# Electrical and Computer: Power Exam problem help



## slarty (Jan 27, 2022)

This problem (#52 in the NCEES practice exam) gives a bunch of extra information for a switchboard feeding a pump motor. I think the pertinent information is that it gives that there are two sets of 400MCM copper conductors in steel conduit with a total circuit length of 500 ft. The problem wants to know the complex impedance for the cable to the motor.

I found the impedance value for that cable in Ch. 9 Table 9 of the NEC code book (0.035 + j0.049 ohms) and multiplied that by 500ft/1000ft for our length of cable. Now is where I get confused. The solution in the practice book says to divide the impedance again by 2 to get the solution...it's not clicking in my head why we need to divide by two here. Can someone help out?

Thanks,
slarty


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## DLD PE (Jan 27, 2022)

slarty said:


> This problem (#52 in the NCEES practice exam) gives a bunch of extra information for a switchboard feeding a pump motor. I think the pertinent information is that it gives that there are two sets of 400MCM copper conductors in steel conduit with a total circuit length of 500 ft. The problem wants to know the complex impedance for the cable to the motor.
> 
> I found the impedance value for that cable in Ch. 9 Table 9 of the NEC code book (0.035 + j0.049 ohms) and multiplied that by 500ft/1000ft for our length of cable. Now is where I get confused. The solution in the practice book says to divide the impedance again by 2 to get the solution...it's not clicking in my head why we need to divide by two here. Can someone help out?
> 
> ...


You divide by two because you have a parallel circuit. The key phrasing in the problem is, "The motor consists of two sets...", therefore you divide 500' by two.


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## main197 (Jan 27, 2022)

Power Ncees 52


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