G98 or G99? Sixteen amps decides it, and it is a design decision

Ask which Engineering Recommendation applies to a new product and the answer usually arrives as a category — micro-generation, small-scale, commercial. It is not a category. It is 16 A per phase, and which side of it you land on is decided at design time by people who are usually not thinking about connection paperwork.

What the threshold actually separates

G98 covers fully type-tested micro-generation up to and including 16 A per phase. Above it, G99 applies, and G99 is a different order of work: Interface Protection, LFSM-O and LFSM-U, RoCoF, Fault Ride Through and Vector Shift each carry their own verification procedure.

The practical consequence is that two products with almost identical output can face very different routes to market. A three-phase unit at 15 A per phase and one at 17 A are not one step apart commercially.

Why this is an engineering decision, not a paperwork one

By the time a connection application is being written the rating is fixed. If the product sits fractionally above the threshold, the options are to redesign, to derate, or to fund the heavier programme — and the first two are only available while the design is still moving.

That is the argument for asking the question early. The cost of the type-test programme belongs in the business case for the rating, not in a compliance budget discovered afterwards.

Issue 2 changed the picture for storage

ENA EREC G99 Issue 2 is dated 10 March 2025, and its requirements for Power Generating Modules incorporating Electricity Storage came into force on 1 March 2026. A hybrid or storage product being sized now is being sized inside that regime, not ahead of it.

Three things worth checking before the rating is frozen

• Whether the 16 A figure is being read per phase or as a total. It is per phase, and reading it as a total has put products in the wrong regime.
• Whether the design has headroom that pushes it over the threshold under any configuration option a customer can order.
• Northern Ireland runs its own variant — NIE Networks publishes ENA EREC G99/NI — so even within the UK the standard forks.