




#191506  12/31/09 03:27 PM
Re: Auxiliary Gutter Sizing
[Re: Webmaster]

Cat Servant
Member
Registered: 01/22/05
Posts: 5305
Loc: Blue Collar Country

Let's look at this as an exercise, for the benefit of those who have never actually 'sized' things.
In choosing a gutter, we have a couple of different criterial to meet. Not only does there need to be room for wires (cable fill), there has to be adequate bending space, and there is a limit to the number of circuits in any particular cross section. Exactly how you count the wires matters, as a spliced wire counts as two for cable fill, but not for the circuit count.
You say that the wires pass 'straight through,' so I will assume there are no splices. I will also assume that the GEC passes through the raceway.
First, the cross sectional rules: For the 4" conduits, I will assume each contains 4 copper wires, 600MCM, and the insulatiion is THWN. Table C8 tells me that 4"RMC has space for as many as six, so we're OK there.
Now .. do we have an 'auxilliary gutter"(Atricle 366), or a 'pull or junction box"(Article 314)? From the tables in 314.16, I think it's safe to assume that this section is aimed at much smaller boxes, and that Article 366 is where we need to look for guidance.
Lest there be any doubt, Article 366 begins it's definition with "enclosure used to suppliment wiring space at meter centers ..."
366.22 limits our wires to 20% of the cross sectional area of the box. Table 5 gives us an area for each 600MCM wire as 0.8676 sq. in. Since, in addition to the circuit wires, we also have a GEC and bonding jumpers in this gutter, we'll just assume 10 wires for the sake of simplicity. 10 x 0.8676 = 8.676 sq. inches. Five times that (20% rule) tells us that the minimum crosssectional area is 43.38 sq. in. You gave the size of the gutter as 14x14", or 196 sq. in ..... so there's plenty of space.
Let's look at bending space. 366.58(B) tells us that where used as a pull box, we need to look at 314.28(A) pull box rules  for the size of the box.
314.28(A) takes us away from wire size, and focuses on the 4" conduit used. For straight pulls, we are told the length has to be 8x the nominal pipe sixe ... 8x4" in this case ... or 32", if we consider these to be 'straight' pulls. If we consider them 'angle' pulls  and a case can be made, as the pulls are not absolutely straight  we need to look at both of the pipe runs. That is, the math is 4 (pipe sixe) x 6 (multiplier) + 4(2nd pipe size). This comes to 28".
Let's back up again, and see where we might 'cheat.' Code would have allowed us to use 31/2 pipe. 314.28(A) also says that the distance will not be less thn 6x the pipe size, no matter what assumptions you make. 31/2 x 6 = 21.
In other words, no matter how you slice it, we need 21" between the two conduit entries. The gutter is two small.
Let's look at the argument that you have a 60" gutter, so you can make the wire 'fold' in. This still gives us a need for 21" for the necessary rightangle and "U" bends, so again, the gutter is too small.
If the pipe entries were truly moreorless in line, and the gutter simply used to adjust for minor differences between the pipe and the KO locations, I'd be willing to let it pass. There's enough offset, though, that I can't really say that you're 'pulling through' the gutter. That would be one ugly pull! I'd almost have to use single rightangle bends, and splice the wires at the far end of the gutter. The gutter, or box, however you slice it, needs to be 21" tall, or 50% larger than it is.
Please note that at no point did we look at the 'minimum bending radius' of the wire. That's really not relevant here, as we are passing through the enclosure, not making connections within it.
If I were the guy on the job .... sometimes life gives you lemons. I'd try to get approval, as I see there are real problems on the site with clearances. Then I'd make a serious note to not let this happen again!

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