Choose the best answer:

1. What size OCPD is required in the primary side (480V, single-phase) of a 2-wire to 2-wire transformer supplying a secondary (120V, single-phase) panelboard rated at 200A? Note that the panelboard is relying on the primary OCPD for protection.

(a) 50A
(b) 60A
(c) 70A
(d) 80A

2. What size OCPD is required in the primary side of a 200kVA, 480V, 3-phase transformer that uses primary protection only? (Round down to size OCPD.)

(a) 200A
(b) 225A
(c) 300A
(d) 350A

3. What size OCPD is required in the primary side of a 200kVA, 480V, 3-phase transformer using primary and secondary protection? (Round down to size OCPD.)

(a) 350A
(b) 400A
(c) 500A
(d) 600A

4. What size OCPD is required in the secondary side of the transformer in question 3, if the secondary is 208/120V, 3-phase? (Round up to size OCPD.)

(a) 500A
(b) 600A
(c) 700A
(d) 800A

5. What size OCPD is required in the primary side of a .8kVA, 480V, single-phase transformer? (Round down to size OCPD.)

(a) 3A
(b) 6A
(c) 8A
(d) 10A

6. What size OCPD is required in the primary side of a .8kVA, 480V, single-phase 2-wire control transformer? (Round down to size OCPD.)

(a) 6A
(b) 10A
(c) 12A
(d) 15A

Answers and Discussion

1. (a). Conductors supplied by the secondary side of a single-phase transformer having a 2-wire (single voltage) secondary shall be considered as protected by overcurrent protection provided on the primary (supply) side of the transformer—providing this protection is in accordance with Sec. 450-3 and does not exceed the value determined by multiplying the secondary conductor ampacity by the secondary-to-primary transformer voltage ratio. Solution: The size of the OCPD is computed as follows:

120V/480V x 200A = 50A

Stallcup’s Code Loop: Secs. 240-3(f), 240-21(c)(1), 384-16(e), Ex., and 240-6(a).

2. (c). A transformer 600V or less, nominal, having individual OCPD on the primary side shall be sized at no more than 125% of its full-load current rating. Solution: The size of the OCPD is computed as follows:

A = 200kVA x 1000/480V x 1.732

A = 241A

241A x 125% = 301.25A

301.25A requires a 300A OCPD (round down).

Stallcup’s Code Loop: Secs. 450-3(b), Table 450-3(b), and 240-6(a).

3. (d). You can provide protection for both the primary and secondary sides of a transformer. A current value of 250% of the rated primary current may be used if 125% of the rated primary current is not sufficient to allow loads with high inrush currents to start and operate. Solution: The size of the OCPD is computed as follows:

A = 241A x 250%

A = 602.5A

602.5A requires 600A OCPD (round down).

Stallcup’s Code Loop: Secs. 450-3(b), Table 450-3(b), and 240-6(a).

4. (b). See explanation in answer to problem 3. Solution: The size of the OCPD is computed as follows:

A = 200kVA x 1000/208V x 1.732

A = 555A

555A x 125% = 693.9A

693.9A requires 700A OCPD (round up).

Stallcup’s Code Loop: Secs. 450-3(b), Table 450-3(b), and 240-6(a).

5. (a). When the rated primary current of a transformer is less than 2A, an OCPD rated or set at not more than 300% shall be used. Solution: The size of the OCPD is computed as follows:

A = .8kVA x 1000/480V

A = 1.6A

1.6A x 300% = 4.8A

4.8A requires 3A OCPD (round down).

Stallcup’s Code Loop: Secs. 450-3(b), Table 450-3(b), and 240-6(a).

6. (a). The primary OCPD may be used to provide the protection for the conductors connected from the secondary side of a single-phase, 2-wire motor control transformer, if its rating does not exceed the secondary-to-primary voltage ratio of the transformer. Solution: The size of the OCPD is computed as follows:

A = 1.6A x 500%

A = 8A

8A requires 6A OCPD (round down).

Stallcup’s Code Loop: Secs. 430-72(c)(4) and 240-6(a).