| 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: