Current)

Secondary Protection (See Note 2.) Primary Protection over 600 -------------------------------------------------------------

Volts Over 600 Volts 600 Volts or Less

Any location ------------------------------------------------------------------------------------------------------------------------------- More than 6% and 400% 300% 250% 225% 125%
not more than 10% (See Note 1.) (See Note 1.) (See Note 1.) (See Note 1.) (See Note 1.) Supervised Any 300% 250% Not required Not required Not required
locations (See Note 1.) (See Note 1.)
only (See ------------------------------------------------------------------------------------------------------------------------------- Note 3.) Not more than 6% 600% 300% 300% 250% 250%
(See Note 5.) (See Note 5.) (See Note 5.) More than 6% 400% 300% 250% 225% 250%
and not more (See Note 5.) (See Note 5.) (See Note 5.)
than 10%
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.

Af -----------------

J Shunt winding(s)

B Figure 450.4 Autotransformer.

Exception: Where the rated input current of the autotransformer is less than 9 amperes, an overcurrent device rated or set at not more than 167 percent of the input current shall be permitted.

(B) Transformer Field-Connected as an Autotrans- former. A transformer field-connected as an autotrans- former shall be identified for use at elevated voltage.

FPN: For information on permitted uses of autotransform-ers, see 210.9 and 215.11.

450.5 Grounding Autotransformers. Grounding au- totransformers covered in this section are zigzag or T- connected transformers connected to 3-phase, 3-wire ungrounded systems for the purpose of creating a 3- phase, 4-wire distribution system or providing a neutral point for grounding purposes. Such transformers shall have a continuous per-phase current rating and a continuous neutral current rating. Zig-zag connected transformers shall not be installed on the load side of any system grounding connection, including those made in accordance with 250.24(B), 250.30(A)(1), or

250.32(B) Exception.

FPN: