A service is supplied by two parallel sets of 250 kcmil copper conductors per phase. The bonding jumper sizing table assigns a 2 AWG jumper where the supply conductors are over 3/0 through 350 kcmil, and a 1/0 jumper where they are over 350 kcmil through 600 kcmil. What size copper supply-side bonding jumper is required for a service raceway?
- A2 AWG copper
Two AWG comes from reading the table with a single 250 kcmil conductor. That halves the basis and produces a jumper undersized for a paralleled service.
- 1/0 AWG copper
- C4 AWG copper
Four AWG is smaller still and corresponds to service conductors of 2/0 or 3/0, far below the equivalent area here.
- D3/0 AWG copper
Three-aught is the largest size the table reaches and corresponds to service conductors well above 1,100 kcmil equivalent, so it is oversized for this service.
Why B is correct
The rule turns on equivalent area, not on the size of any one conductor. Two 250 kcmil conductors per phase equal 500 kcmil, and the table row covering over 350 through 600 kcmil calls for 1/0 AWG copper. The same principle governs the main bonding jumper and the grounding electrode conductor at a paralleled service.
What this question is testing
The item tests whether the candidate combines parallel conductors into an equivalent area before entering a sizing table, and can read the correct row from supplied table values.
On the job
Paralleled services are routine at 400 amperes and above, and the equivalent-area step is the one most often skipped. The consequence is a bonding jumper that cannot carry the available fault current long enough for the utility protection to clear, which is exactly the condition that turns a bolted fault into an arcing fault inside the gear. On the drawing, showing the equivalent area next to the jumper size makes the calculation reviewable.
Memory technique
Parallel means add the circular mils first, then look up once.
Exam tip
Whenever conductors are paralleled, add the circular mils per phase before you open any sizing table.
Where to look it up
Look in the grounding article at the section on bonding jumpers on the supply side of the service, and use the sizing table referenced there.