Transformers Nivel Dos: Revenge of the Xfrmr OCPD w/ Assoc. Tap Rules

Based upon the figure and in accordance with Section 240.21(B)(3), answer the following questions. -------------------------- Calculate the maximum conductor length permitted between the transformer secondary conductor terminals and the line side of the secondary overcurrent protection device. ------------------------- Calculate the minimum size THWN copper conductor permitted on the primary side of the transformer. (Assume terminations are rated for 75°C.) ------------------------- The 2/0 AWG THWN copper conductors have an ampacity that meets the requirements of 240.21(B)(3)(2). (Note: The answer is required to be based upon calculations.)

15 FEET 240.21(B)(3)(3)Total permitted length = 25'Primary length = 10'Secondary length = 25' − 10' = 15'15' maximum -------------- 4 AWG 240.21(B)(3)(1)225 A circuit breaker × 1/3 = 75 ATable 310.16, Next larger wire size 75 A → 85 A using a 4 AWG THWN copper4 AWG THWN copper rated at 85 A ------------------ TRUE 240.21(B)(3)(2) Vratio=(Vprimary)/(Vsecondary)=440/220=2.0 Feeder OCPD = 225 A circuit breakerTable 310.162/0 THWN = 175 A175 A must NOT be less than:2 × 1/3 × 225 A = 150 A175 A is not less than 150 A2/0 AWG THWN copper is acceptable

An installation consists of a 10-kVA, single-phase transformer with a 440-volt primary and a 110-volt, 2-wire secondary using THWN copper conductors. No secondary OCPD is installed. The secondary loads are considered to be noncontinuous loads. Calculate the minimum size secondary conductors required to handle the secondary full-load current for this transformer. (Assume terminations are rated for 75°C.)

3 AWG

A transformer, rated 30-kVA, 3-phase, 480-volt primary and a 208/120-volt secondary, is installed with primary and secondary protection where the primary overcurrent protective device is rated 50-amperes. The transformer supplies a 3-phase, 4-wire, 208/120-volt, 100-ampere main circuit breaker panelboard with secondary conductors which are not longer than 10 feet in length. The panelboard supplies 75 amperes of noncontinuous load.What is the minimum size THWN secondary conductors required for compliance with 240.21(C)(2)(1)?

3 AWG THWN 240.21(C)(2)(1)(a) requires conductors adequate for 75 amperesTable 310.16Select 4 AWG THWN copper = 85 A240.21(C)(2)(1)(b) requires conductors to be adequate for 100 ampere main circuit break in panelboard they supplyTable 310.16Select 3 AWG THWN copper = 100 ATherefore, the correct answer is 3 AWG THWN suitable for both the load and the 100-ampere main circuit breaker.

A transformer, rated 30-kVA, 3-phase, 480-volt primary and a 208/120-volt secondary, is installed with primary and secondary protection where the primary overcurrent protective device is rated 50-amperes. The transformer supplies a 3-phase, 4-wire, 208/120-volt, 100-ampere main circuit breaker panelboard with secondary conductors which are not longer than 10 feet in length. The panelboard supplies 75 amperes of noncontinuous load.What is the minimum size THWN secondary conductors required for compliance with 240.21(C)(2)(4) and 240.21(C)(2)(1)?

3 AWG THWN 240.21(C)(2)(4) requires not less than the 1 to 10 ratio50 × 0.01 × (480/208) = 11.54 ATable 310.16Select 14 AWG THWN copper = 20 A240.21(C)(2)(1) requires a 3 AWG for the 100-ampere main circuit breakerCorrect answer is a 3 AWG THWN

For a transformer less than 1,000 volts, using primary and secondary protection, calculate the maximum number of 40-ampere, 3-pole circuit breakers that will provide the necessary secondary overcurrent protection permitted in the figure.

3 circuit breakers Table 450.3(B), Notes 1 and 2OCPDsec = Isec × 125%= 90.3 × 1.25= 112.8 A450.3(B), Note 1, Next higher size permitted240.6(A) standard sizes112.8 A → 125 A OCPDDetermine the number of 40 A, 3-pole branch circuit breakers permitted Three 40 A, 3-pole branch circuit breakers

An installation consists of a 10-kVA, single-phase transformer with a 440-volt primary and a 110-volt, 2-wire secondary using THWN copper conductors. No secondary OCPD is installed. Calculate the maximum standard size OCPD permitted by Table 450.3(B) to protect the primary of this transformer

30 A max. primary OCPD / 25 A max. primary OCPD Note: In accordance with 450.3(B), a 30 A OCPD is the maximum to protect the transformer. Dropping the size down to a 25 A OCPD still protects the transformer but also protects the secondary conductors.

Based upon the figure and using 450.3(B), calculate the maximum ampacity of a standard-size circuit breaker permitted to protect the primary side of the transformer where a secondary OCPD is provided.

300 A circuit breaker Iprimary=((kVA)x1,000)/(1.73x(Vprimary))=((112.5)x1,000)/(1.73x(480))=112,500/830.4=135.48A Table 450.3(B)OCPDpri = Ipri × 250%= 135.48 × 2.50= 338.7 ANext smaller size; use 240.6(A) standard sizes338.7 A → 300 A300 A circuit breaker The correct answer is: 300

Using the figure from the previous question, calculate the maximum standard size 2-pole circuit breaker permitted where the circuit breaker provides primary only protection for this transformer.

40 A circuit breaker

Using the figure from the previous question and using Section 450.3, calculate the maximum ampere rating of a standard-size circuit breaker permitted to protect the primary of the transformer when using primary and secondary protection.

450 A max. primary OCPD Isecondary=((kVA)x1,000)/(1.73x(Vsecondary))=((150)x1,000)/(1.73x(440))=150,000/761.2=197.06A Table 450.3(B)OCPDpri = Ipri × 250% = 197.06 × 2.50 = 492.65 ANext smaller size; use 240.6(A) standard sizes492.65 A → 450 A450 A OCPDpriNote: The 360 A secondary OCPD selected is less than the 450 A max. OCPD permitted. Thus, the 360 A OCPD qualifies the transformer for primary plus secondary overcurrent protection (250% for primary current over 9 A) as a protection method allowed by Table 450.3(B).

Based upon the figure and using 240.21(C)(3), calculate the minimum size THWN copper conductor required for the protection secondary conductors.

500 kcmill 240.21(c)(3), 25-foot rule Isecondary=((kVA)x1,000)/(1.73x(Vsecondary))=((150)x1,000)/(1.73x(240))=150,000/415.2=361.27 A Table 310.16361.27 = 500 kcmil THWN copper rated 380 ATable 450.3(B)OCPDsec = Isec × 125%= 361.27 × 1.25= 451.58 ATable 450.3(B), Note 1, Next size larger451.58 A → 500 ATable 450.3(B), Note 2OCPDsec = Total all circuit breakers= 40 + 50 + 60 + 75 + 45 + 90= 360 A360 A qualifies as OCPDs at not greater than 125%360 A requires 500 kcmil THWN500 kcmil THWN

An installation consists of a 7.5-kVA, single-phase transformer, a 277-volt primary, a 120-volt, 2-wire secondary using THWN copper conductors and no secondary OCPD. The secondary loads are considered to be noncontinuous loads. Calculate the minimum size conductors required for the secondary full load current of the transformer where there is no secondary overcurrent protection provided. (Assume terminations are rated for 75°C.)

6 awg

Using 240.21(C)(4), calculate the minimum wire size of an outside underground feeder using THWN copper conductors from the secondary of the transformer to the outdoor overcurrent protective device shown in the illustration.

600 kcmil 240.21(C)(4) Table 310.16361.27 A = 500 kcmil THWN copper500 kcmil THWN rated 380 AOCPD = 400 A circuit breaker240.21(C) violation500 kcmil THWN too smallUse 600 kcmil THWNAnswer: 600 kcmil THWN copperNote: The last sentence of 240.21(C) does not permit the use of 240.4(B) (going to the next higher OCPD rating) for transformer secondary conductors. The choice of 500 kcmil THWN copper is rated only for 380 A according to Table 310.16 and is therefore too small for this application according to 240.21(C)(4)(2). The choice of a conductor at or above the circuit breaker rating of 400 A is required. The selection is then changed to a 600 kcmil THWN with a rating of 420 A. The Code does not place a limit on the length of these outside secondary conductors as there is with other transformer tap rules.

Using the figure from the previous question, 240.21(C)(6), and Note 1 from Table 450.3(B), determine the minimum size THWN copper conductor required for the secondary conductors where the primary is protected at 250% and the secondary is protected at 125%.

600 kcmil 240.21(C)(6), 25-foot rule Iprimary=((kVA)x1,000)/(1.73x(Vprimary))=((112.5)x1,000)/(1.73x(480))=112,500/359.84=312.64A Table 450.3(B)OCPDsec = Isec × 125%= 312.64 × 1.25= 390.8 ANext larger size; 240.6(A) standard sizes390.8 A → 400 A240.21(C) i.e., 240.4(B) cannot applyWire size must equal or exceed OCPD sizeTable 310.16500 kcmil THWN at 380 A is too small420 A = 600 kcmil THWN copper600 kcmil THWN copperNote: Additionally, the ratio required to be verified by 240.21(C)(6)(1) may be expressed as 420 A and must be greater than: Solving this equation, the answer is true. The ampacity of the secondary conductors does in fact exceed this ratio.

An installation consists of a 25-kVA, single-phase transformer with a 480-volt primary and a 208-volt, 2-wire secondary using 1 AWG THWN copper secondary conductors without any secondary conductor overcurrent protection provided. Calculate the maximum standard-size OCPD permitted by Table 450.3(B) to protect the primary of this transformer. (Do not account for the size of the secondary conductors.)

70 A max. primary OCPD / 50 A max. primary OCPD

The figure illustrates a transformer supplied from a feeder tap and terminating in a circuit breaker located as primary protection for this transformer. --------------------- Using the figure, calculate the minimum wire size of the eight-foot tap from the feeder to the circuit breaker protecting the transformer. (Assume terminations are rated for 75°C.)

8 AWG

A 3-phase, 75 kVA transformer with a 480-volt primary and a 240-volt secondary supplies a 200-ampere main circuit breaker panelboard with a connected load of 175 amperes, noncontinuous. The secondary feeder conductors are 3/0 AWG THWN copper installed in EMT. The main circuit breaker line terminals in the panelboard are located within 10 feet from transformer secondary terminals. ------------------------------------ These 3/0 AWG conductors are considered acceptable for this installation. (Note: the answer is required to be based upon calculations.) -------------------------------------- The 200-ampere main circuit breaker adequately protects the secondary windings of the transformer and is considered acceptable for this installation. (Note: The answer is required to be based upon calculations.)

TRUE 240.21(C)(2) not over 10 feetTable 310.163/0 THWN copper = 200 AConnect load = 175 A Min. cond. rating = 180.64 A3/0 AWG THWN is acceptable --------------------------- TRUE Table 450.3(B)OCPDsec = Isec × 125% = 180.64 × 1.25 = 225.79Next smaller OCPD is permitted225.79 A → 225 A200 A OCPD is acceptableNote: However, if a 225 A OCPD is desirable, the secondary conductor would need to be increased from 3/0 to a 4/0 AWG THWN in order to comply with the last sentence in 240.21(C), which reads, "The provisions of 240.4(B) shall not be permitted for transformer secondary conductors."

In general, transformer secondary conductors are not considered to be protected by the primary overcurrent protective device.

TRUE 240.4(F)