Where circuits are grouped, there are two availablemethods of calculating the rating factor C_g to be applied:

• Subject to simultaneousoverload:Assumes that more than one of the grouped circuitsmay be overloaded at any one time –applies grouping using: I_n/C_g.

[BS7671:2018,Appendix 4,5.1.2:Equation 2]

• Not subject to simultaneousoverload:Assumes that only one of the grouped circuits will normally be overloaded at any one time (a reasonable assumption for general distribution loads) – uses two calculations to determine C_g from which the highest calculated value is to be used. Usually gives a reduced level of derating compared to ‘Subject to simultaneous overload’.

[BS7671:2018,Appendix 4,5.1.2:Equation 3,4]