CONTROL TRANSFORMER

[pilot]

HELLO EVERY ONE

THERE WAS A QUESTION OF WHERE WE WOULD FIND THE RULE THAT DETERMINES THE CORRECT WIRE SIZE FOR THE PRIMARY SIDE OF THE 250 VA CONTROL TRANSFORMER (480 x 120 VOLT) TO POWER THE START/STOP STATION FOR THE MOTOR STARTER.
WE HAVE A 60 AMP DISCONNECT/STATER BOX FEED WITH # 6 WIRE WITH 60 AMP FUSES,AT THE POWER SORCE( NO FUSES INSIDE THE STATER ONLY OVER LOADES FOR THE MOTOR)WE WANT TO FEED THE TRANSFORMER FROM THE LOAD SIDE OF THE DISCONNECT,USEING NO. #16 WIRE ,IS THIS O.K.
THANKS FOR THE HELP

[Tony Moscioni]

14-100 Overcurrent Devices Required.

Each ungrounded conductor shall be protected by an overcurrent device at the point where it receives its supply of current and at each point where the size of conductor is decreased, except that such protection shall be permitted to be omitted:

(e)Where the smaller conductor is No. 14 AWG or larger, is in a control circuit, and is located external to the control equipment enclosure, and:

(i) The rating or setting of the branch circuit overcurrent device is not more than 300% of the ampacity of the control circuit conductor; or

(ii) The opening of the control circuit would create a hazard; or

(f) Where the smaller conductor supplies a transformer, and:

(i) The conductor supplying the primary of the transformer has an ampacity not less than one-third that of the larger conductor; and

(ii) The conductor supplied by the secondary of the transformer has an ampacity not less than the ampacity of the primary conductor multiplied by the ratio of the primary to the secondary voltage; and

(e) Control circuit conductors No. 14 AWG and larger that are run externally from the control equipment enclosure are considered to be protected

(1) if they are connected on the load side of a set of branch circuit overcurrent devices; and

(2) if the rating of the branch circuit overcurrent device is not more than three times the ampacity of the control circuit conductors. Overcurrent protection should not be required where the control circuit performs a critical function and its opening would cause a hazard (eg, fire pump control); or

(f) In installations where the tap conductor feeds a transformer,

(1) the minimum primary conductor's ampacity is at least 1/3 of the feeder (larger) conductor it is being fed from, or the demand load (see Rule 26-258(3)), whichever is greater; and
(2) the minimum secondary conductor's ampacity is calculated by multiplying the primary conductor's ampacity (see Step 1) by the ratio of the primary to secondary voltage; and

(3) the length of the primary and secondary conductors is not more than 7.5 m; and

Note: When the primary conductor is protected at its ampacity rating, the maximum length of the secondary conductor could be 7.5 m.

(4) the primary and secondary conductors are protected from mechanical damage; and

(5) the secondary conductors terminate in a single set of overcurrent devices having a rating not greater than the secondary conductor's ampacity;

Tony Moscioni
Electrical Inspector
Electrical Safety Authority