sub panel

[xara]

Why must the neutral bus bar and ground bus bar be separate in a sub panel and not connected by a main bonding jumper?

 

[augie47]

short answer: because 250.24(A)(5) says so.

 

[journeyman0217]

if your neutrals and grounds are bonded together in a sub panel, it will give a parallel path for neutral current to travel back to its source.

 

[Dennis Alwon]

if your neutrals and grounds are bonded together in a sub panel, it will give a parallel path for neutral current to travel back to its source.

Exactly and the equipment grounding conductor should only carry current in case of a ground fault.

 

[Dennis Alwon]

Definition equipment grounding conductor

Grounding Conductor, Equipment (EGC). The conductive
path(s) that provides a ground-fault current path and connects
normally non–current-carrying metal parts of equipment
together and to the system grounded conductor or to the
grounding electrode conductor, or both.

 

[petersonra]

Exactly and the equipment grounding conductor should only carry current in case of a ground fault.

but only because the code says so.

otoh, the code also allows for some exceptions to this rule.