved for use with the circuit breaker.. .SIZING CONDUCTORS BASED ON TEMPERATURE RATING When selecting a conductor for a circuit. one has to be selected to accommodate the temperature termination rating rules outlined in NEC 110-14(c). This includes the compatibility of a conductor type as to the OCPD’s 62 & other devices the conductor terminates inside an enclosure. The wire temperature rating is determined by testing the circuit breaker under full-load current with conductors sized for the appropriate temperature rating. 63 .SIZING CONDUCTORS BASED ON TEMPERATURE RATING Underwriter Laboratories Inc. (UL) standards require that molded-case circuit breakers rated at 125 amperes or less be marked with the conductor insulation-temperature rating. 60°C or 75°C. 64 .SIZING CONDUCTORS BASED ON TEMPERATURE RATING Conductors with 90°C rated insulation (THHN) can be used on circuit breakers rated for 60°C or 75°C wiring only if their size is based on the ampacity of the lower temperature-rated wire. If th 90°C (THHN) wire size were to be the i i t b selected based on the ampacity allowed in the 90°C column of the Ampacity Table. the smaller resulting wire size would generate additional heat at the circuit breaker terminals and possibly cause nuisance tripping. SIZING CONDUCTORS BASED ON TEMPERATURE RATING 65 . 16 66 .NEC Table 310. NEC Table 310.16 67 . wire sizes shall be based on the ‘60 deg C 60 temperature rating’ listed in NEC Ampacity Table 310-16.IMPORTANT!!! SIZING CONDUCTORS BASED ON TEMPERATURE RATING POINT 1: For device or equipment terminals rated 100 A or less. 68 . 16 69 .NEC Table 310. POINT 2: Circuits over 100A [NEC Sec. 70 . 110-14(c)(2)]: SIZING CONDUCTORS BASED ON TEMPERATURE RATING Unless the terminals are marked otherwise. equipment/ device i t/ d i terminals rated over 100A shall be sized according to the ‘75 deg C temperature rating’ listed in NEC Table 310-16. The advantage of 90 deg C wire (THHN) is that it can keep the designer from using a larger wire (when ampacity adjustments are needed).POINT 3: What then is the purpose of 90 deg C wire if we cannot use its higher ampacity? SIZING CONDUCTORS BASED ON TEMPERATURE RATING This is now the catch! Ninety deg C rated conductor ampacities like the THHN’s can not be typically used for sizing circuit conductors. greater labor & increased material costs. 71 . its rating only comes into play when adjusting conductor ampacity for elevated ambient temperature (correction due to temperature) or when bundling more than three current-carrying conductors together (derating). which require larger conduits & raceways. However. 72 . (Reference 110.14 in the NEC® for specific requirements. rated ampacity. the ampacity for a conductor with 90°C insulation (THHN) is generally greater than of a conductor of the same size but with 60°C (TW) insulation.TO REVIEW: Important in the electrical & thermal relationship for circuit components are the conductor size.) 73 . the greater ampacity of a THHN conductor with 90°C insulation is not always permitted to be used due to limitations of the terminal temperature rating and/or the requirements of the NEC®. For instance. However. the insulation temperature rating and the permissible connector device temperature limits. blocks. 74 . there are some simple rules to follow for circuits of 100A and less. holders.However. etc. disconnects. These simple rules generally should be followed because these are the norms for the device component product standards and p p performance evaluation to these standards for fuses. circuit breakers. it has to be rated for ampacity as if it were a 60° 60°C conductor [110. Higher temperature rated conductors can be used.less: Simple rules for 100 amps and less: 60° 1. even if a 90°C THHN conductor is used. 90° In other words. but the ampacity of these conductors must be as if 60° conductors. 75 . This 60° assumes all terminations are rated for 60°C rated conductors. they are 60°C rated conductors.14(C)(1)(a)(2)].. 2.14(C)(1)(a)(1)]. Use 60°C rated conductors [110. A 6 AWG. 90°C conductor has an ampacity of 75 amps per (NEC® Table 310. 90°C conductor. (Ampacities are from NEC® Table 310.For instance.16). but this ampacity can not be used for a 60°C termination. if a 90°C. For this circuit. assume an ampacity of 60A is needed in a circuit that has terminations that are rated for 60°C conductors. the ampacity of this conductor must be according to the 60°C conductor ampacity.16. which is 55A. 6 AWG conductor is evaluated.) 76 . If a 90°C conductor is to be used. what is the minimum conductor size required? The answer is 4 AWG. Conductors with higher temperature ratings can be used at their rated ampacities if the terminations of the circuit devices are rated for the higher temperature rated conductor [110. 77 . However. disconnects and circuit breakers.3.14(C)(1)(a)(3)]. such as blocks. the industry norm is that most devices rated 100A or less. have 60°C or 75°C rated terminations. 78 . If a conductor is run between two devices that have terminals rated at two different temperatures. 5. conductors with insulation rating of 75°C of higher are permitted as long as the ampacity of the conductors is not greater than the 75°C rating [110. C or D. the rules above must be observed that correlate to the terminal with the lowest temperature rating. For motors with design letters B.4.14(C)(1)(a)(4)]. For circuits greater than 100A. 100A. use conductors with at least a 75°C insulation 75° 75° rating at their 75°C rating. ampacity rating. 79 . So why would anyone ever want to use a conductor 90° 105° with a 90°C or a 105°C rating if they can’t be applied at their ampacity ratings for those temperatures? The answer lies in the fact that g p y g those higher ampacity ratings can be utilized when derating due to ambient conditions or due to exceeding more than 3 current carrying conductors in a raceway. 80 . where the ambient is 45°C. First. the lowest one must be used. Ambient: 45°C 45° Assume that an ampacity of 60A is needed in a circuit with a 75° 60° 75°C termination at one end and a 60°C termination at the other 45° end. which is 60°C.Example: Circuit ampacity required: 60 amps. since one termination temperature rating is higher than the 60° other. 81 . The first choice might be a 4 AWG TW conductor with an ampacity of 60° 70A at 60°C. which is less than the required 60. looking at the table at the bottom of Table 310. by a factor of 0. due to the 45°C ambient. This is where a conductor with a higher temperature 90° rating becomes useful.65.16. Again. This yields a new ampacity of 49.71.87 must be used. Ambient: 45°C 45° However. 82 . in the NEC® the Correction Factors reveals that the 70A 45° ampacity must be derated due to the 45°C ambient.Example: Circuit ampacity required: 60 amps. A 4 AWG THHN conductor has a 90°C ampacity of 95A. This yields a new ampacity of 82. which is adequate for the required 60A ampacity. a factor 45° of .7A. Ambient: 45°C 45° Could a 6 AWG THHN conductor be used in this application? Its 90° 45° 90°C ampacity is 75A. and therefore overheating problems could result. The amount of copper associated with a 4 AWG conductor is terminal. which seems adequate for a required ampacity of 60A.Example: Circuit ampacity required: 60 amps. 83 . a 6 AWG conductor of any insulation rating could never 60° be used in this application because the 60°C terminal requires that the smallest amount of copper is a 4 AWG for a 60A ampacity.25. required to bleed the right amount of heat away from the terminal. However. The use of less copper won’t bleed enough heat away.87 for the 45°C ambient gives a new ampacity of 65. Using the factor of 0. which require l d d) hi h i larger conduits & raceways. greater labor & increased material costs.This is now the catch! The advantage of 90 deg C wire (THHN) is that it can keep the designer from using a larger wire (when ampacity adjustments are needed). 84 . 14(C)(1) 85 .2008 NEC 110. 2008 NEC 110.14(C)(1)(a)(1…4) 86 . 2008 NEC 110.14(C)(1)(a)(4)(1…2) 87 . IS SIZING OF CIRCUITS FINISHED ? FINISHED…? NOT QUITE…! 88 .AT THIS TIME. BECAUSE THE CIRCUIT OVER-CURRENT PROTECTORS OVERMAY BE THE ONES TO START FULLA FULL-BLOWN FIRE…! How Come…? 89 . SIZING OCPD’S BASED ON TEMPERATURE RATINGS 90 . 91 . the fault duty could be kilo-amperes. kilo-amperes. if a 230v lighting panelboard is receiving supply threefrom a 500 KVA transformer. 92 . 230v For instance. transformer delivers a three-phase short circuit current at 12.INTERRUPTING RATINGS FOR OCPD’s Fault Duties (in a simplistic view) depend largely (among other factors) on the size of the source transformer and the impedance to. In like manner that a 100 KVA three-phase three12.500 KVA.0 kilo-amperes. of the cables before the points where the fault is subjecting to. secondary terminals could be as high as 30 kilo-amperes. three70 kilo-amperes. If the source transformer is 1. the three-phase fault duty at its kilo-amperes. it has to be remembered is that the use mcb’s are limited only if the enough. source transformer are small enough. are available. molded case circuit breakers (MCCB’s) available. For source transformers that are large where short circuit levels are higher. necessary.Care however on the use of miniature cb’s because they have relatively low capacities. short circuit capacities. In this case. Although models are available with ratings up to 16 KA. 93 . In any case. majority of mcb’s only have a KA. maximum breaking capacity of 9 KA. Fault Calculations are necessary. IT IS BEST TO SHOW THE EFFECTS OF UNDERRATED CIRCUIT BREAKERS IN THE CONTEXT OF THEIR INTERRUPTING CAPACITIES…! 94 .ALTHOUGH FAULT CALCULATION IS NOT WITHIN THE SCOPE OF THIS MODULE. the breaker disintegrated because the fault current is too much for the breaker to handle. THE CRCUIT BREAKER AFTER A FAULT 95 . The ‘injury’ means the condition where after interrupting a fault. the breaker ceased to be operable .6 KV MINIMUM OIL CRCUIT BREAKER BEFORE A FAULT The IC rating is the maximum amount of current that the device will open safely to relieve a fault condition without injuring itself. a voltage rating and an interrupting capacity (IC) rating.or worse.INTERRUPTING RATINGS FOR OCPD’s What is KAIC all about? Each circuit breaker has three most important ratings – a continuous current rating. A 3. INTERRUPTING RATINGS FOR OCPD’s What is KAIC all about? The IC rating is the maximum amount of current that the device will open safely to relieve a fault condition . the breaker disintegrated because the fault current is too much for the breaker to handle. 96 .or worse.without injuring itself. The ‘injury’ means the condition where after interrupting a fault. the breaker ceased to be operable . INTERRUPTING RATINGS FOR OCPD’s 97 . INTERRUPTING RATINGS FOR OCPD’s 98 . INTERRUPTING RATINGS FOR OCPD’s 99 . INTERRUPTING RATINGS FOR OCPD’s 100 . INTERRUPTING RATINGS FOR OCPD’s 101 . INTERRUPTING RATINGS FOR OCPD’s 102 . INTERRUPTING RATINGS FOR OCPD’s 103 . INTERRUPTING RATINGS FOR OCPD’s 104 . the engineer must specify the KAIC ratings of devices. it is not enough to specify the Continuous Current Rating and Voltage Rating of the breaker but most importantly.INTERRUPTING RATINGS FOR OCPD’s Therefore. these protective devices. This does not only cover the large breakers but must transcend to all breakers including the smallest branch circuits at the end points system. 105 . of the system. Molded Case Circuit Breakers can reach up to 4.INTERRUPTING RATINGS FOR OCPD’s LV Power Circuit Breakers LV PCB’s are recommended for mains & distribution feeder applications from 800 A to 6.000 A rating but good only for two interruptions 106 only. .300 A. 107 . 108 . 109 . 110 . 111 . 112 . 113 . IS SIZING OF SAFE CIRCUITS FINISHED ? FINISHED…? 114 .AT THIS TIME. NOT QUITE…! 115 . Because. DESIGNING CIRCUITS Must Consider CONDUCTOR DAMAGE DURING FAULTS! How come when Circuit Breakers are there…? 116 . DESIGNING CIRCUITS FROM CONDUCTOR DAMAGE Overcurrent protection is to open a circuit before conductors are damaged when an exists. During short circuits. point of conductor insulation is reached. overcurrent condition exists. 117 . currents to th the D i h t i it t t conductors are tremendously high that it must be removed quickly before the damage reached. ‘letb) how much ‘let-through’ current it allows to flow into the conductor.DESIGNING CIRCUITS FROM CONDUCTOR DAMAGE Assuming that the OCPD’s has sufficient interrupting capacities. 118 . a) the speed of the clearing. there are still two actions of the OCPD’s that are important in protecting circuit wires & cables. Damage ranging from slight degradation of insulation to violent vaporization of the conductor can result if the short-circuit shortwithstand is exceeded.DESIGNING CIRCUITS FROM CONDUCTOR DAMAGE Although conductors do have allowable ampacity ratings. they also have maximum shortallowable short-circuit current withstand ratings. ratings. 119 . DESIGNING CIRCUITS FROM CONDUCTOR DAMAGE 120 . DESIGNING CIRCUITS FROM CONDUCTOR DAMAGE CurrentCurrent-limiting Circuit Beaker The I2t associated with the asymmetrical current is required to be reduced to the equivalent I2t of a symmetrical current or less 121 Behavior of a Typical Current Limiting Fuse 122 123 is the wire protected when the available short40.40 KA 1. the LOW-PEAK® YELLOW™ Dual-Element fuse (30 ampere CLF) is fast acting. amperes for one-half cycle. letThus the above example is safe.850 amperes. The opening onetime of the fuse is less than one-half cycle (less than 0.000 exceeds the wire withstand? shortLOWDualUnder short-circuits.020 onecycle. Conductor protection is not a problem when the conductor is protected by currentcurrent124 limiting fuses which have an ampere rating that is the same as the conductor ampacity rating. In the circuit below.300 amperes for one cycle and 6. the prospective current let-thru by the fuse is less than 1. . Opening time & current let-through of the fuse is far lower than the wire withstand rating. short-circuit current of 40.008 seconds). In letthis particular example.85 KA Case 1: A # 10 AWG conductor can only withstand 4. It will clear & limit short circuit current before it can build up to a level higher than the wire withstand. from # 10 to # 1/0 AWG). The 40 KA short-circuit current far exceeds the withstand of the # 10 THW wire. 125 .Case 2: Does the circuit below represent a misapplication? (10 AWG THW insulated copper wire can withstand 4.. The slow acting ordinary circuit breaker (clearing time of 1 cycle) makes the circuit misapplied. one with a withstand rating greater than the short-circuit for 1 cycle (See Chart). What can be done to correct the above misapplication? 1) Use a larger size conductor (i. 2) Use an OCPD which is a current-limiting type (CLF or CLCB) such as that shown in the previous case.020 onecycle). 40 KA Yes.e.300 amperes for one cycle and 6. amperes for one-half cycle). 126 . A FULL-BLOWN FIRE ONLY NEEDS AN IGNITION. AND IGNITION IS.REMEMBER GENTLEMEN. 127 . YES – IT’S THE RESPONSIBILITY OF THE ELECTRICAL ENGINEER. SO THEN. IN SIZING CIRCUITS & FEEDERS. OTHERWISE. WE ALSO NEED TO CONSIDER THE SIZE OF THE SOURCE TRANSFORMER. WHAT SHOULD HAVE BEEN A NICELY-DESIGNED CIRCUIT ACTUALLY TURNS OUT TO BE GROSSLY WRONG & FAULTY! 128 . Therefore. circuit designing is not complete without fault calculations… The question is…: ‘ARE WE DOING IT…?’ 129 . END… THANK YOU THANK YOU THANK YOU 130 .