ECN Forum Forums Photo Discussion Forums Violation Photo Forum Will this installation get "Red Hot" soon?

Sent from the field:

In a pic so neat, it's hard to see what the fuss is all about. All those black wires kind of blend together, you know!
It it as I suspect- that each phase has its' own conduit?

I agree with reno, it sure looks like each phase has it's own private conduit!!

Too bad such clean workmanship is fouled up by such an error!

As for the AHJ missing it, it's not impossible. As was explained to me, the AHJ (intentionally or otherwise) will tend to make the depth of his/her inspection based on the workmanship of the site. Really neat work would tend to get an inspection not subject to the "fine-tooth comb." Not-so-neat work would make for a deeper inspection to see if there are issues hidden within. Still, one would hope the AHJ in this case is aware of a little thing called induced current!

Gregg, I too am wondering how you could possible correct this and keep the site on-line. When you figure it out, please share (and maybe post "after" pics?)

My original thought was to correct the genny side first, transfer to genny power then fix the utility side. But one small (sarcastic) flaw with my idea is that the load side connections left alone would make for a phase-to-phase short if left as-is? And of course, how would you correct the load side if it had to stay fed? Hmmmm...

How about making temp hook-up directly from utility side to switchgear, (at 1600 amps no small feat), correcting the transfer switch then re connecting?

Wow. Now I remember why I stick with cinema stuff.

Is that supplied by 1600A? That doesnt look like 4 500MCM on each phase to me.

HOLY CRAP!!!!!!!


This looks like a new install.
Backcharge!
There is no statuate of limitations on code and this contractor needs to be stopped!

Hopefully he is a Janitor at a JC

[This message has been edited by sierra electrician (edited 06-13-2005).]

Yes, indeed all of the phase conductors go through the same conduit. And to top it off, there are two matching transfer switches both wired this way in the same electrical room. When you are on a roll.... There are also grounding and ground bonding issues, as if that would surprise anyone reading this.

The solution is to bypass one transfer switch by using the other, since the load is still minimal by backfeeding breakers in the distribution panels that are adjacent to the transfer switches. Then pull out wiring as necessary and re-feed new, the correct way.

Four sets of 500kcmil is not enough copper for a 1600A circuit.
Don

Actually, what size would be needed for 1600A? I always thought parallel runs were based on amperage, but I looked it up and its based on cross sectional area (ashamed to admit that all my parallel runs were engineered and I never bothered to look it up =\). I have no idea what cross sectional area is needed for 1600A.

I would assume 600MCM would be enough since the divided rated amperage would be enough, but the cross sectional area for that high of amperage is throwing me off.

[This message has been edited by dmattox (edited 06-14-2005).]

IMHO no need to be quite so harsh on this guy. My read of the pictures (and thus insufficient data) is a fine electrician who had a _serious_ brain fart.

The neat workmanship suggests that sort of pride that comes of wanting to do a good job, and the mistake was one that was missed by the inspector as well. In fact, I'd bet that this installation is reasonably safe because all of the bonded connections are probably made up perfectly, and can thus carry the induced currents without excessive heating. Not to say that this shouldn't be fixed, nor that the original electrician shouldn't be learning a costly lesson...just that this is probably a fine guy who should be more than welcome in this forum, and that this is an expensive mistake that would be appropriate in https://www.electrical-contractor.net/ubb/Forum1/HTML/001764.html

-Jon

dmattox,
Parallel ampacity is based on the sum of the individual conductor ampacities, but the ampacity of 500kcmil is 380 amps, not 400.
Don

Don,
I thought that also, but I looked it up this morning and its based on the cross sectional area, not amperage. Though I am sure there is a direct relationship between area and allowed ampacity, but thats what I am not clear on.

BTW, excuse my brain fart as Jon put it on 500 MCM being rated for only 380A

Dave

Dave,

The relationship between conductor cross section and ampacity is nowhere near linear. If you look at tables of ampacity, you will notice that the cross section increases far more rapidly than the current being carried.

The factors to consider include the heat generated in the conductor, the thermal resistance to heat being carried to the environment, the temperature rating of the insulation, etc.

If you maintain the same relationship of copper cross section to current, then you get the same 'current density' and thus the same heat production per amp. But in a round wire, the cross sectional area goes up as the square of the diameter, while the surface area goes up linearly with diameter. This means that if you were to double the wire diameter, the cross section to carry the current would go up by a factor of 4, but the surface area to dissipate the heat would only go up by a factor of 2. If you tried to keep the same current density, you would push 4x the current through the wire, and get 4x the heat production, but not have sufficient surface area to dissipate the heat. So you need to reduce the current density, so that the increase in heat production in a given length of wire will match the increase in heat dissipation capability.

-Jon

Dave,
Where does the code say to use the conductor area and not its ampacity for parallel conductors?
Don

The code doesn't actually say it, but the handbook does.

At the end of the paragraph in 310.4, the handbook states (I know this then isn't code)

So if I am reading this right, let’s say we have a 400A run. We could run 500 kcmil (380A second column, but round up to next breaker size).

Let’s instead run this in parallel, according to the note in the handbook, running in parallel is equivalent to adding the cross-sectional area.

By table 5:
500 kcmil area = 456.3 mm^2

Finding a wire size that is at least half that would be:
250 kcmil area =256.1 mm^2 * 2 = 512.2 mm^2

4/0 is less that the area of 500 kcmil.

On lots of projects, its called out to parallel 2 3/0s to get 400A. This seems wrong to me if this note is correct.

The only thing I see in the code itself is a requirement that parallel conductors comply with 310.15(B)(2)(a). That appears to say that when there are three or fewer current-carrying conductors in a raceway or cable, you can use 100% of the Table 310.16 ampacity (subject of course to the other derating factors, like ambient temperature, etc.) for each of the paralleled conductors.

Intuitively, that makes sense--if the paralleled conductors are spaced for cooling, they should be able to carry their normal current without overheating.


[This message has been edited by SolarPowered (edited 06-16-2005).]

The handbook statement is very misleading. You directly add the ampacities of the parelleled conductors to get the total ampacity. If it was based on the cross sectional area the advantage of paralleling would disappear. The main advantage of paralleling is that you use less total copper for the same ampacity.
Don

Just because a transfer switch is rated 1600 amps doesent mean that it has to have feeder breakers or wiring to support the 1600 amps. When this was installed, in the dot com boom, anything in the way of switchgear that you could get your hands on was fair game as long as it would meet code.

Oh man, I cringe in fear at the sight of each phase having it own conduit. There's another set of pictures that I came across, same situation, each phase had its own conduit and the conduits heated up to 52 Degrees Celsius, according to the guy's temperature reader. I don't remember where those pictures are now.

One interesting detail as to sizing parallel feeds is that you don't really get to 'divide' the ground wire.

As I understand it .... and feel free to correct me ... every pipe is supposed to get a full sized ground wire, with the exception that the ground wire need not be larger than the 'hot' wires.

Assuming all those wires for the feed are at least the correct size and number, what is the appropriate way to avoid the mistake in this photo? I'm assuming everything run through one big monster conduit would be a compliant solution (would derating be needed?). But could you run one wire of each phase, plus a neutral, in each of the conduits (under the notion that the current on each would be equally divided among the wires)?

The correct way is to run a complete set of current carrying conductors in each pipe.
As for the equipment bonding conductor it can be sized to each set of conductors in each pipe
250.102(C)Where the service-entrance conductors are paralleled in two or more raceways or cables, the equipment bonding jumper, where routed with the raceways or cables, shall be run in parallel. The size of the bonding jumper for each raceway or cable shall be based on the size of the service-entrance conductors in each raceway or cable.

That's it ... each phase, neutral, and ground in each run of pipe.

I am curious just how "hot" this installation was actually running. A FLIR picture would be great but simply looking with a non contact thermometer or a touch with a wet finger would still be interesting. I know when I moved in here and really got a good look at the panel (behind a pretty wood chase) they had three service conductors entering through 3 romex connectors. I didn't see any particular heating.

I've always had a worry about splitting the wiring like that. In an ordinary circuit, if one wire comes loose, you will usually know it because things don't work. But with parallel conductors, one of them coming loose doesn't break things (yet). So the remaining conductors carry more of the load. If they are spread over multiple conduits, it causes things to move towards a scenario like in the photo.

Do you depend on really good workmanship to reduce the risk? Would alternate methods like compression terminals help?

pdh, check your NEC. The code has plenty of detailed specifications as to how parallel feeds are to be done. Yes, you're right, good workmanship is key.

The primary reason for parallel feeds is that eventually wire gets too heavy, too stiff to work with.

The fault is not with the wiring, it is with the cabinet, it is too small!

I'm currently working a job (I just moved into training for line voltage, still VERY new) where they have 7 or 9 MC cables paralleled to a 5000A 480V delta switchboard from a 35kV xfmr. Very nice work, cable tray and RMC 20' in the air (heavy industrial site). If I read the ampacity tables correctly, not only does wire get too heavy, there just isn't really any wire to handle 5000A.

I would think that if the transfer switch frame and the cabinet come together as a single unit, or if they are intended for each other, and are listed for use at the current levels involved, that they would have engineered it, and checked it for listing, for the appropriate wiring installed.

But it certainly looks to me like it is stuffed on the right side.

A closer look and I see the top feeds have 1 phase (brown) and what I assume is neutral in the leftmost conduit, and the other 2 phases (orange and yellow) in the 2nd conduit from left. What comes from or goes to the conduits on the right are not marked within the frame of the picture.

Yes, pdh, that's ecactly what you're supposed to do per 300.20(A). That way, the magnetic fields of all the phases cancel out, and there is very little inductive heating of the conduit.

The key to avoid the mistake is to take the # of conduit into account when sizing/selecting parallel conductors. With 2 conduits, they should have selected 2 or 4 wires per phase, and not 3. In some cases, the # of conduits may be dictated by the size and # of lugs. (5 wires does not appear to be an option in this case, and the lugs may not support the larger conductors required for 2 wires per phase.) And yes, derating and conduit fill must always be considered.

Reno: NEC 2008 is not clear in 310.4(D) as to whether they want EACH ground wire to be sized for the OCP, or that all ground wires in parallel add up to the OCP. From the wording, I believe the total simply has to match 250.122, not each parallel ground wire.

I've seen a much smaller version of this: A UPS distribution board with all the phases and neutrals separated out. The UPS was very heavily loaded; most of the circuits were less than 50A, but near capacity.

The four nipples between the wire-way and the panel were so hot you couldn't keep your hand on them for more than about a second. I'd wager something like 130-140 degrees F.

I'd be interested to see how hot this installation actually does get....

-John

Each ground wire must be able to handle the full OCPD.

The reason is that in the event of a short in one of the pipes... ALL of the fault current floods the short, which is going to be huge. The grounding conductor in the undamaged pipe has to be able to handle the surge.

Example, a forklift or crane loses a heavy load that crushes surface mounted pipes in parallel... earthquakes... vehicle strikes... errant backhoes... subsidence crushes pipes... landslides...

If something can go wrong, it will. The instantaneous fault energy that is released from a paralleled XFMR faulting is truly astounding.

Reminds me of a hoist that we repaired once...two phases in one conduit third phase and neutral in another, only the neutral and the lone phase conduit took another route and was substantially longer to boot.

Just because it looks neat doesn't always mean it's right.

Overcurrent Protection:
240.4(B) 800Amps or less next highest breaker. .
240.4(C) over 800 amps next lowest breaker.
Thus Parallel 500 mcm 380x2 = 760 next highest 800.
800 amp breaker per code.
Thus 4 sets of Parallel 500 MCM = 380 x 4 =1520.
240.6 standard breaker rateings next lowest 1200 .
1200 amp breaker per code.
4 sets 600mcm . 600mcm = 420x4=1680 . next lowest breaker 1600.
1600 amp breaker per code.
When you parallel MC cable above 800 amp you have to either order special cable(larger equipment ground) to comply with the code.
Equipment Ground in EACH conduit per 250.122F
Sized per breaker. As stated in pervious post.