# sizing mbj



## 480sparky (Sep 20, 2007)

What size are the raceways, and what material?

And what's an mbj?


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## highlandele (Aug 9, 2008)

pvc, the question doesnt give me a size for a conductor.. mbj is main bonding jumper


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## 480sparky (Sep 20, 2007)

Have you rambled through 250.28(D)?


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## highlandele (Aug 9, 2008)

ya but its rather hard to understand.. basically if i have four paralleled conduits with sets of 350 kcmil in each conduits i multiply 350 x 4= 1400, 1400 x 12.5 % =175 kcmil... so id go with a 250 kcmil main bonding jumper, is that correct? gotta love grounding and bonding!


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## GDK 13 (Oct 6, 2009)

highlandele said:


> ya but its rather hard to understand.. basically if i have four paralleled conduits with sets of 350 kcmil in each conduits i multiply 350 x 4= 1400, 1400 x 12.5 % =175 kcmil... so id go with a 250 kcmil main bonding jumper, is that correct? gotta love grounding and bonding!


Re-Read 250.28 (D)(1) through (3). You base it off the size of the largest phase conductor in the raceway. In your case, 350 kcmil, you use a #2 copper (250.66). There is no need to multiply it by 4 wires.

edit: the largest main bonding jumper would be 3/0, so 250kcmil is out.


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## raider1 (Jan 22, 2007)

GDK 13 said:


> Re-Read 250.28 (D)(1) through (3). You base it off the size of the largest phase conductor in the raceway. In your case, 350 kcmil, you use a #2 copper (250.66). There is no need to multiply it by 4 wires.


Nowhere in 250.28(D)(1) does is say "largest conductor In the raceway".

When you size a main bonding jumper it must be based on the largest ungrounded phase conductor. For a parallel installation that would be the combine area of the parallel conductors.

So for example 4 350's in parallel would require that you multiple 350,000 circular mils by 4 to get the area of the largest phase conductor.



> edit: the largest main bonding jumper would be 3/0, so 250kcmil is out.


Not so, 250.28 requires that when the size of the ungrounded conductors exceeds Table 250.66 then the main bonding jumper must be sized not smaller than 12 1/2% of the largest ungrounded conductor.

So for 4 350's in parallel the main bonding jumper would need to be 175 kcmil and since that is not a standard size you would need to go with a 250 kcmil.

Chris


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