Cable sizing of a power feeder 1

[nkn5]

Incase of a cable feeding power from one switch-gear to another one, I understand that the cable better shall be sized as per the least of outgoing breaker of later and incoming breaker of the second one. But sizing as per the connected load on the downstream swgr is not a good practice as we are trying to size on some thing which can change.

Now, Can any one suggest if any standard (IEC / ANSI) requires to be done this way or is it just a good practice followed?

Thanks in advance

 

[alanElec]

The upstream breaker is the one meant to protect the cables and down stream equipment/switchgear..I think it would be better to size the cable to have a current carrying capacity of atleast 1.25 times FLA it has to feed.Voltage Drop also has to be kept in mind.

 

[alanElec]

http://myelectrical.com/tools/cable-sizing-calculator

This link is one of the most reliable links for Cable Sizing.I usually cross check my calculations with this one.Feel free to try.

 

[MikiBg]

It should be sized according to upstream circuit breaker protection unit. The best approach is to use protection unit rated current, because then cable can carry "all that CB can pass through", but you are allowed to use actual current setting as well, which may resulting in smaller cable diameter. This can be seen in IEC 60364 set of standards, but implicitly.

 

[7anoter4]

In my opinion, if both switchgears are provided with a coupler breaker at each end of the interconnection cable, then the cable will be loaded always from only one side, so the overcurrent protection [the breaker] of the supply side will protect the cable.
NEC [NFPA 70] art. 240.4 Protection of Conductors (B) up to 800 A and (C) for above states:
(B) “The next higher standard overcurrent device rating (above the ampacity of the conductors being protected) shall be permitted to be used”[ if the next higher is less than 800 A.]
(C) “Where the overcurrent device is rated over 800 amperes, the ampacity of the conductors it protects shall be equal to or greater than the rating of the overcurrent device”

 

[cherryg]

It appears from your statement..."something which can change" that your load list is not firm. Please list down all your loads (kW, KVAR, kVA). Next apply the required load factors, as per engineering judgement (or BHP values if available from the mechanical folks), and seprately list the connected and operating loads. Add suitable margin, which should match the anticipated load growth. This is the tricky part.
Now you have a good operating load and you can select a suitable breaker AF/AT rating both in the source and destination Switchgear. Based on the upstream breaker trip setting, the cable needs to be selected. If the Switchgears are far apart, a voltage drop calculation is required, usually limiting the drop to 3%. A check should be done to see that the selected cable can be terminated on the breakers / terminating points.