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Joined: Oct 2004
Posts: 265
D
Member
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 [Linked Image]

Dave

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Joined: Sep 2003
Posts: 650
W
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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

Joined: Nov 2000
Posts: 2,148
R
Member
Dave,
Where does the code say to use the conductor area and not its ampacity for parallel conductors?
Don


Don(resqcapt19)
Joined: Oct 2004
Posts: 265
D
Member
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)
Quote

Conductors connected in parallel, in accordance with 310.4, are considered a single conductor with a total cross-sectional area of all conductors in parallel.

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.

Joined: Jul 2004
Posts: 625
S
Member
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).]

Joined: Nov 2000
Posts: 2,148
R
Member
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


Don(resqcapt19)
Joined: Jun 2005
Posts: 2
G
Junior Member
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.

Joined: Mar 2008
Posts: 76
P
Member
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.


I have a sense of adventure, I just keep it leashed with common sense.
Joined: Jan 2005
Posts: 5,445
Likes: 2
Cat Servant
Member
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.

Joined: Jan 2005
Posts: 354
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pdh Offline
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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)?

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