Fuse Boxes, Old Panels, and Service Upgrades

Old houses often still have a screw-in fuse panel somewhere, sometimes alongside a newer breaker panel. Here’s what the code says about the old equipment, and how a service upgrade works.

Plug fuses and Type S adapters

The round screw-in fuses in an old panel are Edison-base fuses, the same thread as a light bulb. They’re rated for no more than 125 volts and 30 amps, and the code permits them only as replacements in existing installations, and only where there’s no evidence of tampering or overfusing ¹.

Overfusing is the danger. Every Edison-base socket accepts every Edison-base fuse, so when a 15-amp circuit keeps blowing its fuse, it’s tempting to screw in a 20 or a 30. Then the fuse no longer protects the wire, and the wire can overheat inside the wall long before the fuse blows.

The fix is a Type S adapter. It screws into the Edison-base socket and can’t be removed once installed. It accepts only a Type S fuse of its own amp rating, and Type S fuses and adapters are designed so tampering would be difficult ¹. With a 15-amp adapter on a 14-gauge circuit, the usual wire for 15 amps ², nobody can ever put a 30-amp fuse there again. Check each circuit’s wire size before choosing its adapter.

A fuse panel that’s sound and properly fused is existing work that may stay ³. One that’s overfused is a hazard the inspector can order corrected ³.

A service upgrade, step by step

Adding electric heat, a heat pump, a car charger, or just a modern kitchen often means more power than a small old service can deliver. Replacing the service is new work, held to the current code ³.

1. Plan. Size the new service and panel for the loads you have and expect.
2. Permit. Get the electrical permit before work begins. Stoughton’s building inspector issues electrical permits under Chapter 10 ⁴; state rules require a permit before a new or added service for some building types and describe how inspections work ⁵.
3. Call the utility. Stoughton Utilities asks for reasonable notice of any substantial load increase that needs a larger transformer, service, or meter ⁶. The utility makes all connections to its distribution system, and its meters are sealed; only utility employees may remove or relocate a meter ⁶. Schedule the disconnect with them.
4. Install. New meter socket, service entrance conductors, panel, and the grounding and bonding system, all to the current code. In an old house this is usually where the grounding gets brought up to date: the service is where the neutral is bonded to earth and the equipment grounds start ⁷.
5. Inspect. The inspector reviews the work.
6. Energize. State law requires the utility to have the electrical inspector’s certificate of approval before it furnishes service ⁶.

What a new panel doesn’t fix

A new panel doesn’t upgrade the branch circuits behind it. Old two-wire circuits are still ungrounded, a switch loop is still a switch loop, and knob-and-tube is still knob-and-tube, all allowed to stay if they’re sound ³. The new breakers protect the old wire only if each breaker matches the size of the wire it feeds.

Try it

Open your panel’s door (not its cover) and make a circuit list: each fuse or breaker, its rating, and what it feeds. If you have plug fuses, note any 20- or 30-amp fuses on circuits that seem to be ordinary lights and outlets; those are the first candidates for a look at the wire size and a Type S adapter.