Troubleshoot circuit problems in old Philadelphia home

A recent circuit troubleshooting job at 44xx Locust St. probably started when a plug-in electric space heater overloaded a knob-and-tube circuit serving a dozen locations besides the one where the heater was connected. Space heaters typically consume 100% of permissible circuit load, although when connected to a knob-and-tube location, they may be sharing with a dozen or more lights and other receptacles.

The overload might have had no consequence if a wire-nutted splice in a basement ceiling junction had been better made-up. Unevenness in the wire twist combined with absence of any metal bonding sleeve inside an old ceramic wire-nut probably allowed oxidation to partially insulate the spliced wires from each other. When the heavy heater load subjected this poor connection to overload, the connection failed entirely — probably due to arcing and burning of the tiny contact-points that remained. A problem such as this could be identified before it manifested as a complete fault, by use of a loading receptacle tester to analyze voltage drop under a load of 12A or 15A (see FAQ-Voltage Drop by SureTest(TM)).

More on causes and fix for circuit overload in an old West Philadelphia house >>

How to not burn down your old house with a plug-in space heater

It’s that chilly, blown fuse and tripped breaker time of year again.

Based on my experience as an electrician serving residential customers, electric space heaters cause more electrical trouble tickets in Philadelphia than any other single connected appliance or event. With the cold weather comes the seasonal task of explaining to homeowners and tenants that they simply should not be using their space heaters — at least not unless their entire house has been rewired top to bottom, or until they have their breaker distribution panel mapped and make up a plan to balance loads with extreme care. The difficulty is that knob-and-tube circuits remain in any older (pre-WWII) home that hasn’t been rewired, and that these circuits tend to serve a dozen locations — much too much to be able to handle the additional load (overload) of a space heater.

The risk of fire from a circuit overloaded by a space-heater is like the risk of cancer from smoking: you may have done it for decades without obvious damage, but cancer/fire could strike any day. You need to find a way to pay for a better solution than putting everything you own and everyone you love at risk just to stay warm.

If your house lacks dedicated circuits appropriate for space heaters, then it may be worthwhile to just keep the central heating system’s thermostat dialed comfortably and/or invest in weatherization to avoid the risk of catastrophic circuit failure and fire, and convenience issues such as 90% of the lighting in the house being unsafe to use after a near-fire on the shared circuit where you plugged in a heater. Space heaters largely just shift heating costs from gas or oil to electric, rather than save money, anyway.

But I need to use my space heater, because…

Read further on steps to avoid fire hazard from electrical overload

Fuse cabinet as distribution panel

Fuse cabinet as distribution panel

Many homes in West Philadelphia feature a distribution panel remote from the main service equipment. In older installations, this may be a wooden cabinet with a picture-framed wooden or glass-pane door, usually located in a stair- or hallway, and lined with a felt-like friable material that may be asbestos.

Several fuse-holder modules provide for branch circuit over-current protection. Unlike with modern wiring, the fuses may protect both the grounded (neutral) and ungrounded (‘hot’) conductors of 120V circuits, so a single circuit may have two fuses in it.

These fuse cabinets may not meet the demands of modern electrical usage

More on working with, or replacing old fuse cabinet type distribution panels >>