Generator Main Cable connection standards in the USA 1

[genRman]

I am looking for authoritive 'best practice' information on generator output connections, i.e. where a contractor would use something akin to steel wire armoured flexible cable to connect through a bushing box onto the generator output bushings. This is primarily for generators up to 13.8kV, in the range of 2 to 5MVA.

There seems to be a confligration of 'Code' plus local standards, and I wonder if anyone has seen a summary to suit the whole of the USA?
can anyone help please?

Thanks.

 

[MMAshwin]

Hi,

This is not really my field but try the following links they may be helpful

http://necdirect.org/

http://www.joetedesco.com/

http://www.mikeholt.com/

regards

 

[redtrumpet]

genRman - is there something fundamentally different about a generator as opposed to other equipment, say a transformer, that would have undue influence on the cable selection? Otherwise I would say look at the same factors as you would when designing a cable system for other electrical equipment. A good overview of cable systems is given in IEEE Std 141 (Red Book), Chapter 11 - I might start there. I don't think you will find one "best practise" that covers all situations. For example, you may find the ideal conductor to be MV busduct, unarmoured cable in duct, armoured cable in tray, etc. depending on the installation.

Steel wire armour is usually used to support vertically run cables. It does give a high degree of mechanical protection, if that is what you are looking for.

 

[redtrumpet]

genRman - I may have misunderstood your question. Is your concern the actual entry into the generator terminal box and subsequent bushing connection? Since you cited steel wire armour, is your concern mechanical protection of the conductor at the point of entry?

 

[genRman]

redtrumpet,
The fundimental difference on the type of genset system I am considering, is the potential for transverse generator movement when on isolators, either soft mounted to a genset skid or the skid isolators mounted to a concrete floor.
In either case, under start up, the set may roll quickly through different resonances. In addition, under fault conditions, say for instance full load rejection due to grid trip, then there will enevitably be some relative movement between the anchor point for the cable as it leaves the tray, or floor ducting, and the bushing box of a generator. This is the crux of the question and it is my belief that it is an overlooked but extremely essential consideration, when designing & installing the genset.
Hence my original question - albeit possibly too broad ranging, but then I need to start somewhere!
Experience tells me that where using steel wire armoured cable, it is good practice to include a cable sweep through a 90 Degree angle, something akin to 15- 20 times the diameter of that cable, with secure points of clamping at either end.

My recent review of some installations has shown what I believe is a horror of poor practice, where cables are being fully supported on bushings alone, with no cable clamping. I would like to understand where this practice comes from, & thought some esteemed engineers would be able to provide some direction to me.

 

[redtrumpet]

This is akin to connecting cables to motors - flexible connection to allow for vibration, and on motors with slide bases - you have to leave enough slack in the cable to allow the motor to freely move within the travel of the slide base. I don't know about best practises, but the following works for me.

1. Ideal entry to terminal box is from the bottom to improve sealing against dust and water hosedown. This is regardless of whether or not the cable tray or raceway feeds from above or below the terminal box. Top entry is least preferable.

2. Cable obviously must be supported, but 3 to 5 feet of unsupported cable won't hurt anything. If feeding from tray located above, extend channel iron or angle iron support from the floor to the tray. Clamp cable to support, run cable beside equipment pad and turn upward to enter terminal box. You may have to mechanically protect cable on the ground. Obviously, try to route where it will not be a tripping hazard or exposed to mechanical injury. Bend radius of 15 to 20 times cable diameter is excessive, and is typically the minimum radius for pulling (actually 24 times for shielded armored MV cables), not training into final position. Shielded MV cables with tape interlocking armour typically require 12 times bending radius for final training.

3. If feeding from tray or trench located below, the cable will not require support at all unless the tray or trench is more than 5 feet below the terminal box. In that case, mount a piece of channel strut or angle to available support steel or concrete at 5 foot intervals and clamp the cable to it.

4. If feeding wire in conduit run the last 3 to 5 feet in flexible metal conduit or liquid-tight flexible conduit.

5. Remember to leave enough slack to account for expected movement of equipment.

 

[redtrumpet]

Additional points I forgot - the bushing should not have to support the weight of the cable, this should be supported by the cable connector used to enter the terminal box. The connector will typically terminate the outer jacket and armour. The phase conductor will continue on to the bushing. Also, under short circuit conditions, 3 conductor cable constructions move very little if at all. Single conductor constructions will move, and if this is your case, you may have to put more thought into where you clamp the cable and how much slack you can leave. Still, the magnetic force will drop off as the square of the distance between the cables, so if your separation is great enough the movement under short circuit conditions may be negligible.