The phase current in a grounding autotransformer is one-third the neutral current.
(A) Three-Phase, 4-Wire System. A grounding autotransformer used to create a 3-phase, 4-wire distribution system from a 3-phase, 3-wire ungrounded system shall conform to 450.5(A)(1) through (A)(4).
(1) Connections. The transformer shall be directly con- nected to the ungrounded phase conductors and shall not be switched or provided with overcurrent protection that is independent of the main switch and common-trip overcur-rent protection for the 3-phase,
4-wire system.
Table 450.3(B) Maximum Rating or Setting of Overcurrent Protection for Transformers 600
Volts and Less (as a Percentage of Transformer-Rated Current)
Primary Protection Secondary Protection (See Note 2.)
Protection
Method
Currents of 9 Amperes or
More
Currents Less Currents Less Than 9
Amperes Than 2 Amperes
Currents of 9 Currents Less
Amperes or More 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.
(2) Overcurrent Protection. An overcurrent sensing de- vice shall be provided that will cause the main switch or common-trip overcurrent protection referred to in
450.5(A)(1) to open if the load on the autotransformer reaches or exceeds 125 percent of its continuous
current per-phase or neutral rating. Delayed tripping for temporary overcurrents sensed at the autotransformer overcurrent device shall be permitted for the purpose
of allowing proper operation of branch or feeder
protective devices on the 4-wire system.
(3) Transformer Fault Sensing. A fault-sensing system that causes the opening of a main switch or common- trip overcurrent device for the 3-phase, 4-wire system shall be provided to guard against single-phasing or internal faults.
FPN: This can be accomplished by the use of two sub- tractive-connected donut-type current transformers installed to sense and signal when an unbalance occurs in the line current to the autotransformer of 50 percent or more of rated current.