312.8 !!! Really!?!

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rjfell02

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I have an inspector that said I was not allowed to run conduits from one panelboard to another and I pointed to section 312.8 and said "YES I can." well he returned back and said "prove to me that you are not using 40% of the cross sectional area then"... I was speachless. Can anyone help me in proving this.
I have one 42space panel thatis being used as the chase for a smaller 30 space panel. Both have almost every breaker filled.

Thanks
 
raider1

raider1

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Welcome to the forum.:)

Can you add up the cross sectional areas of the conductors and then find the cross sectional area of the space to the side of the panel? This might be the only way to show the inspector that what you have is code compliant.

Chris
 
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rjfell02

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Here was my thinking and let me know if this sounds right.

Per 314.16(B) Volume Allowance required per Conductor
1 - #10 wire requires 2.5in3 (Which is the average size conductor in the panels)

Therefore, Hypothetically lets say that the box is 12x12x6 (which really its more like 36x24x6). This yields a 864 cubic inch space.
Then 864/2.5 = 345 - #10 wires that could potentially be in that box.
but then 345 x 40% = 138 - #10 wires would be the cut off to the 40% rule

Sound Right?

Furthermore, if everybreaker was being used and each circuit had its own neutral (which it doesn't) then I would be right about at the 40%.
 
raider1

raider1

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I wouldn't use the volume allowance of 314.16 because 312.8 deals with cross sectional area not volume.

I would begin by getting the cross sectional area of the conductors used from Chapter 9 Table 5 of the NEC. Then I would add all the cross sectional areas together and see if they exceed 40% of the cross sectional are of the wiring space on the side of the panel.

Chris
 
raider1

raider1

Senior Member
Staff member
Location
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For example from Chapter 9 Table 5 #10 THHN has an approximate area of 0.0211 square inches.

If the panel had a wiring space of 6" by 4" then that would be 24 square inches. So 40% of that would be 9.6 square inches. using #10 THHN I could have almost 455 #10 conductors in that space. (Assuming my math is correct:))

Chris
 
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