Diesel Generator wire size

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ads74

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Hi, I need some advise regarding the cable sizing calculation of a diesel generator. The D-Gen is 1200kW, 415V, 50Hz, 2087Amps, 3-ph. The existing wire size for this D-Gen is 3 sets of 3Core x 300 sq.mm XLPE/Cu/PVC/SWA/PVC cable, apparently this cable size is wrong because the cables were melting/burned. Any idea why this happen? Any professional advise on this matter is very much appreciated. Thanks
 
300mm2 CU XLPE/PVC/SWA/PVC is rated about 600 amps for unenclosed, equating to 1800 amps for three in parallel. If the cables are in ducts or in a higher ambient temperature the current rating would be reduced. The generator is 2087 amps. This might be the problem.

Frank2004
 
So, an increase in the wire size is the solution? would it be practical to use 2 nos. of 1Core x 630mm2 XLPE/Cu/PVC/SWA cable per phase? Does armoured and unarmoured cable is a factor? Any idea about cross bonding method? Thanks!
 
more info

more info

how much load was on the cables when they melted? what is the rating of the breaker which protects these cables? the capacity of the generator doesn't really matter. the breaker needs to be properly sized for the cables. Did all of the cables melt or was there maybe 1 per phase or only one phase melting?
 
ads74...Does the nameplate on the generator say 1200KW and 2087 amps?
If this is a 3 phase generator, 1200 KW equates to about 1670 amps at 415V. or about 560A per leg (paralled bundle).
frank2004 gives the rating of the wire at 600A per bundle. Wire length and operating temperature are controlling factors and must be considered. Are the conductors paralled correctly? Same length, material and terminations? There are some recent posts on this web site that give good information on this topic.
steve
 
Hillbilly,

You are using 1200KW as if it is KVA. A typical rating of a generator is based on a 0.8 pf, therefore the KVA rating will be approximately 1500 KVA. The 2087A would be correct.

The (2 sets) 1-Core, 630 mm2 may work, but it will be dependent on the installation method, ambient temperature, cable rating etc, as Frank mentioned.

I would contact the cable manufacturer you are intending to use, and have them give you some guidance.
 
Unfortunately, this problem was given to me today and i have not gone through all the details yet. The first thing that comes to my mind when my colleague told me that the cables were melted was to change the wire size. I have made some rough computation on it,apparently the existing cables 3C x 300mm2 has a current capacity of 481A which is in sufficient. These D-Gen are being used by the US military camp in Kuwait. Any suggestion or advised to solve the problem, except for changing the cable size? I also thinking to use 2runs of 3C x 300mm2 XLPE/Cu/unarmoured/PVC cable per phase, considering all the parameters such as ambient temp., termination, and a lenght of 50meters, any advise on this? I appreciate all the help guys. Thanks
 
ads74 said:
... apparently this cable size is wrong because the cables were melting/burned.


ads74 -

Yes they are sized wrong - but unless the gen is running fully loaded, 24/7, I suspect this is not the cause of the cable burning/melting.

The copper is good for 1800A (frank2004) and the gen output is 2100A. If the gen is loaded to less than 90%, the copper might run hot, but should be okay. And, it's rare for gens to be loaded to 90% or more.

As I understand, the cables are burned/melted, so you are changing them out no matter what - good choice. The NEC specifies 115% ampacity, or 100% if the gen can not be overloaded (Art 445). Depending on the rating (Prime, Continuous, Standby) could be either way. Check the gen specs. Interestingly, you can overheat the gen and the copper feeder and not be overloading the engine. A low power factor will do it.

So size copper for 2100A or 2400A. Pay really close attention to the terminations. With the information you have given us, that is the first place I would look for a problem. Also check the gen leads where they terminate. If the leads are hot coming out on the generator, that can cook the feeder.

If you are still running on the existing feeder, use the opportunity to look for other issues. Are the cables burned on one end or both ends? If you have access to a thermographic scanner, look for hot spots. Look for termination issues. Even if you are changing the cables, I would do this. Mostly cause if the same person installs the cables that did the first set, you may end up with the same problems.

Here is the professional advice part. Changing the copper will fix the symptom, but maybe not the problem. You are focused on the size of the copper - and while that may be the problem, or one of the problems, there may well be other root causes. The best advice I could give is to broaden your scope, look for other contributing issues. It might not be just the size of the copper.

Then again, I could be all wet. If you are running the gen flat-out, and the cables are burned in the middle, not the ends, then hummmm.... more copper, good clean installation is about it. In this case, fixing the symptom will fix the problem.

Let us know how it comes out.
 
coulter said:
Here is the professional advice part. Changing the copper will fix the symptom, but maybe not the problem. You are focused on the size of the copper - and while that may be the problem, or one of the problems, there may well be other root causes. The best advice I could give is to broaden your scope, look for other contributing issues. It might not be just the size of the copper.

Then again, I could be all wet. If you are running the gen flat-out, and the cables are burned in the middle, not the ends, then hummmm.... more copper, good clean installation is about it. In this case, fixing the symptom will fix the problem.

Let us know how it comes out.


Thanks for the advised. Tomorrow i will visit the site and have i look on the gen and the burned cables. I was informed that the cables were very hot and melted completely, but i will check whether it was burned on both ends or in the middle. The US army want to solved the problem immediately for they need power there in the camp, thats why i focus on the cable size thing. Still open for some advise. Thanks!
 
just some things that i would take a look at when i get there are
1)are all the cables burned up or just a couple?
-If only a few what does this generator power? if it is not powering a delta transformer take a look at the size of the neutral and loading from phase to phase.
-obvious stuff, cam-lock fittings if temporary cable, tightness of connections, are these in the sun?
2)did the generator ouput breaker do it's job?
-if this was caused by high current your breaker should have tripped. Rate the cable to the size of the feeder breaker, not the generator. it's the CB's job to protect the cables. if you have a 2000A gen with a 3000A output breaker (for whatever reason...) you should have 3000A capable cables.
 
I have been to the site, i have seen the cables were totaly melted whole lenght of it. I think there's more to it than a wire size issue. The cables were 4core x 300mm2 XLPE/Cu/PVC/SWArmoured with a current capacity of 481A. The generator is 1.7MVA 3ph 440V 60Hz 2230Amps. Gen loading is only 80%. Does how to check if the 2230A is per phase? if this is per phase, at 80% the 4core X 300mm2 should have not melted. In the cable manufacturer catalogue it states the current capacity for 4core x 300mm2 is 481A, does it means that each core has 481A (481 x 4) or the cable it self has 481A? These things confuses me. The cables that were melted were not laid underground properly and touching each other. Any advise for the termination procedure and laying of cables? What causes this cable to burn is still a mystery.
 
It would thake 5 cables per phase to carry the full load of this generator, and that is without derating for installation, and temperature. Even at 80% loaded, thats 1784A worth of load, which is 4 cables per phase, minimum wihtout considering temeprature. With temperature derating, I would predict you need 5 cables per phase to operate this unit successfully @ 80%. At full load you may need 6 per phase. The three cables melted simply due to high temperatures. The fact that they were damaged the full length tells me that terminations were good, and the load was split evenly between phases.

Since all the cables need to be replaced, I would probably jump up to 400mm2to decrease the number of cables per phase, unless they keep a lot of 300mm2 on hand, ready to install. Just make sure the generator terminal enclosure and switchgear can take that many cables per phase.
 
I have made some calculations for the replacement of the burned cables. For a Gen 1.7MVA, 440V, 2230A, 3ph. i will use 2 nos. of 3Cx300mm2 XLPE/Cu cable/phase with a current rating of 312A (applying all the rating factors), but this will carry only 80% of the load. If i use 3 nos. of 3Cx300mm2/phase it will cover the whole load. But the problem is the copper price have gone up. Is it feassible to use 2nos. instead of 3nos.? The generator will not be working more than 80% anyway. Need some advise, thanks!
 
How do you figure 2 cables per phase @ 312A is going to carry 80% of the load. 2 cables per phase at 312A is only 624A, when 80% 0f 2230A is 1784A.

I recommend that you entertain the idea of contacting an electrical engineer familiar with this type of installation and who can perform the necessary calculations. I am getting the impression that this is outside your area of expertise.
 
kingpb said:
How do you figure 2 cables per phase @ 312A is going to carry 80% of the load. 2 cables per phase at 312A is only 624A, when 80% 0f 2230A is 1784A.

I recommend that you entertain the idea of contacting an electrical engineer familiar with this type of installation and who can perform the necessary calculations. I am getting the impression that this is outside your area of expertise.


Please have a look again on my post. i said i will use 2 nos. of 3C x 300mm2 per phase. Now you figure it out how it will carry 80% of the load?
 
Please excuse me if I am miss understanding something, but the way I am interpreting what you are saying is that you are using a 3 Core, 300 mm^2 cable and running 2 of these cables in parallel, and you said that after derating factors, each conductor in the cable can carry 312A.

That means, each set of parallel conductors, per phase, is capable of 624A (312A x 2 = 624A) I am not following you on how this equates to 80% of 2230A.
For clarification, the 2230A is NOT divided by three to get each phase current. The 2230A is PER PHASE. That means each phase needs to have 2230A worth of cable to provide 100% load capability.

Assuming your 312A is correct, that means you need far more than 2 cables, per phase, to carry 80% of the phase current.
 
kingpb said:
Please excuse me if I am miss understanding something, but the way I am interpreting what you are saying is that you are using a 3 Core, 300 mm^2 cable and running 2 of these cables in parallel, and you said that after derating factors, each conductor in the cable can carry 312A.

That means, each set of parallel conductors, per phase, is capable of 624A (312A x 2 = 624A) I am not following you on how this equates to 80% of 2230A.
For clarification, the 2230A is NOT divided by three to get each phase current. The 2230A is PER PHASE. That means each phase needs to have 2230A worth of cable to provide 100% load capability.

Assuming your 312A is correct, that means you need far more than 2 cables, per phase, to carry 80% of the phase current.


For clarification, the 2230A is NOT divided by three to get each phase current. The 2230A is PER PHASE. That means each phase needs to have 2230A worth of cable to provide 100% load capability.

I agree with you with these statement, 100%.

The 312A current capacity is for each core of cable, which means that in one phase of cable it can carry of about 1872Amps. If we say 3core x 300mm2 cable with a current capacity of 312A it means that each core of 300mm2 can carry 312A, not the whole 3 core. Always open for correction. Thanks!
 
ads74-

Your original (first) post said this:

ads74 said:
The existing wire size for this D-Gen is 3 sets of 3Core x 300 sq.mm XLPE/Cu/PVC/SWA/PVC cable, apparently this cable size is wrong because the cables were melting/burned.

Then you posted this at the conclusion (last):

ads74 said:
i will use 2 nos. of 3Cx300mm2 XLPE/Cu cable/phase with a current rating of 312A (applying all the rating factors), but this will carry only 80% of the load. If i use 3 nos. of 3Cx300mm2/phase it will cover the whole load.

It is very clear to many that this will not work. If 3 of whatever burned up because of overloading then 2 of the same stuff will burn up faster, or sooner. You need to be open to installing it right before anyone else can help you.

kingpb said:
How do you figure 2 cables per phase @ 312A is going to carry 80% of the load. 2 cables per phase at 312A is only 624A, when 80% 0f 2230A is 1784A.

Take another look at this, he's onto something here.
 
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