Calculations: Transformers and Service Conductor Calculations

Calculations: Transformers and Service Conductor Calculations
©Ted “Smitty” Smith 2008, 2011, 2014, 2017

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Transformer Overcurrent Protection Article 450
Primary Overcurrent Protection Secondary Overcurrent Protection Find the rated kilovolt-amperes or "KVA" of the transformer. Refer to transformer specifications. As a example, assume 20 KVA. Find the primary voltage of the transformer or "Vprimary." Refer to transformer specifications. As an example, assume the primary voltage is 480-volts. Calculate the primary current flow, or "Iprimary," using the formula Iprimary = KVA x 1000/Vprimary. Using the example numbers: Iprimary = (20 x 1000)/480 = 20,000/480 = 41.6 amps. Note: If you had a 3-phase transformer, the formula would be Iprimary = KVA x 1000/(Vprimary x 1.732). The accounts for the 3-phase configuration. Find the circuit breaker size for the primary side of the transformer by multiplying Iprimary by 1.25. Continuing with the example: Primary circuit breaker size = 41.6 x 1.25 = 52 amps Note: Be sure to look at Table 450.3(B) for the increase because less than transformers with currents less than 9 amperes are multiplied by 1.67 and transformers with currents less than 2 amperes are multiplied by 3. Also this is all based on Primary protection only. Please keep in mind that if the transformer will have overcurrent protection on both the primary and secondary, then the primary current is multiplied by 2.5 according to Table 430.3(B) Find the secondary voltage of the transformer or "Vsecondary." Refer to transformer specifications. As an example, assume the secondary voltage is 240-volts: Calculate the secondary current flow, or "Isecondary," using the formula Isecondary = KVA x 1000/Vsecondary. Using the example numbers: Isecondary = (20 x 1000)/240= 20,000/240 = 83.3 amps. Note: If you had a 3-phase transformer, the formula would be Isecondary = KVA x 1000/(Vsecondary x 1.732). The accounts for the 3-phase configuration. Find the circuit breaker size for the secondary by multiplying Isecondary by 1.25. Continuing with the example: Secondary circuit breaker size = 83.3 x 1.25 = 104 amps. NOTE: Please keep in mind that if a transformer has overcurrent protection in the secondary side, it also requires overcurrent protection in the primary side. The overcurrent protection on the primary side can exceed 250% if the rated primary current The overcurrent protection in the secondary side can not exceed 125% for current of 9 amperes or more or 167% for current less than 9 amperes for the secondary rated current.

Secondary Conductor Tap Rules
10’ Tap Rule (C)(2) 25’ Tap Rule (C)(6) Secondary conductors can be run up to 10 ft without overcurrent protection, but they must be installed in accordance with the following requirements: (1) The ampacity of the tap conductor is: Not less than the computed load in accordance with Article 220, and Not less than the rating of the device supplied by the tap conductors or the overcurrent protective device at the termination of the tap conductors. (2) The secondary conductors do not extend beyond the switchboard, panelboard, disconnecting means, or control devices they supply. (3) The secondary conductors are enclosed in a raceway. Secondary conductors can be run not over 25 ft without overcurrent protection at the point they receive their supply, but they must be installed in accordance with the following requirements: (1) The secondary conductors have an ampacity that (when multiplied by the ratio of the secondary-to-primary voltage) is at least 1/3 of the rating of the overcurrent device protecting the primary of the transformer. (2) The secondary conductors terminate in a single circuit breaker or set of fuses that have a rating not greater than the conductor ampacity. (3) The secondary conductors are protected from physical damage. Copyright 2005 Ted "Smitty" Smith

Phase and Line Relationships
Delta Wired Transformer Wye Wired Transformer Coils are wired in series Line voltage is equal to phase voltage Line current is times greater than phase current Coils are wired in parallel Line voltage is times greater than phase voltage Line current is equal to phase current

Service Conductors Dwelling Units Non- Dwelling Units
A load calculation is needed to determine the service size rating. Service conductors are based on the size of the service rating. The service rating is based off the size the main breaker for the service. Remember to base the conductor ampacity off of 83% of the service rating according to (B)(7). Everything remains the same as dwelling units except you can no base the ampacity of 83% of the service rating. That only applies to dwelling units.

Question 1 What is the maximum sized inverse time circuit breaker that can be used to protect the primary side of a 30 kVA, three phase transformer which will be wired Delta, Wye and fed 480 volts three phase? The transformer will have only primary protection. A. 45 B. 40 C. 70 D. 55

Answer 1 45 ampere Table 430.3(B) 30,000/(480x1.732)= 45.1 amperes
Copyright 2005 Ted "Smitty" Smith

Question 2 What is the maximum standard rated inverse time circuit breaker which can be used to protect the primary of a 240V to 120V, single phase transformer which is rated at 2 kVA? The transformer will have only primary protection. A. 10 A B. 15 A C. 20 A D. 25 A

Answer 2 B. 15 A Table 430.3(B) and Note 1 2,000/240= 8.33 A
8.33 A x 1.67= A Answer 2 Copyright 2005 Ted "Smitty" Smith

Question 3 What is the maximum sized standard fuse that can be used to protect the primary side of a 45 kVA, three phase transformer which will be wired Delta, Wye and fed 480 volts three phase? The transformer will have both primary and secondary overcurrent protection. A. 150 A B. 135 A C. 125 A D. 110 A

Answer 1 C. 125 A Table 430.3(B) 45,000/(480x1.732)= 54.128 A
A x 2.5= A Table 430.3(B) does not allow us to use note 1 for Primary and Secondary protected transformers, therefore you have to go down to the next size fuse or CB which is 125A. Answer 1 Copyright 2005 Ted "Smitty" Smith

Question 4 What is the maximum standard sized fuse which can be used to protect the secondary of a single phase 120V to 24 V, 0.5 kVA transformer? The primary overcurrent protection does not exceed 250% of the primary current rating. A. 20 B. 25 C. 30 D. 40

Question 5 Transformers with ventilated openings shall be installed so that the ventilated openings are not blocked by walls and no more than _________ grounding lugs can be placed on the ventilated openings. A. 1 B. 2 C. 3 D. 0

Question 6 What is the minimum sized THHN conductor which can be used to feed a 208V, three phase, 200 A panel which is fed from a 480V/ 208V, 80 kVA, three phase transformer? The secondary conductors are 8 feet in length, there is no secondary overcurrent protection and the primary overcurrent protection is a 125A inverse time circuit breaker. Assume all terminations are rated at 75 degrees C. A. 1/0 AWG B. 3/0 AWG C. 4/0 AWG D. 250 kcmil

B. 3/0 10 foot tap rule. Must have an ampacity equal to the rating of the equipment terminated at which is 200 A (C) also states we have to use the termination temperature rating column of (B)(16) which is 75 C. Answer 6 Copyright 2005 Ted "Smitty" Smith

Question 7 What is the minimum sized RHH aluminum conductor which can be used to feed a three phase, 480V to 208V, delta-wye, 45 kVA transformer which has primary only protection of 70 amperes? Termination temperature ratings are unknown. A. 4 AWG B. 3 AWG C. 2 AWG D. 1/0 AWG

C. 2 AWG Must have an ampacity which his large enough to handle the load and the overcurrent protective device rating can not exceed the conductor ampacity. Therefore we need a conductor which can handle 70 amperes. 110.14(C) requires us to coordinate using the lowest temperature rating. We do not now the temperature ratings, therefore we have to use the defaults in (C). The transformer is rated less than 100 A therefore we have to base the ampacity on the 60 C column of (B)(16) Answer 7 Copyright 2005 Ted "Smitty" Smith

Question 8 What is the minimum sized USE, aluminim service entrance cable which can be used to feed a 200A service for a dwelling unit that has 2800 square feet of habitable space and is equipped with a 12 kW a 5.2 kW electric clothes 240V, and a 3.2 kW electric water 240 V? The service is 240/120 single phase. Assume all terminations are rated at 75 degrees C. A. 250 kcmil B. 4/0 AWG C. 3/0 AWG D. 2/0 AWG

Answer 8 B. 4/0 AWG Table 310.15(B)(16) 310.15(B)(7) 200 x .83= 166 A
All terminations at 75 degrees C and USE is a 75 C conductors so we will use the 75 C column Answer 8 Copyright 2005 Ted "Smitty" Smith

Question 9 What is the demand on the service conductors of a dwelling unit for the fastened in place appliances if it is equipped with a .5 hp, 120V garbage disposal, a 3.8 kW water 240 v, a 1.2 kW 120V and a 2.5 t capacity Air Conditioner that has a minimum circuit ampere rating of V? A VA B VA C VA D VA

C VA 220.53 Air conditioner is not a fastened in place appliance according to Current for GD is based on Table which is 9.8 A. 9.8 x 120= 1176 = 6176 VA does not allow a squeeze unless there are four or more appliances on the feeder. Answer 9 Copyright 2005 Ted "Smitty" Smith

Question 10 What is the minimum wire bending space required at the ungrounded conductor terminal lugs of a 480V/ 208V, three phase , 30 kVA transformer which is fed with 3/0 THHN conductors? The conductors will enter the transformer on the side not opposite the terminations. A. 6 inches B. 8 inches C. 4 inches D. 2 inches

Answer 10 C. 4 inches 450.12 Table 312.6(A)

Question 11 What is the minimum sized THHN conductor which can be used to feed a three phase, 480V service for a restaurant using the optional method of calculation? The restaurant will have a total connected load of 300 kVA and it will be an all electric restaurant. Assume all terminations are rated for 75 degrees C. A. 1/0 AWG B. 2/0 AWG C. 4/0 AWG D. 250 kcmil

Answer 11 C. 4/0 220.88 Optional restaurant calc
Total connected load of 300 kVA is 100 kVA over 200. 10% of 100 is is 170 kVA 170,000/ (480 x 1.732)= amperes A 225 Ampere service required. Using the 75 C column of (B)(16) due to (C) requirements a 4/0 THHN will be required. Answer 11 Copyright 2005 Ted "Smitty" Smith

Question 12 A 40 kVA, dry type transformer is installed in the hollow space of a building and all ventilation requirements are met. What type of access is required to this transformer? A. Accessible as applied to equipment B. Readily accessible C. Accessible In a transformer vault D. None of the above

Answer 12 A. Accessible as applied to equipment 450.13(B)
Definition of Accessible as applies to equipment in Article 100 Answer 12 Copyright 2005 Ted "Smitty" Smith

Question 13 The floors of a transformer vault that are in contact with earth shall be of concrete that is not less than ____________ inches thick. A. 2” B. 4” C. 6 “ D. 12”

Question 14 A dry type transformer installed indoors that is rated 30 kVA shall have a separation of at least _______________ inches from combustible material. A. 3” B. 9” C. 12” D. 24”

Question 15 A three phase, four wire autotransformer shall be equipped with a ____________________ that causes the opening of a main switch or common-trip overcurrent device to guard against single-phasing or internal faults. A. Ground Fault Interrupter B. Fault-sensing system C. Universal Equilibrium Adjustment D. Short Circuit, Branch Circuit Protection

Answer 15 B. Fault-sensing device 450.5(A)(3)

Question 16 What is the primary line current for a three-phase, delta-wye, 480V to 208V, 30 kVA transformer? A A B C A D A

Answer 16 C. 36.09 Amperes 30,000/ (480 x 1.732)= 36.09 amperes

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Question 1 What is the maximum sized inverse time circuit breaker that can be used to protect the primary side of a 30 kVA, three phase transformer which will be wired Delta, Wye and fed 480 volts three phase? The transformer will have only primary protection. A. 45 B. 40 C. 70 D. 55

Answer 1 45 ampere Table 430.3(B) 30,000/(480x1.732)= 45.1 amperes