If you are familiar with IEC or IEE wiring, but no so with NEC, read through.

Cable selection in NEC is not same as IEC.

When you select a LV cable in a project which uses NEC as the wiring code, the following must be
observed.

1      125% rule ( or  80% rule)
1.25 times current carrying capacity is required.

In NEC and UL, MCCB, MCC and panel board  are 3 hours rating, not continuous. For continuous load,
which is continuous for more than 3 hours, MCCB rating must be greater than 1.25 times the load
current. For instance, continuous load of 100A requires 125AT MCCB at least.  If you select 100A MCCB
for 100A load, it may trip if the current of 100A continues more than 3 hours. The power cable connecting
this load is protected by this 125A MCCB, thus the current carrying capacity of the cable is 125A
minimum, which means you need a 125A cable for 100A load.  

Cable current carrying capacity >=  MCCB rating  >= 1.25 times continuous load current

Refer to NEC 210.19(A)(1), 210.20(A), 240.4

IEE Note
Cable current carrying capacity >=  MCCB rating  >= Continuous load current
Refer to IEE 433-02-01 (1) and (2)

2      Many ACBs and some MCCBs are 100% continuous rating.

If your ACB or MCCB together with the enclosure is 100% continuous rating, the item 1, above 125%
rule does not apply.  Such equipment or device has normally large rating current such as 500A or larger.

Cable current carrying capacity >= MCCB rating >=  Load current

However, as far as I know, most of LV loads are fed from 3 hours rating panels. Practically, 100%      
continuous rating is only for ACB.

Refer to NEC 210.19(A)(1) Exception No.1, 210.20(A) Exception

3      For cable protection, the next higher standard over-current device permitted.

NEC permits a cable is protected with next higher OC device. For instance cable with 100A current
carrying capacity must be protected by 100A MCCB, however, 101A current carrying capacity cable can
be protected by 110A MCCB which is next higher standard of 101A.

The rule shown in item 1 above needs to change to the following way.

Cable current carrying capacity >= 1.25 times continuous load current
And
MCCB rating >= 1.25 times continuous load current.
And
Cable current carrying capacity >= next higher MCCB rating

Refer to NEC 210.19 (A)(1), 210.20(A), 240.4 (B)



      
  Continued.....
NEC (NFPA 70) Wiring
Useful link to learn NEC

One of the best NEC
instructor in USA.
www.mikeholt.com
PDF file for detail
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