The rating is used to indicate how much “headroom” a given substation (or supply area) has to accept additional demand or generation connections without network reinforcement.
Green: healthy headroom – e.g. “greater than 5% available headroom” or “greater than 5 MVA” in some cases.
Amber (Yellow): moderate headroom but starting to be constrained – e.g. “between 0% and +5% headroom” (or up to some small positive margin) or “1 MVA to 5 MVA headroom” in some cases.
Red: little or no headroom (or overloaded) – e.g. “less than 0% headroom” or “headroom less than 1 MVA”.
