NEC Code say about splice

[paulengr]

You mostly talking about continuity of the splice and insulation of the splicing method. OP is asking if it is OK to have lesser ampacity conductor spliced into a run of higher ampacity conductor. Like I told him, if the original run or the bulk of the main run is higher ampacity than the load needs and is selected for voltage drop reasons, it might be feasible as long as it doesn't contribute too much to overall voltage drop in the circuit.

In his case I believe the motor in question needs at least 4 AWG as minimum size conductor ampacity wise so his smaller conductor could never be less than that size

I thought that the ampacity issue was covered. The question of joint integrity and what effect different size cables has wasn’t. The constriction effect is a current dependent impedance (increases with current) when you have a change in the circular mils. However it occurs even at horizontal bolted joints of identical thickness bus bars because at the joint itself unless it is welded together the total surface area in direct, intimate contact is only a small fraction no matter how high the contact pressure is. It shows up in extreme con

 

[kwired]

I thought that the ampacity issue was covered. The question of joint integrity and what effect different size cables has wasn’t. The constriction effect is a current dependent impedance (increases with current) when you have a change in the circular mils. However it occurs even at horizontal bolted joints of identical thickness bus bars because at the joint itself unless it is welded together the total surface area in direct, intimate contact is only a small fraction no matter how high the contact pressure is. It shows up in extreme con

OP could have same size conductor throughout but a hundred joints in it, joint integrity still important.