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#92938 - 04/18/05 01:48 PM Cable Feeding lower ampacity busway
greg2468 Offline
Junior Member

Registered: 01/13/05
Posts: 7
Loc: Houston
My application is that I have an existing application installed in the 70's where a 1000A cable is feeding an 800A Busway and I would like to keep the existing 1000A fuses for overcurrent protection at the primary of the 1000A cable. In Busway - 368.17(B), the exception is that you don’t have to provide overcurrent protection for the reduced amperage busway if the total length of the reduced ampacity bus is less than 50 ft, the reduced ampacity is at least 1/3 the size of the overcurrent protection for the larger bus, etc.
My problem is that the NEC does not specifically address my situation one way or the other since it is not covered in section 240 either. If I had a 1000A busway feeding a 800A busway, I could claim this exception and fuse it at 1000A. However, since I have a 1000A cable feeding a 800A busway, my application is not specifically covered even though the principle is the same as is covered in the bus-bus exception. My question is how do I determine my minimum requirement? Note that I am in an industrial plant that does not have local inspectors come in. Thanks!

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#92939 - 04/18/05 04:10 PM Re: Cable Feeding lower ampacity busway
renosteinke Offline
Cat Servant
Member

Registered: 01/22/05
Posts: 5305
Loc: Blue Collar Country
Forgive my confusion...since you are transitioning from cable to bus....why not have an OCPD (fuse) there? What am I missing?

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#92940 - 04/19/05 05:27 AM Re: Cable Feeding lower ampacity busway
greg2468 Offline
Junior Member

Registered: 01/13/05
Posts: 7
Loc: Houston
This is an existing installation and there are around one hundred of them at different plant around the country. It would be very expensive to retrofit an OCD in each application. They were installed in the late 60's or early 70's.

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#92941 - 04/19/05 06:20 AM Re: Cable Feeding lower ampacity busway
George Little Offline
Member

Registered: 01/18/04
Posts: 1492
Loc: Michigan USA
I guess I'm with Reno- Why don't you fuse the whole system at 800a especially if there is nothing else on the cable feeder. Is there any reason to fuse it at 1000a? Are you having motor starting issues?
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George Little

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#92942 - 04/19/05 06:27 AM Re: Cable Feeding lower ampacity busway
greg2468 Offline
Junior Member

Registered: 01/13/05
Posts: 7
Loc: Houston
I didn't want to get into the details but there is a good reason why I need to fuse up to 1000A. The cable feeder feeds out to the center of a longer bus (bus goes one way for 20 ft and also goes the other way for 20 ft) and there are multiple taps off of the bus on each leg. The total load exceeds 800A but if you total the load on each side of where it splits, it is lower than the 800A bus rating.

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#92943 - 04/19/05 07:13 AM Re: Cable Feeding lower ampacity busway
renosteinke Offline
Cat Servant
Member

Registered: 01/22/05
Posts: 5305
Loc: Blue Collar Country
I guess we have to fall back on the basic question of: Is the equipment being used, and was it installed, as the manufacturer intended? Or, was something "field engineered" at some time in the past?

Your statement that it's carrying more than the 800 amp rating concerns me- that's why I would look at the original purpose of this equipment.
If this is something cobbled together from bits & pieces and 'made to work,' I'd say it was time to budget replacement. After all, your electrical needs are only going to increase with time.

That said, assuming that the two sides of the 800 amp bus are not electrically connected, this would be a straightforward application of tap rules, and the only issue is whether the bus mfr intended the equipment to be used this way. We're back to that 'field engineering' issue.
Keep in mind that the folks at UL are not psychic, and have to rely upon the equipmet manufacturer's intent in order to decide how to evaluate a product. When you start adapting equipment, you need some idea as to what standards would apply, before you can make a good judgement.

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#92944 - 04/19/05 07:24 AM Re: Cable Feeding lower ampacity busway
greg2468 Offline
Junior Member

Registered: 01/13/05
Posts: 7
Loc: Houston
The two sides of the bus are electrically connected. It is one 40 foot long bus with a cable feeder directly connected to the center without an OCPD in between. Since the total amperage of the connected taps (each has it's own OCPD) to each bus end are below the 800A rating, there is no way the total load on each end can exceed the rating. However, the total load connected to the two bus ends together can exceed 800A and that is why I would like to fuse at 1000A. Thanks.

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#92945 - 04/19/05 07:28 AM Re: Cable Feeding lower ampacity busway
winnie Offline
Member

Registered: 09/15/03
Posts: 652
Loc: boston, ma
IMHO, 'I don't want to go into details' means that you won't get any useful advice. The devil is in the details, and doling them out in dribs and drabs means that you are wasting people's time trying to guess at your situation.

The point about the busway being fed in the middle changes the picture, in a big way. What other little features might tip the scales in one direction or the other, hmm?

I do not believe that the NEC directly addresses your installation, but that you could make a case based upon guidance from the NEC that your installation is acceptable, or acceptable with small modification. Since the NEC is not directly applicable, you may be in a situation where you use it as the basis for an engineering analysis, and then use that engineering analysis to get an exception granted by whichever AHJ is involved, if any.

The first principle that you need to observe is 210.19(A)(1) 'ampacity not less than the maximum load served'. This is not an absolute rule; for some _specific_ non-continuous loads, the continuous ampacity of the conductor can be less than the short term loading created by the load device. 210.19 is about branch circuits, but it is a general principle followed all over the place.

The second principle is that if the conductors are sufficient for the load served, then in some specific limited cases, the OCPD may be sized larger than the ampacity of the conductors. Off the top of my head, the OCPD for motor wiring and welders are examples, and the reduced ampacity busway exception is another example. In general, the OCPD _must_ be sized to protect the conductors, but there are specific exceptions.

Clearly you couldn't safely put 900A of load on an 800A busway with 1000A of OCPD, regardless of the exception in 368.17(B). I don't see an explicit statement in 368 that says that a busway can only carry its rated current...presumably that is in the listing of the busway components

The exception in 368.17(B) clearly permits an installation where you might have an 800A busway with less than 800A of load on it, extending off of a 1000A busway, with the entire system is fed with 1000A OCPD. If additional devices are attached to this busway without proper supervision and load calculation, then there is a risk that the 800A busway would be overloaded without tripping the 1000A OCPD, and it is interesting to note that the 368.17(B) exception mentions industrial situations but does not mention 'engineering supervision'.

In your situation, if we imagined a slightly modified system, where a short length of 1000A busway was fed by the cable with the 1000A OCPD, and than two lengths of 800A busway were fed from the 1000A bus, the exception would clearly apply and your installation would be to code. Two 800A busways, each < 800A of load, 1000A OCPD shared by the two busways via a 1000A intermediate connection.

As I see it the problem is right at the connection between the cable and the busway. Here you have a connection that is presumably designed for the _800A_ busway, where you may have between 800A and 1000A of current. If you have 450A feeding one busway, and 475A feeding the other length of busway, then right at this junction you have 975A. I don't see how you can use the NEC to determine if this is safe. Presumably you would need some sort of engineering analysis to determine that this short length of busway can reliably and safely operate as an extremely short length of 1000A busway feeding the two 800A sections. This doesn't sound too far-fetched to me; while the actual bus bars are presumably 800A bars, you have considerable build-up of materials for actually making the junction. But this is no longer a question of code interpretation; it is a question of engineering analysis.

Please note: this is an opinion based on reading of the code and physics, not experience with these installations.

-Jon

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#92946 - 04/19/05 05:46 PM Re: Cable Feeding lower ampacity busway
renosteinke Offline
Cat Servant
Member

Registered: 01/22/05
Posts: 5305
Loc: Blue Collar Country
"The devil is in the details"

I've been thinking about this some...and I think we need to look at our starting points some more.

First: What are the bussway ampacities? I know you said 1000 and 800 amps, but we need to know if the rating is for maximum current, continuous current, or what. If you're going by the nameplate, UL says busways are marked with the "rated continuous current." (White Book)
Now, how do you "know" you're exceeding the rating? Is it based upon the current actually measured (rms)....and is that load continuous (over three hours a day)? Or is it thee maximum current measured at start-up? Or is it based upon equipment FLA rating, or by adding up the fuses, or some other method?

Where a load is continuous, code requires all conductors, including bussbars, to be sized based upon 125% of the continuous load. This means that a buss way rated at 800 amps of continuous current can handle intermittent loads as high as 1000 amps.

Fuses are often sized quite a bit larger than the current that the load actually requires. "Full Load Amp" ratings on motors are likewise much higher than the current the motor actually draws, as the motor is not fully loaded.

If the actual measured continuous load is in fact more than 800 amps, then the connection to the busway is unsafe- regardless of whether the bussway is connected at the end, in the middle, or anywhere else.
I would go so far as to say that if there are any signs of overheating, the installation is unsafe, and should be replaced.

If, however, your fears are based upon a few moments of more than 800 amps, say, at start-up, or upon some sort of calculation that shows a load greater than that measured , I say the fears are unfounded.

One last note...I am assuming that this is a three-phase installation. If this installation serves variable speed frequency drives, it is critical that you pay particular attention to your neutral bus, and the load on it. That is why I specified measuring RMS amps....just in case there is a hermonics issue here.

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