In the United States in many state jurisdictions, wiring that operates either exactly at or below 100 volts is not required to be installed by a licensed electrician, and no site plan, building permit, or followup inspection is required for electrical work involving these voltages.
I am trying to determine where this concept first originated, because I've tried looking through various editions of the National Fire Protection Association (NFPA) National Electric Code (NEC) going back about 1980, and I don't see a specific designation of "circuits above 100 volts" and a separate one for below.
There are many electrical standards that seem to be based on this..
- about 90 volt AC ringing voltage of landline telephones
- 25 volt / 70 volt constant voltage speaker wiring systems (70.7v RMS * 1.41 = 99.6v .. so it's just barely "low voltage", no license needed to install in some jurisdictions)
- 48 volts DC for phone battery backup in central offices and remote terminals
- up to 48 volts DC for phantom power in balanced audio microphone systems
- up to 48 volts DC for Power Over Ethernet (though there has been a recent change to the NEC regarding network POE in excess of 60 watts per device)
- up to 48 volts for signalling on computer communications cables.
It is also directly written into the statutory law of some states. For example, Wisconsin statutes 101.80 to 101.862:
- 101.80 Electrical Wiring and Electricians
- (paraphrasing the weirdly written language) 101.862(4)(d) for equipment or systems that operate at 100 volts or less:
- Anyone may engage in the business of installing, repairing, or maintaining this electrical wiring. They are not required to be licensed as an electrical contractor with the WI DSPS (Dept of Safety and Professional Services).
- Anyone may engage in installing, repairing, or maintaining these systems. No licensing as an electrician or enrollment as a registered electrician with the WI DSPS is required.
- A master electrician is not required to be at all times responsible for the person working on these systems.
In a search for historical references to 100 volts, I found this:
The Electrical Experimenter, August 1915, Page 134: How Electricity Kills - [...] The average man can take a direct current of 100 volts and scarcely notice it. 200 to 400 volts will give rise to muscular cramp, while respiration is suddenly stopped at 550 volts. [...]
Is there a specific definitive point in history where this division of "0 - 100 volts is safe, but 101+ volts is unsafe" started?