In that pic, it would appear that the NM simply runs through the gutter, hence all the bare ground wires that the installer had a problem terminating. They could have installed a ground bar in the gutter, and run a single ground back to the panel.
It looks like the installer had more homeruns to the panel than there were 1/2" KOs in the top of the box, and went with the gutter rather than start going into the sides of the box. Makes for a neater install, until you look at how he tried to ground everything...
A gutter like that is sometimes useful when replacing an old panel/fusebox during an upgrade, if the old cables are too short to make it to the new panel location.
Re: Gutter box for NM installs.#57032 10/04/0509:05 PM10/04/0509:05 PM
I have used a gutter with residential romex a few times. One time I do this is when I'm doing a service change, and the existing wires either won't line up, or won't reach, the new panel. A gutter makes it possible to splice the wires, and route as I wish.
Another use is in a place such as a crawl space or attic, where I run the assorted romex lines to the gutter, then run in pipe to the panel. When the panel is outside -common around here- this results in one pipe on the outside, rather than several. It also makes for easire additions later.
To simplify things, I usually mount a ground bar in the gutter, and run a single wire from the bar to the panel. This is in addition to the ground path provided by the conduit. My reasoning is that the metal of the gutter and conduit is not easily "converted" to any particular wire size. Just how much current can it handle? Remember, by using that ground bar in the gutter, you're "putting all your eggs in one basket." I size the ground wire as I would any bonding wire....which here our local code says must be #8 or larger. So #8 it is.
I cannot say the NEC requires these practices on my part....there are other arguments that have some merit. I feel, however, that my method is certainly straightforward, and complies, even surpasses, the code.
Now, some will say "why not do the same with the neutrals?" I believe that the code technically would allow you to do this- but I consider such a move unwise. It is just too easy to wind up with a poor neutral connection, in some place that would result in 200+ volts on some circuits. In my opinion, the only place to "land" the white wires is at the panel itself.
Re: Gutter box for NM installs.#57034 10/05/0508:32 AM10/05/0508:32 AM
What exactly is derating? Sorry, apprentice barely gtting into the field.
IMO, I'm thinking it's more in the line of sizing the wire? Take for example the 40% factor for that install, meaning figuring out 40% of the total load per circuit, then going with a bigger gauge wire to compensate for the ampacity, or am I completely off?
Re: Gutter box for NM installs.#57037 10/05/0508:31 PM10/05/0508:31 PM
I'm glad you're asking questions, I'm learning from the "simple" questions too. Derating is what you think it is. When you have that many conductors together, cooling starts to become an issue and it's better to use larger wires for the same current. the 40% comes from Table 310.15(B)(2)(a) for having 31-40 current-carrying conductors in a raceway or cable. You then apply this value to the tables on ampacity. In this section in the Handbook there are some examples that make it much clearer.
Re: Gutter box for NM installs.#57038 10/05/0509:38 PM10/05/0509:38 PM
So, let's say there is between 31-40 current carrying conductors in this install. Since 310.15(B)(2)(a) states the derating to be at 40%, we'll multiply this percent into the total load on the circuit.
Let's say we have a continuous load of 16A on a single circuit. #12 THHN is rated for 30A. 30A x .40 = 12A. This will not do. So, we move onto #10 THHN, rated for 40A. 40A x .40 = 16A. This means that on a continous load of 16A, we'd need to use a #10 for the install, correct?
But, what happens when it isn't a continous load? The NEC states a continous load being "on" for more than 3 hours. What happens if this is a residential application? Most people don't leave things on for more than 3 hours, during a "normal" day, and if they did, it's probably something that is not coming close to or exceeding the total ampacity of the wire or circuit breaker.
Would this type of residential install need derating, even though most to all circuits won't be a continous load, everyday? Or will it be derated for the sole purpose of "just in case"?
Re: Gutter box for NM installs.#57039 10/05/0510:54 PM10/05/0510:54 PM
Most people don't leave things on for more than 3 hours, during a "normal" day, and if they did, it's probably something that is not coming close to or exceeding the total ampacity of the wire or circuit breaker.
Don't forget air conditioners in windows. They'd be on all day long and as we all know draw lots of current.