Cable sizing - Cable is undersized?

Duncan Brown — Thu, 27 Jul 2017 10:47:28 GMT

Current Revision posted to Hevacomp Wiki by Duncan Brown on 7/27/2017 10:47:28 AM

	Product(s):	Hevacomp Electrical Designer
	Version(s):	V8i (N/A)
	Environment:	N/A
	Area:	N/A
	Subarea:	N/A

Problem

It may appear on occasion that a conductor is undersized. The methods used for cable sizing are described below.

Solution

The Bentley Hevacomp Connect program has a number of considerations when sizing phase conductors.

Where:

Ib	circuit current
In	nominal device rating
It	required current for circuit
Ca	Correction for ambient temperatures (Tables 4B1 and 4B2)
Cg	Correction for grouping (Tables 4C1, 4C2, 4C3, 4C4 and 4C5)
Ci	Correction for insulation (specified with circuit data)
Cd	Correction factor for BS3036 fuses (=.0725)
Cc	Correction for buried cable (= 0.9)

For fixed power, radial, lighting and sub main circuits:

It = ln / Ca / Cg / Ci / Cd / Cc

Where grouping is considered, if the group is not liable to simultaneous overload, then the higher of the following two values for required current is taken:

It1 = Ib / Ca / Cg / Ci / Cd / Cc

It2 = 1 / CA / Ci / sqr ( In x In / Cd / Cd / Cc / Cc + 0.48 x Ib x Ib (( 1 - Cg x Cg ) / Cg /Cg ))

(Please see BS 7671: 2015, Appendix 4, Section 5 for more information)

For ring circuits, In is replaced by 20 Amp rating, Ib is replaced by 20 Amps.

For motor circuits, the following is used.

It = Ib / Ca / Cg / Ci / Cd / Cc

You should note that for motor circuits, the circuit current is used. This information is from Practical Guide to the Wiring Regulations (Stokes) Section 7.3.2 - Protection against overload - motors: