What is the point to having a gutter box such as this one in this thread:
https://www.electrical-contractor.net/ubb/Forum4/HTML/000433.html

for? I've seen them on commercial applications, but not on residential. Does the Romex run all the way through the gutter or does it get spliced in there?

Thanks.

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.

I see. So when the maximum amount of circuits will be reached, enough not to have enough KO's available, a gutter box will make it easier for the installation?

BTW, how would you size the ground coming from the main service, if you were to place a ground bar inside the gutter box?

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.

I see, thanks for the information.

There are more than 30 conductors in that wireway. Any crossection in a metallic wireway can have up to 30, but after that, derating is required.
See310.15
40%

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?

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.

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"?

Don't forget air conditioners in windows. They'd be on all day long and as we all know draw lots of current.

Don't those window A/C units only draw in the range of 4A-8A, depending on size?

Computers, lights, tv's (for some people), a/c's (as mentioned), refrigerators, stereos... I'm sure the list can be expanded, but all of these things are often on for more than 3hrs. And a lot of homes have central air, those units can get a little big. But, the only real continuous load on an a/c is the fan. The compressor only runs when the a/c cycles which can vary in frequency depending on the conditions.

So, as DS has asked, in residential (or any application for that matter) how do you determine "continuous loads"? Or do you just forget about it and do it anyway regardless of the code?

So, even if the total load isn't close to the rating of the circuit breaker, or percent thereof, it must be derated anyways?

derating is for the conductors, not the breaker. you're giving the same current a larger wire to run on, but the current is the same for the breaker. the issue is heat dissapation.

Say you're running a 20A circuit on #12 wire, but due to derating you have to drop down to #10, you're still running a 20A circuit with a 20A breaker. And as far as I can tell, the derating is only applied where you have the wires running together, so you can have #12 in free air, but have #10 in conduit for the same circuit. Am I wrong here?

The "adjustment factor" (derating) changes the maximum permitted ampacity of the conductor. The size of the overcurrent protective device cannot not exceed the maximum permitted ampacity of the conductor.
Don

In reality, there are almost no continuous loads in a dwelling unit.

Think of it this way: How many circuits are loaded up to their full ampacity for 3 hours or more? I would say none.

Peter

I see. Would derating apply to conduit as well?

well, once you've derated, you're now dealing with larger wires, which will require larger conduit.

Derating does apply to conduits.

Also derating is done when you have more than three current carring conductors, regardless if it is a continuous load or not.

Keep in mind that you can use the temp rating of the wire for derating. So for THHN you can use the 90 degree column of 310.16 for you derating. If you notice that column lists the ampacitiy of # 12 as 30 amps.

Chris

Another question, let's say you've caluclated the derating of the circuits and come up that you'd need to use #10 wire. Does this mean that the entire install needs to be in #10 wire, or does it mean that only the homeruns that'll end up in the gutter box need to be #10 wire?

Only the wires in the gutter will be derated. Do you own a codebook? I am required to have one for my classes.

I do, but it makes it a tad easier to interpret by asking the question, rather than assume it to be one way, only to be wrong.

I use gutters like this a lot! And yep <30 conductors at any cross section. If I have a lot of room to work with, I'll take a 6' and put it above or below the panel, with about 2' either side, so I dont have to figure out how to squeeze all the romex, or MC into one stud bay. This work fantastic for MM w/ dist 200a panels with a 3" concentric, and a few halfs at the bottom. Otherwise how would they all fit in there? This way you leave all the grounds in the gutter. (I like cans too)

I put a ground bar, and bond with a #6, only 'cause I never use #8, and have tons of 6 on the truck.

Anyway, while on the subject of derating, and using fewr KO's...

I have seen alot of this done lately.

42 circ panel with 4 1" emt's to cans and gutters (like 50' away) with the whole panel going through it all in #10's, all derated. Late one I saw, I did the math, and it was all pretty close... I don't like it, but seems to be a growing trend with a few companies around here. I guess it saves some mat's and labor on bending a few more conduits, but just don't feel right?

When we last had a discussion on this topic, I had the following idea:

A large shallow gutter, perhaps 1" thick, 2 feet tall, as wide as necessary. Spaced through the inside on perhaps 2" centers would be insulating stand-offs.

The kicker would be that every 'bay' (horizontal or vertical channel defined by the stand-off array) would be tested and _listed_ to carry a certain number of conductors _without derating_. You would have a large grid, and could run wires as needed to get from the breaker panel to the appropriate conduit, and as long as you kept to fewer than X conductors per bay, you would be in the clear. So rather than being limited to 30 conductors across the entire cross section, you might be limited to 20 conductors per bay, but have space for 240 conductors in the 8 horizontal bays.

I have not worked out the physics of how many runs would actually be reasonable in a 'bay'.

-Jon