can you copy those few pages and email it to me?
thanks.[/quote]
450.3 Overcurrent Protection. Overcurrent protection of transformers shall comply with 450.3(A), (B), or (C). As used in this section, the word transformer shall mean a transformer or polyphase bank of two or more single-phase transformers operating as a unit.
Informational Note No. 1: See 240.4, 240.21, 240.100, and 240.101 for overcurrent protection of conductors.
Informational Note No. 2: Nonlinear loads can increase heat in a transformer without operating its overcurrent protective device.
(A) Transformers Over 600 Volts, Nominal. Overcurrent protection shall be provided in accordance with Table 450.3(A).
(B) Transformers 600 Volts, Nominal, or Less. Overcurrent protection shall be provided in accordance with Table 450.3(B).
Exception: Where the transformer is installed as a motor control circuit transformer in accordance with 430.72(C)(1) through (C)(5).
Table 450.3(A) Maximum Rating or Setting of Overcurrent Protection for Transformers
Over 600 Volts (as a Percentage of Transformer-Rated Current)
Location Limitations Transformer Rated Impedance Primary Protection over 600 Volts Secondary Protection (See Note 2.)
Over 600 Volts 600 Volts or Less
Circuit Breaker (See Note 4.) Fuse Rating Circuit Breaker (See Note 4.) Fuse Rating Circuit Breaker or Fuse Rating
Any location Not more than 6% 600%
(See Note 1.) 300%
(See Note 1.) 300%
(See Note 1.) 250%
(See Note 1.) 125%
(See Note 1.)
More than 6% and not more than 10% 400%
(See Note 1.) 300%
(See Note 1.) 250%
(See Note 1.) 225%
(See Note 1.) 125%
(See Note 1.)
Supervised locations only (See Note 3.) Any 300%
(See Note 1.) 250%
(See Note 1.) Not required Not required Not required
Not more than 6% 600% 300% 300%
(See Note 5.) 250%
(See Note 5.) 250%
(See Note 5.)
More than 6%
and not more
than 10% 400% 300% 250%
(See Note 5.) 225%
(See Note 5.) 250%
(See Note 5.)
Notes:
1. Where the required fuse rating or circuit breaker setting does not correspond to a standard rating or setting, a higher rating or setting that does not exceed the next higher standard rating or setting shall be permitted.
2. Where secondary overcurrent protection is required, the secondary overcurrent device shall be permitted to consist of not more than six circuit breakers or six sets of fuses grouped in one location. Where multiple overcurrent devices are utilized, the total of all the device ratings shall not exceed the allowed value of a single overcurrent device. If both circuit breakers and fuses are used as the overcurrent device, the total of the device ratings shall not exceed that allowed for fuses.
3. A supervised location is a location where conditions of maintenance and supervision ensure that only qualified persons monitor and service the transformer installation.
4. Electronically actuated fuses that may be set to open at a specific current shall be set in accordance with settings for circuit breakers.
5. A transformer equipped with a coordinated thermal overload protection by the manufacturer shall be permitted to have separate secondary protection omitted.
Table 450.3(B) Maximum Rating or Setting of Overcurrent Protection for Transformers
600 Volts and Less (as a Percentage of Transformer-Rated Current)
Protection Method Primary Protection Secondary Protection (See Note 2.)
Currents of 9 Amperes or More Currents Less Than 9 Amperes Currents Less Than 2 Amperes Currents of 9 Amperes or More Currents Less Than 9 Amperes
Primary only protection 125% (See Note 1.) 167% 300% Not required Not required
Primary and secondary protection 250% (See Note 3.) 250% (See Note 3.) 250% (See Note 3.) 125% (See Note 1.) 167%
Notes:
1. Where 125 percent of this current does not correspond to a standard rating of a fuse or nonadjustable circuit breaker, a higher rating that does not exceed the next higher standard rating shall be permitted.
2. Where secondary overcurrent protection is required, the secondary overcurrent device shall be permitted to consist of not more than six circuit breakers or six sets of fuses grouped in one location. Where multiple overcurrent devices are utilized, the total of all the device ratings shall not exceed the allowed value of a single overcurrent device.
3. A transformer equipped with coordinated thermal overload protection by the manufacturer and arranged to interrupt the primary current shall be permitted to have primary overcurrent protection rated or set at a current value that is not more than six times the rated current of the transformer for transformers having not more than 6 percent impedance and not more than four times the rated current of the transformer for transformers having more than 6 percent but not more than 10 percent impedance.
(C) Voltage Transformers. Voltage transformers installed indoors or enclosed shall be protected with primary fuses.
Informational Note: For protection of instrument circuits including voltage transformers, see 408.52.
