Sizing Bonding Jumper with parallel runs

[cflava]

I am currently studying for my electrical exam and NC is still testing off the 2011 book. Anyway while studying from Mike Holt's Electrical Exam Prep book, the section on sizing the system bonding jumper for transfomers confused me with on the answers.

System Bonding Jumper Example 3:
Question: What size bonding jumper is required for a 112.50 kVA, three-phase, 480-120/208V transformer, when the secondary conductors are two parallel 3/0 AWG conductors?
Answer: 2 AWG
Cmil of 3/0 = 167,800 mil X 2 = 335,600 cmil
Table 250.66 based on 335,600 cmi = 2 AWG

My question is why would you wan to use the paralleled conductor size when the conductor is not paralleled would be sized at 400kcmil? Don't you have to size the system bonding jumper by 250.66 based on the original calculated size of the conductor despite whether or not it is run in parallel? The figure 12-61 shows the system bonding jumper to be the wire that goes from the transformer neutral (XO) to a grounding terminal block on the transformer.

 

[infinity]

The question gives you the secondary conductor size which is parallel #3/0's. You would simply use the total circular mil area of the parallel conductors, in this case two sets, to determine the system bonding jumper size. Parallel #3/0's would have an ampacity of 400 amps, quite a bit larger than that of a single set of 400 kcmil conductors.

 

[Carultch]

My question is why would you wan to use the paralleled conductor size when the conductor is not paralleled would be sized at 400kcmil?

You get better amp performance out of parallel small conductors, than you get out of larger conductors of the same amount of copper. The reason is that smaller conductors have more surface area per unit of volume, and thus have more area available for releasing their heat.

Parallel 3/0 in separate raceways has an ampacity of 400A. If you wanted to use a single conductor for each phase, you'd use 600 kcmil, which is 78% more copper.

Parallel 3/0 in the same raceway have an ampacity of 360A. If you wanted to use a single conductor for each phase, you'd use 500 kcmil, which is 49% more copper.

Don't you have to size the system bonding jumper by 250.66 based on the original calculated size of the conductor despite whether or not it is run in parallel?

The amount of copper in your bonding jumper is a lot less than that which will be in all of your phase conductors.