Detailed rules in the energy sector are crucial for realising the transformation of energy systems to sustainability, but are often overlooked. This paper explores institutional design in governance arrangements for detailed rules, through the case study of British energy industry codes. The British codes governance system involves a high degree of delegation and associated risks of regulatory and informational capture. There is also a gap between code objectives and wider policy goals, producing regulatory inertia. A reform proposal to address these problems is presented. Wider lessons for the governance of detailed rule systems in other countries are drawn out.