Parallel 750MCM Conductors

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What size is the main? 4 sets of 750's are good for 1900A. 240.4(B) says we can't go 2000A main. So it must be a 1600A main. Sounds like the EE has some play built in to this install.

If OP could give us some better measurements and a load calc., we could see if the short conductors would be close to 475A each.
 
Chris,
The code does not require the use of standard size OCPDs....you can use an OCPD with an rating of 1900 amps.
 
This is where you needed to think the install out so you would be same length. No not easy but its part of the job. The inspector is a pain for you now but lot better than melt down later. I would ask him for suggestions that he will pass.
 
The parallel cables are inherently self regulating. As one cable draws more and gets hotter its resistance will go up transfering the load to an easier path on one of the other cables. Tell the inspector to get a job in a car wash.
 
quogueelectric said:
The parallel cables are inherently self regulating. As one cable draws more and gets hotter its resistance will go up transfering the load to an easier path on one of the other cables. Tell the inspector to get a job in a car wash.
Click to expand...

The problem with this statement is that you can't quantify 'hotter.' If 'hotter' is greater than the temperature rating of the cable, there is a problem. Can you be certain of the temperature at which the cables will start regulating themselves?

I agree that what you say is true, but that doesn't change the code.
 
Where oh where is the OP?

KarlS.....give us some facts.

Everyone has a opinion, and no one has the facts.:smile:

steve
 
quogueelectric said:
The parallel cables are inherently self regulating. As one cable draws more and gets hotter its resistance will go up transfering the load to an easier path on one of the other cables.
Click to expand...
That is what is described, in the engineering world, as a "second order effect." That phrase means that the concept you are describing is correct, but the numbers are so small that they don't change the situation by much.

In the following example, I am making up numbers, just for illustration. Suppose that due to extreme differences in lengths, the currents in the three parallel conductors that comprise Phase A are 180 amps, 200 amps, and 220 amps. As you say, the conductor carrying 220 amps will be hotter than the other two, and its resistance will go up because of the temperature. When the system settles into a state of equilibrium (i.e., no more temperature changes; all is steady), the currents might be 180.02 amps, 200.01 amps, and 219.97 amps.

Whatever the actual numbers might be, it will not be much of a difference at all.
 
Parallel 750MCM Conductors

Sorry I have not responded. I did not expect to get so many reponses this quick.

This is Eaton 4160V Switchgear with a 2000A main. I have bushings set on the backside of a building for connection to a substation. The Bushings are located above the next section of MV gear. The rear of the main breaker section has a top hat with bus bar extensions. I have connected a 6" x 36" cable tray 90 to this tophat with the other side extending thru a wall penetration. The bushings are then elevated above this cable tray and enclosed. Taking one phase of cables, the inspectors problem is the lengths which are 13'-5", 15'-3", 15'-6" and 15'-5". Remember in the cable tray I need to bundle ABC, ABC, ABC. Therefore, the last set of cables will need to extend all the way back to the beginning phase. This is why the cable lengths are different. Otherwise, per the inspector I should have just piled them up on one side.

I stacked the cable tray correctly but the problem is the parallel cables and running the same length. This is impossible in cable tray.
 
Karl,
Are the footages in you last post correct? If so, I would expect to see the following currents assuming everything else is identical. This is based on a 1600 amp load, but the total is only 1599 as a result of some rounding short cuts.
13'5" at 442 amps, 15'3" at 389 amps, 15'6" at 383 amps, and 15'5" at 385 amps.
750kcmil is good for 475 amps so this should not be a real problem.
 
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