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Question about industrial wiring (distribution)

ewlsey

Diamond
Joined
Jul 14, 2009
Location
Peoria, IL
I need advice from someone with some experience in industrial wiring.

See the attached drawing. Basically, if I have a branch circuit, say 100A, and I want to further distribute the power using fuseable switches, what is the proper procedure?

More specifically, if I use a wiring trough with a 100A feeder, do I need wire capable of handling 100A for each sub circuit? It seems like systems I have seen use smaller wire for the sub circuit on both sides of the fused switch. Is this correct?

Basically, you have 100A potential on one side of the switch and 30A max load on the other side. So does the 30A fuse protect you all the way back to the source, or do you need to be able to handle the 100A on the feeding side of the switch?

Wiring.JPG
 
S_W_Bauch has nailed it, and he's not being a smartass by giving your the NEC Articles rather than the gist, because 240.21 is a rather involved article. The rules are different if the conductors you are asking about are less than 3 meters (10 feet), or between 3 and 7.5 meters (10 to 25 feet), or longer than 7.5 meters (25 feet), or outdoors, etc, etc.

In the case of less than 3 meters, the ampacity of the tap conductors must be not less than the rating of the device or overcurrent protection supplied. That's the 30A fuses in the fusable disconnect in your diagram. There are four other conditions that apply in the less-than-3 meters case, so understand what I just said was the gist and not the Code.
 
S_W_Bauch has nailed it, and he's not being a smartass by giving your the NEC Articles rather than the gist, because 240.21 is a rather involved article. The rules are different if the conductors you are asking about are less than 3 meters (10 feet), or between 3 and 7.5 meters (10 to 25 feet), or longer than 7.5 meters (25 feet), or outdoors, etc, etc.

In the case of less than 3 meters, the ampacity of the tap conductors must be not less than the rating of the device or overcurrent protection supplied. That's the 30A fuses in the fusable disconnect in your diagram. There are four other conditions that apply in the less-than-3 meters case, so understand what I just said was the gist and not the Code.

OK, I have a copy of the NEC here from 1971. I will buy a newer copy one of these days (it's damn expensive).

In my version, this is article 240-15 and it is damn confusing.

As far as I can tell you use the ampacity of the overcurrent device (fuse) to determine the tap conductor ampacity in most cases.

If you want to feed a panel board or switch board, you are limited to 3m and there are a few other restrictions. Not sure what a panel board is exactly.

If you want to further subdivide the circuit, you are limited to 7.5m. You still use the ampacity of the overcurrent device, but it has to feed just one fuse, then can be subdivided after the fuse.

Hopefully this is correct.
 
Since the latest version of NEC is 2011, you should be able to pick up a used 2008 Handbook fairly inexpensively.

If you have any doubt about the relevant Articles, the key phrase is "feeder taps".
 
A "panelboard" is a "word of art" which means a fully-enclosed switchboard which is generally limited to 42 breaker positions (84 breaker positions with the very latest edition of the NEC). Enclosed on all sides and with NO exposed energized parts. Intended for operation by homeowners or semi-skilled operators. The most popular models are especially designed to fit within a 2 x 4 stud wall.

A "switchboard" is also a "word of art" which means a semi-enclosed switchboard, not necessarily a fully-enclosed one. Not necessarily enclosed on all sides, and may have exposed, energized parts. Intended for operation by especially skilled operators.

At the very large electric utility where I was an EE in another lifetime, we tried ... once ... what is called "metal-clad switchgear", which is basically a switchboard which is fully enclosed. The experiment was not particularly successful and all previous and all subsequent switchyards were made with non-enclosed switchgear. "Metal-clad switchgear" generally is limited to 34,500 volts, or less.

We had 230,000, 287,500 and 500,000 volts in many of our switchyards, as Los Angeles is the largest municipal utility in this Nation, and is a city with a VERY large aggregate load.


Now, as to "bus-ways" ...

These systems are VERY expensive, but these are also VERY flexible.

You may use "bus-way" as service conductors.

You may also use "bus-way" as feeders within your premises.

The sending end of a "bus-way" might have a very large breaker (hundreds or even thousands of amps) at the head-end, but the various "taps", which may be placed at almost any convenient location, usually have a breaker within these, and this protection is intended for the feeder, which may be 15 amps to several hundred amps.

"Bus-ways" are unaffordable, if purchased new.

However, good, used "bus-ways" may often be had for free or for scrap value.

"Bus-way" is especially good for commercial premises where reconfiguration is going on over time.
 
I have never seen a bus way used in a machine shop.

In most shops what I see are huge power distribution cabinets that divide the main power into 100 to 400 amp branches that are send out to various locations in the shop. These branches feed either a larger junction box, or a long trough. Sub circuits are pulled from these boxes and fed to the equipment using fused disconnect switches. A few branches will have small transformers attached to feed small breaker boxes for the 110V circuits.
 
"I have never seen a bus way used in a machine shop."

Possibly because it is, by far, the HIGHEST COST option for distribution.

But, it is also the most flexible and is almost instantly reconfigurable.

You need another branch circuit, just plug-in a "tap" and go.
 
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I can list quite a few machine shops with Buss ways, Yes they are expensive but if you search the Web you can find them cheap. HGR Surplus Usually has some. Once installed they make connecting machinery easy!

Frank
 
Anyone that would like an e-mail copy of the 2008 NEC, send me an email address and I will return copy to you. The copy I have was posted online by Idaho.

Tom
 
What people are alluding to here is what we call at work "the tap rule"...... smaller wires can be tapped off a larger feeder under certain conditions and restrictions.

But 100A is not so high that it should be horribly troublesome to just run the wire to a local "box" with "some" breakers in it. You may be able to use a local 'fused disconnect" to make a machine connection, so that you can either run the 100A feeder to each of them, or locate the fused disconnect in an appropriate place without needing much of the feeder size wire, and not needing to measure for the various "tap rules".

In a new installation, you may be able to pre-place some disconnects 'for expansion". Some inspectors don't like that, others will allow it.

Bus duct is generally for 500A or more. The woodshop at a prior employer had it run down the middle of the bay, suspended. I think they had 400A capability.

Whenever they wanted a new machine, or to move an existing one, they unplugged the drop and it's box, moved it to the new place (or added one), and plugged it in. No muss, no fuss, no bother, and no inspection.
 
There was 90 feet of good-looking duct for sale on the local craigslist, for $2000. It did not stay advertised for long.

I have been advised that busway/bussway/buss duct/bus duct less than 400 amps (say, 100 amps) is not so economical if you can't find the accessory plugs, ends, etc. And it's not that rugged.

There's a local used equipment place called Midwest Machinery Mart, 317 357 2336, http://mid-westmachinery.com/ , that carries used transformers, duct, panel boards, etc. at reasonable prices.

A clever person just comprehends the NEC, a lazy person buys buss duct :)
 
I might misunderstand post # 14...but a problem arises when you attempt to stick #3 wiring into the lugs on a 600 volt/30 amp disconnect as it wont' fit. The box and its lugs weren't designed for wire that large so it's almost required that you invoke the 'tap rule'.

Stuart
 
I'm also a bit confused by #14.

I don't see the point of using a breaker panel in this situation. The machines need lock out switches anyway, so now you have to pay for a panel and the disconnect switches.

I have seen breaker panels used for cells of machines where there may be 20 or 30 circuits in a small area. However, a switch is still used before the panel so everything can be locked out.
 
Don't get hung up over post #14, please. JST was just suggesting an alternative option, which doesn't appear to be a cost-effective way to achieve ewsley's intent. However, it probably is a much more typical installation. Regardless, the feeder tap rules exist and are applicable to the original request.
 
It varies..... no confusion intended

if you have a "cluster" of machines at a distance from the main service panel, it can make sense to put a small panel there at the end of a feeder.

if you have them spread out, then a disconnect at each, tapped off the feeder, much as you would for a bus duct. Ditto if you don't know where they may end up, or expect to change the setup.

it depends on how you want to do it..... does it make sense to run fat wire/big conduit all over, or does it make better sense to distribute smaller wires at teh end of the feeder?

For that matter, the original picture doesn't make sense if you take it literally, since he shows a 100A distribution going to ONE machine that takes much lower current.... you have to assume there are more (which he basically says there are), and if so how/where are they set up?

And also..... if the setup is a high bay with the distribution way up, some of the tap rules might not apply, since you may not be able to locate the fused disconnect where you would need to AND satisfy the shorter length tap rules. Details, details.
 
Anyone that would like an e-mail copy of the 2008 NEC, send me an email address and I will return copy to you. The copy I have was posted online by Idaho.

Tom
Warning, doing so is a violation of copyright laws and puts you in a position of violating the rules of this (and any other) website. Please do not get the site owners in hot water by facilitating copyright infringement.
 








 
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