Sales tax for an electrical contractor: you are the consumer of wire and the retailer of light fixtures
In one paragraph
California treats you two ways at once. For wire, conduit, boxes and devices that disappear into the building you are the consumer: you pay sales tax to the supply house on your cost and charge the customer nothing labeled tax. For light fixtures, alarm fixtures, standby generators and switchgear that keep their identity, you are the retailer: you owe tax on the selling price and need a seller's permit. Getting this wrong in either direction is an audit finding; getting it right is a bookkeeping habit.
The rule
Under CDTFA Regulation 1521, construction contractors are consumers of materials (property that loses its identity to become part of the real property) and retailers of fixtures (accessories to a building that do not lose their identity when installed). Conduit and electric wiring and connections are materials (Appendix A); lighting fixtures, alarm fixtures, affixed electric generators, transformers and switchgear are fixtures (Appendix B). Tax on materials is owed on your cost; tax on fixtures is owed on the selling price, or on your cost if the contract states no price (CDTFA).
Why it matters
Almost every electrical job mixes the two. A panel change is mostly materials; a kitchen remodel with twelve recessed fixtures is a retail sale of fixtures with an installation attached. If you never registered for a seller's permit, the CDTFA can assess the tax on every fixture you ever installed, plus penalties and interest. If you charge tax on materials you already paid tax on, you are collecting tax you are not entitled to.