If you have electrical questions you’d like answered in a future edition of this column, send them to the Editor at northshorejournal@gmail.com, or email John directly at john@clovervalleyelectric.com.
By Clover Valley Electric’s Master Electrician, John Christensen
When Breakers and Fuses Don’t Match the Wire
A reader emailed this week about two things that show up in North Shore homes and cabins: breakers that are oversized for the wiring – like a 30-amp breaker feeding 14/2 cable – and whether old fuse systems can remain and be safe.
Breakers and fuses are overcurrent protection devices, and their primary job is to protect building wiring from overheating. For typical copper conductors in dwelling wiring, the NEC “small conductors’ rule” requires: 15-amp overcurrent protections for 14 AWG, 20-amp for 12 AWG, and 30-amp for 10 AWG. When that relationship is violated – like a 30-amp breaker on 14/2 NM-B – the wire can overheat before the breaker trips. That extra heat raises fire risk – most commonly at splices and device terminals.
There’s an important exception to this set of rules. The 2023 National Electrical Code (and many editions before this version) allows certain equipment applications to use larger overcurrent protection. The specific types of equipment can be found in the table associated with 2023 NEC 240.4(G). Homeowners most often see this with HVAC equipment (mini-split or air conditioner), but they might also encounter it on some motor circuits or electric welders. These devices can have “starting” or inrush current. The normal current from a mini-split condenser might be small enough for a 14 AWG wire to handle, but in the moment that the condenser motor is started, the current can exceed 15 amps by a large enough margin to trip a 15-amp breaker. In this case, the nameplate on the equipment will outline an appropriate breaker size, which might be 20- amp or even 25-amp.
A larger breaker can be correct on a dedicated equipment circuit if the conductor size and terminations are chosen to match the equipment’s required circuit ampacity and maximum overcurrent protection as outlined by the manufacturer. That’s very different from putting a larger breaker on a general lighting or outlet circuit just to stop nuisance trips. If the circuit serves rooms or general receptacles and the breaker seems big for the cable, that mismatch deserves attention.
Similar to breakers, fuses themselves can be safe and reliable when they’re correctly matched to the wiring. There are still Edison-base fuse boxes in-service on the North Shore. We have replaced a few in the last year for customers due to convenience and insurance requirements. The common hazard with these older systems is misuse – primarily with screw-in Edison-base plug fuses – when a larger amp fuse is installed “to stop a circuit from blowing,” defeating the protection and letting the wiring overheat.
Anies are refusing new policies or are dropping coverage when older Edison-style fuse boxes are still in service. We just replaced an old Edison-base fuse box on the shore for someone who had their cabin insurance policy dropped over the older load center. While the technology behind Edison fuses is still safe and effective after all this time – the individual installations are getting older and have a potential for misuse – putting pressure on everyone to upgrade to breaker-based overcurrent protection.
Regardless of overcurrent protection type – breaker or fuses – it is important to have the correct size installed for the connected wiring. If anything does not look right, it is worth discussing it with a qualified professional for confirmation.
This column is open for reader feedback and questions. If you have an electrical question or are curious how something works in your electrical system, please send over a question.
John Christensen is a licensed Master Electrician in Minnesota and has a bachelor’s degree in electrical engineering from the University of Minnesota – Duluth.
The advice provided in this column is intended for general informational purposes only. If you have specific concerns or a situation requiring professional assistance, you should consult with a qualified professional for advice or service tailored to your individual circumstances. The author, this newspaper, and publisher are not responsible for the outcomes or results of following any advice from this column. You are solely responsible for your actions.
