Cable Ratings 1

[keithc]

I am having to upgrade an existing 630A CB, to 1000A due to new equipment arriving. However, the existing 630A breaker supplies two drives/motors via 2 X 150mm 3core cables. We are keeping these existing drives, and adding extra cables onto the bottom of the new 1000A CB, which go off to the new equipment. I have had a couple of quotes to do this work, but one company has said we need to add another 150mm 3c cable to the new CB, to the existing drives, to meet current legislation, whereas another has said we don't need to. Who is correct? or how can I find out?

 

[uno]

keithc

One philiosophy is to size the cable based on the circuit breaker rating , so if the breaker size increases the cable size/no. of runs has to be increased so that the cable always remains protected by the circuit breaker.

Can you give more details on how you are supplying power to different drives from 1000A CB. Is it through a separate MCC/PCC bus connected to the CB Outgoing feeder?

 

[keithc]

Uno,
We are taking 3 single core 300mm cables from the outgoing feeder of the new CB to the new drives. These are going over the top of the cubicles on perforated tray, whereas the existing 150mm cables are in trunking under the floor. We do have room for another 150mm if we need to do it under the floor.

 

[rbulsara]

Are you saying that you are feeding three motors/drives through one 1000A CB? If so it is a very unprofessional setup and may not meet NEC (if it applies to your installtion).

A good engineerig practice will be to have separate OCPD for each drive. I would have an intervening distribution panel fed by the 1000A CB and three brach breakers, sized for each drive, and feed the each drive from them.

Regardless of the Code, I would not associate my name to such a design/installtion.

 

[keithc]

Hi Rbulsara,
Sorry for any confusion.Each drive does have its own CB (Existing drives 400A CB's, and new drives 352A CB's). As you stated this 1000A CB is a distribution panel only which feeds these drives.

 

[rbulsara]

Keithc:

You need to be more clear:

1. Are the 400A drive breakers in the drives or in the distribution panel? They have to be in the distribution panel.

2. Where is the 1000A CB? Is there a feeder between the 1000A CB and the distribution panel?

3. What is the distance from this 'distribution' panel to the drives.

4. What is the distance between the 1000A CB and the distribution panel?

150mm cable is good for about 320A at 90*C. For a 1000A feeder you will need 4-150mm conductors per phase (four 3-core cables). Most terminations are good only for 75*C, at which the ampacity may be less like 285A. 3 cables sounds close enough but you need to check your Codes and actual ampacity.

The cable between the branch brakers (400A) and the drive can be sized to 1.25% of the drive input current or the motor FLA.

If the 400A breakser are in the drive then, you have lot of fixing to do. Each feeder has to be protected at the source.

 

[jbartos]

Suggestion: This kind of project is better to model by software, since there are cable sizes and ampacities involved and possibly harmonics. Some engineers and designers tend to oversize power distribution system rather than to use the software. This implies that the power analysis software can actually pay for it-self handsomely.

 

[alehman]

Use of software to optimally size conductors may be good engineering practice in some situations, however code requirements always take precedence. Codes tables are commonly conservative when it comes to sizing conductors because they are must be applied to many conditions.

 

[jbartos]

Suggestion: Visit

http://www.etap.com/nuclear.htm

or equivalent,
and check the sofware details since it can save a lot of money in the cable ratings, and it is more official than the paper and pencil calculation/design.

 

[keithc]

Thanks for all the suggestions gentlemen/ladies, very helpful.
In answer to rbulsara's questions, yes the 400A breakers are in the drives (FLC = 429A). The 1000A CB is in a panel, which is at most 5m from the drive cubicles. We currently feed from this panel to the drives via a 630A CB using the 2 X 150mm cables per phase. These are attached to a set of copper busbars, which supply our existing drives. The plan was to add the extra 150mm cable per phase, to these busbars, as well as attaching another 300mm cable to the bottom of the new 1000A CB, to feed the new drives. We do have power factor correction on this supply already. This is being done in the UK, if that makes any difference.

 

[uno]

keithc

Even if you are in UK and do not follow NEC , I think the reliability of this system is at stake. Any fault in you new equipment can also result in loss of power to the other two drives.

 

[rbulsara]

Keithc:

With all due respect, the installtion you are describing sounds utterly unproffessional, nevermind codes and regulations, regardless of where you are located.

If I understand correctly," you have a large panelboard in which there is 630A breaker with (2)-150mm, 3-core cable terminated on a busbar. From this bubar two 400A drives are fed. How? Are these both cables 'daisy chained 'to the two drives ? or from the 'busbar' you are splitting the cables to each drive?

Now you plan to replace this 630A CB with a 1000A CB, the existing cables remains as is and a third 300-mm cable, directly from the 1000A CB's load termnials, will feed the third 400A drive!"

If the above is accurate interpretation, you have a shabby installtion, I am sorry to say.

Ideally parallel cables originating from a breaker should be terminated only in one set of fuses/breakers or the input terminals of the equipment.
If they terminate in different pieces of equipment/breaker/fuse, each 'branch' shall be rated for full rating of the breaker (1000A). (not a recommended practice)
If you apply tap rules than, atleast according to NEC, taps longer than 10 ft (+/- 3 meters) but not more than 25 feet (+/- 8 meters) shall be rated at least 1/3 of the feeder breaker rating, which is 333A in this case. But at that point you have a far less than a professional set up in my opinion. Even from maintenance point of view, in order to work on one of the drives, you will have to open 1000A breaker killing power to all the three drives!

If were you , I would install a 1000A or 1200A subpanel with three 400A branch breakers in it and feed the each drive with cables rated for 400A. (or install two new 400A disconects/enclosed CB right next to the panel, tapped of the panel bus, replace the existing 630A to a 400A CB and feed each drive separately)

I hope the size of the 1000A CB is arrived at after duely considering the maximum demand of the three 400A drives.

 

[jbartos]

Suggestion: In some instances, where long runs are involved and conductors oversized, two or more runs from the circuit breaker output to separate load destinations are normal solutions. Also in houses, a 15A 1-pole circuit breaker output terminal is often overcrowded by branch circuit runs to various outlets, which often leads to a circuit breaker trip. However, as the homeowner learns about this shortcoming, one carefully applies loads to avoid the circuit breaker trips.