Sizing a gutter for the proper fit is relatively easy. I get a lot of calls to make sure that the gutter that they are using is up to code.
We are allowed by code to fill a gutter to 20%. of its interior cross-sectional area (366.22(A),(B)) This is for both sheet metal and Nonmetallic gutters
Figuring the 20% is easy, for example.
If you have a 6 X 6 gutter, that would have total of 36 sq. in. and 20% of that is 7.2 sq. in. (36 X 20% = 7.2 sq. in.) of space to use. Table
5 is used for the area per conductor. Take a 3/0 THWN conductor it is listed at .2679 sq. in.for each. if you wanted to know how many could be
used in a 6 X 6 gutter, you can take the 7.2 sq. in. and divide it by .2679 to get the maximum number that would be allowed to be installed in
that gutter. 7.2 / .2679 = 26 conductors.
You would also have to check the wire bending space minimum from Table 312.6(A). which would require a minimum of 4’” for a 3/0 conductor.
Till next time, work safe always check the circuit with a tester.