a $4,000$A switchboard requires a minimum distance from the phase bus to the closest steel member of$\_\_\_\_\_.$

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Part $1$

A$.$

$12$double prime$$

$(300$ mm$)$

B$.$

$4$double prime$$

$(100$ mm$)$

C$.$

$6$double prime$$

$(150$ mm$)$

D$.$

$2$double prime$$

$(50$ mm$)$