NEC 240.86 Series Ratings?

[chuck@apsinc.biz]

I am working on a project where the main distribution board is 65K AIC rated and feeds a 200HP motor(240FLA), a 250HP motor(302FLA)AND MCC Section the has 437FLA worth of motor load. VFD contollers are used for the 2 large motors, the controllers are fed from the main distribution board and are only 18K AIC rated. The Available fault current at the VFD is 23K amps.

240.86(B): Series rating shall not be used where motors are connected on the load side of the higher rated overcurrent device and on the line side of the lower-rated device, and the sum of motor full-load currents exceeds 1% of the interrupting of the lower-rated circuit breaker.

As far as I can tell, series rating can not be used to valadate the 18K AIC VFD. The VFD must be replaced with a VFD that is fully rated(Min 23K). I need some conformation that I am reading the code correctly before I give the owner a heart attack with this information. Can any one out there provide this conformation?

 

[ron]

Re: NEC 240.86 Series Ratings?

Your interpretation is right on target.
The VFD's must be pretty far from the Main Dist Board to have only 23k available. Contribution from almost 1000 FLA of load side motor load and the source contribution is surprisingly low.

 

[ron]

Re: NEC 240.86 Series Ratings?

Chuck,
While your at it, also be sure that the withstand ratings for the safety switches are adequate. Most non-fused disconnect switches have a withstand of 5 or 10 times the ampacity rating of the switch.

 

[nvcape]

Re: NEC 240.86 Series Ratings?

If you have a handbook handy, look at the example and discussion. The concern is a fault that has contribution from the motors. 1% of 437 is 4 amps. You do not have a 200 and 250HP motor connected, but electronic VFD's, unless they have a hard bypass.

 

[ron]

Re: NEC 240.86 Series Ratings?

nvcape,
1% of 18k (the VFD is rated at 18k) is 180A. Even if you don't count the VFD load, the motor load of 437 is more than the permissable 180A. The code paragraph applies.

 

[gwz2]

Re: NEC 240.86 Series Ratings?

Some questions;

1) How was the Available Short Circuit Current (ASCC)
determined at the MDP and at the VFD ?
2) What KVA size of Poco transformer ?
3) What is the percent Impedance of the tranformer ?
4) What type of Service Entrance (SE) conductors,
copper or aluminum ?
5) What is length of SE conductors ?
6) what size SE conductors ?
7) What type of raceway for the SE conductors,
magnetic or non-magnetic ?
8) What size feeder conductors to each load from
the MDP to each load ? (use same criteria as
for questions 4 thru 7 above for each load).

Appears you have 979 amps of motor load, 240A + 302A and 437 amps.

9) Are all these off of the same feeder from the MDP ?

Point I am trying to determine, is there actually 65K ASCC at the MDP ?


As others have mentioned, the 18KAIR OCPD seems way too low.

 

[nvcape]

Re: NEC 240.86 Series Ratings?

Ron, sorry about the math, was trying to beat traffic out the door. Is there any chance there are isolation transformers in front of the VFD's?

 

[chuck@apsinc.biz]

Re: NEC 240.86 Series Ratings?

Some Answers:


gwz2)Available Fault Current:1,500KVA Trans with 5.75% IMP, equals 31,400 Amps at Transformer. 350LF of 6 RUNS-500MCM to MDP puts an available fault at MDP of 25,174 Amps. Another 20LF of 1 Run-500MCM to the VFD puts an available fault at the VFD of 23,288 Amps.

ron-gwz2)As far as motor load contribution goes, I have not accounted for it. Does any one have a refrence in how to calculate the Motor load contribution? I will double check the withstand rating of the disconnect switches.

nvcape) There are no isolation transformers in front of the VFD's only reactors. Each VFD is backed up/bypassed by a "soft start" controller also connected to the MDP.

gwz2)
9) Are all these off of the same feeder from the MDP ?
This is an interesting question, the answer is NO all the motors are not on the same feeder from the MDP. The MCC has it's own feed, each VFD has it's own feed and each back up Soft Start has it's own feed. Are you saying that since there are no motors on the load side of the 65K breaker feeding the VFD from MDP and no motors on the line side of the VFD back to the MDP that series rating can be applied to this circuit? If Yes, is it permissiable to mix with-in your system, for instance the MCC section can not be series rated but the VFD's can? That doesn't seem right, am I missing something?

 

[steve66]

Re: NEC 240.86 Series Ratings?

OK, I'll be the first to admit I don't completely understand series ratings, but it seems like a breaker installed on the line side of the VFD should be enough (if it is rated for the fault current available there). I would think the VFD should be able to withstand any fault currents comming from the motor it runs. So, it seems like you should only have to protect the VFD from line side faults. A breaker with the correct AIC rating should do this.

Even if the VFD couldn't handle the fault current from the motor, could a breaker be placed on the load side of the VFD with a high enough fault current? Maybe even a VFD with a breaker on the line side, and one on the load side?


Steve

 

[nvcape]

Re: NEC 240.86 Series Ratings?

Series rating is explained at www.bussman.com. Essentially, testing is done per UL489 to determine series ratings. If the upstream breaker opens soon enough, the down stream breaker does not self destruct. Full rating is always the safe way. Not everything out there is a tested combination. Bussman offers their product with different circuit breaker combinations for a series rating.

As far a motor contribution, Bussman's Electrical Protection Handbook has a short circuit calculation. It states that morto short-circuit is about 4 times the full load amps.

As you are fairly close, instead of assuming an infinite primary on the transformer, try to get the available on the primary.

 

[BAHTAH]

Re: NEC 240.86 Series Ratings?

My recomendation would to be very careful in the application of series rating on power distribution systems in industrial installations. You may find that a series rated system is no longer viable because a motor has been added causing you to now exceed the 1% motor FLA allowed in 240.86. If your system is one where motor loads may be added their effect on your series rated design may be an issue.UL489 (testing of series rated combinations) does not include motor contribution in its testing, this is why the NEC limits you to the 1% FLA of contribution which under fault will be 4-6 time that level. The Bussman "SPD" has some excellent comentary on this subject that may be of interest.

[ October 06, 2003, 02:12 PM: Message edited by: grant ]

 

[chuck@apsinc.biz]

Re: NEC 240.86 Series Ratings?

Due to the motor load NEC 240.86 says series rating can not be utilized.

IEEE Standard 1015-1997 States:

"In cases where increases in available short-circuit current necessitate a system upgrade, a second approach, shown in Figure 4-4, may be used for retrofitting existing older systems where recognized series rating is not available. A line-side current-limiting circuit breaker or fuse, which limits peak current and let-through energy, may be added only if the existing load-side breakers do not exhibt dynamic inpedance within the first half cycle....."

Is this still a violation of NEC 240.86?

Does the gear have to be replace with fully rated equipment?

Is the up over and down method valid with the use of load-side breakers that do exhibt dynamic inpedance within the first half cycle?

 

[bob]

Re: NEC 240.86 Series Ratings?

Chuck
Before you go any further go back and get the available fault from the utility. I used 150 mva which is 7000 amps at 12.46 volts and the sym fault at the transformer was about 22 ka. The motor contribution will be about 5 x FLA. Recaculate the fault at the drive and then decide if series is require.

 

[don_resqcapt19]

Re: NEC 240.86 Series Ratings?

Chuck,
That would be a code violation. The code only permits the use of listed and tested series combinations. Calculated combinations are not permitted by the code. I believe the violation would be of 110.22 and not 240.86.
Don

 

[bob]

Re: NEC 240.86 Series Ratings?

Don
If you are referring to my post, the point I was making was to caculate the fault again as I said and it may be lower and not require any changes at all. You are correct in your comment about series rated equipment be tested and approved for series installations.

 

[ron]

Re: NEC 240.86 Series Ratings?

Chuck,
You cannot use the up-over and down method, as every circuit breaker will exhibit dynamic impedance (changing its impedance as the contacts open). As described before, you cannot use a series rating because you have exceeded the % of motor load that may contribute to the downstream breaker.

 

[gwz2]

Re: NEC 240.86 Series Ratings?

Do as Bob suggested, get the Available Short Circuit Current (ASCC) at the transformer secondary from the Poco.

Using the Bussmann Point to Point method I get:

1500KVA * 1000 / 480 volts / 1.732 = 1804.2725 FLA.

1804 * 100 / 5.75 %Z / .9 = 34860 ASCC at the

transformer Secondary.

Guessing from the data provided:

6 parallel 500Kcmil Copper in magnetic raceway

350 feet from Transformer to the MDP,

The ASCC at the MDP is 26190 Amps.

If adding the motor contribution at 979FLA times 5 = 4895 Amps plus the 26190 Amps = 31091 Amps at the MDP.

If not adding the 979FLA motor contributions at the MDP and having 20 feet of 500 Kcmil to the MCC, the ASCC at the MCC would be 24139 Amps.

The 18Kaic OCPD just does not make this a series rated system.

What are the cable lengths and size and raceway type for the 200 HP and the 250 HP motors ?

Are these two motors on CB's in the MDP ?

You noted 437 FLA on the MCC, and yet only have 1
500Kcmil ( 380 Amps ) supplying the MCC.

A detailed description of a one line diagram for this installation would be helpful.

Magnetic or Non-magnetic raceways was not indicated.

gwz2

 

[gwz2]

Re: NEC 240.86 Series Ratings?

Bussmann had changed the Motor contribution multiplier from several years ago which was 6 then 5 the last I heard, the multiplier of the total motor FLA is now 4. The program I used in the previous post had the " 5 " multiplier.

The motor contribution should be 4 * 979 FLA = 3916 Amps plus the MDP ASCC = 30106 ASCC with the motor contribution instead of the 31091 Amps of my preious post.

Again, if the Poco ASCC is less than the 34860, this may lower the load end, but I doubt that it will be safely below the 18Kair OCPD.

And, as noted before, any series rated system must be labeled on the factory enclosures.

gwz2

 

[charlie]

Re: NEC 240.86 Series Ratings?

IEEE Standard 1015-1997 States:

"In cases where increases in available short-circuit current necessitate a system upgrade, a second approach, shown in Figure 4-4, may be used for retrofitting existing older systems where recognized series rating is not available. A line-side current-limiting circuit breaker or fuse, which limits peak current and let-through energy, may be added only if the existing load-side breakers do not exhibit dynamic impedance within the first half cycle....."

Is this still a violation of NEC 240.86?

I had to jump in since this question from the IEEE Blue Book wasn't answered to my satisfaction.

Yes, it is a violation unless approved by the AHJ. The only time this can be used is where you have the old style, large, airframe circuit breakers where you can be assured the breaker will not introduce dynamic impedance into the circuit.

Bussmann is trying to get this method approved for the 2005 edition of the NEC in 240.86. I managed to get it accepted in the Supervised Industrial Installations portion of Article 240. After all, if it can't be used there, where can it be used?

 

[bob]

Re: NEC 240.86 Series Ratings?

Chuck
Using your conductor information and assuming a utility available fault at 100 mva the fault levels are as follows:

Transformer - 20.3 ka sym 34.5 ka asy
End of secondary - 16.9 ka sym 23.8 asy
At the VFD - 15.9 ka sym 21.4 ka asy

Check with the mfg to deetermine if the drive rating should be symmetrical or asymmetrical

[ October 07, 2003, 05:16 PM: Message edited by: bob ]