HV Phases in separate conduits

Status
Not open for further replies.
steve66

steve66

Senior Member
Location
Illinois
Occupation
Engineer
A factory is planning to replace five 480V services with a primary high voltage meter and high voltage feeders.

For the high voltage feeders, a salesman reccomended running each phase in a separate conduit. That way, if one cable develops a fault, it won't damage the other two phases.

I was just wondering what opinions people had on this. Is it better to run all three phases in a single conduit?

Steve
 
Separate Phases in separate conduits

Separate Phases in separate conduits

Ask that saleman who is going to pay for the single-phasing event that shorted out multiple motors, condensers, fried a few computerized machines, etc. If you lose a conductor, what good does that do for a large factory whether or not it is a single phase or 400 to 600A of all three phases?


Are you able to keep all wire lengths the same for each phase? Is there any metal (unistrut, etc) holding the conduits together? I have seen a 4000A service to a hospital blow up 12 hours after energizing because of the fact that the electrical foreman decided to do exactly what you are talking about. What he didn't know was that when the existing conduit was installed the previous contractor had used unistrut below grade to hold the pvc conduits together. This caused a magnetic field issue (according to the electrical engineer hired to investigate) and smoked the B phase. This then knocked down the entire hospital as he fried the switchboard (the B phse shorted at the lugs in the switchboard - taking out the entire B phase bussing), which wasn't available as a load for the emergency generators. Several issues on the project, but it was all uncovered by putting a single phase in each conduit.
 
For the primary incoming service, if is is medium voltage they are more than like going to be shielded cables. The main point is that the cable is shield, which confines the magentic field within the cable. Utilities run high voltage phase conductors in individual conduits all the time.
 
This would be new duct banks, so exisitng unistrut will not be a problem, and yes, all the conduits would basically be the same length.

Sheilded cables are basically required per 310.6, and I am assuming the contractor will comply with 300.20 for preventing the magnetic heating.

As far as the benefits go, the idea is that if a cable goes bad, we would only have to replace that single cable instead of all 3 cables. As far a single phasing goes, I don't really see any difference between having all the cables in one conduit, or 3 conduits. With all cables in the same conduit, there is no guarentee that single phasing won't happen if one cable goes bad.
 
hillbilly said:
I suggest that you show the salesman 300.3(B) in the NEC.
Click to expand...

I would also suggest looking at 300.2(A). Wiring methods in this chapter are for 600V or less. MV and HV cable can be separated by phases but I would also recommend shielded cables.
 
The main point is that the cable is shield, which confines the magentic field within the cable.
Click to expand...
The shield on the medium voltage cable does not confine the magnetic field. You need a ferrous material to provide shielding from magnetic fields.
The following is from this documtent that was filed as part of a power line siting hearing in Hawaii.
Street and sidewalk locations with underground power lines typically ranged from about 1 mG to a maximum of over 25 mG directly above the underground power line in the street.
Click to expand...
Those underground lines would be shielded lines.
Don

edited to add link
 
Last edited:
Mr. Bill said:
I would also suggest looking at 300.2(A). Wiring methods in this chapter are for 600V or less. MV and HV cable can be separated by phases but I would also recommend shielded cables.
Click to expand...

I also recommend looking at 300.20.
 
kc8dxx said:
I also recommend looking at 300.20.
Click to expand...

I don't know what point you were trying to make. 300.20(B) does allow individual conductors if proper precautions are made to minimize the inductive effect. The wiring distance might be minimized when not grouping the phases. Look at the bottom of this link.
 
Mr. Bill said:
I don't know what point you were trying to make. 300.20(B) does allow individual conductors if proper precautions are made to minimize the inductive effect. The wiring distance might be minimized when not grouping the phases. Look at the bottom of this link.
Click to expand...


That is an interesting note to be at the very bottom of a lot of fine print. 595 foot limit on 750KCM cable at 15KV?? I'm not following their note. Is that if I ground one end of each shield only, or if both ends are grouned?

Since isolated phase installations are more common at Medium Voltage, I wonder if MV switchgear has provisions to comply with 300.20?
 
One very specific question I was wondering about:

Assume I install with one phase cable in each conduit. If a fault does happen on one cable, what are the odds that that cable can be pulled out, and the conduit will be suitable for reuse?

If the conduit is messed up, or the cable is damaged beyond removal, now I don't have any way to reinstall that phase. If that happens, I would have been better off with (3) cables in one conduit, and a spare conduit.

Steve
 
steve66 said:
If a fault does happen on one cable, what are the odds that that cable can be pulled out, and the conduit will be suitable for reuse?
Click to expand...
Slim to none. This is among the reasons why you always run empty spare conduits in duct banks.
 
Cable Ampacity

Cable Ampacity

If I install 3 separate conduits, with one single conductor in each conduit (isolated phases), how do I determine the ampacity? Do I use Figure 310.60 Detail #1 or Detail #2?

Reading Table 310.77, Detail #1 is (3) single conductor cables in a single conduit (duct.) So that's not exactly what I have

But Detail #2 is (3) sets of (3) single conductor cables. Thats not what I have either.

The only details that show isolated phase arrangements is details 9 and 10. But they are for directly burried cables.

Steve
 
steve66 said:
If a fault does happen on one cable, what are the odds that that cable can be pulled out, and the conduit will be suitable for reuse?
Click to expand...

It will all depend on the nature, severity and duration of the fault. Most primary cables fail due to imperfections/small voids on the insulation of the cable or ingress of water. The imperfections and water ingress tend to result in creation of "water trees", which will grow over time. Those "trees" are a conductive path between the main conductor and the concentric neutral of the conductor. Of course, the concentric of the cable is grounded at both ends, so if a "water trees" becomes a "conductive tree", the result will be a fault in the cable.

Like I said, depending on the nature, severity and duration/"time to clear the fault", the conduit might be usefull, and it might or might not affect other cables on the same conduit. Utilities constantly run three phases on a single conduit, and while I have seen some un-faulted conductors being affected by the faulted one, the vast majority is not affected. This happens because a good protection system is in place and limits the fault duration, etc.

By the way, there are conduit repair products on the market, designed specifically, so that a utility/contractor can dig a trench, open up the conduit, splice & repair a faulted cable, then repair the conduit.
 
Last edited:
5KV Shielded Single Conductors

5KV Shielded Single Conductors

I have a question....

I am installing 3 5KV shielded single conductors in a switchgear using a OZ type grommet connector which is large enough for all conductors that is not aluminum. With all conductors routed through one hole will that installation avoid any magnetic fields? The switchgear cabinet also has a aluminum plate installed.
 
Status
Not open for further replies.
Top