What is voltage drop?
A voltage drop in an electrical circuit normally occurs when current is passed through the wire. The greater the resistance of the circuit, the higher the voltage drop.
How much voltage drop is acceptable?
A footnote in the National Electrical Code states that a voltage drop of 5% at the furthest receptacle in a branch wiring circuit is acceptable for normal efficiency.
What are the causes of “excess voltage drop” in a branch circuit?
Typically:
1. High resistance connections at wiring junctions or outlet terminals, usually caused by:
- poor splices anywhere in the circuit
- loose or intermittent connections anywhere in the circuit
- corroded connections anywhere in the circuit
- inadequate seating of wire in the slot connection on back-wired “push-in type“ receptacles and switches.
2. The wire does not meet code standards (not heavy enough gauge for the length of the run).
What are the consequences of “excess” voltage drop in a circuit?
Excess voltage drop can cause the following conditions:
- Low voltage to the equipment being powered, causing improper, erratic, or no operation - and damage to the equipment.
- Poor efficiency and wasted energy.
- Heating at a high resistance connection/splice may result in a fire at high ampere loads.
At what % voltage drop does a circuit become hazardous?
It is difficult to say at what point excess voltage drop will cause a fire, because it depends on how much current is flowing through the high resistance connection, what is the resistance of that connection and because many factors must be considered regarding at what point ignition will occur, e.g.:
- Is the high resistance connection in contact with a combustible material?
- Is there air flow to dissipate the heat?
- Is the area around the connection insulated, so that heat cannot escape?
The NFPA reports that from 1988-1992, there was an average annual total of 446,300 fires in homes, resulting in 3,860 Deaths and $4.4 Billion property damage. 42,300 (9%) of these fires were caused each year by Electrical Distribution Systems.
The largest portion of fires caused by electrical distribution systems (48%) were caused by faulty fixed wiring, receptacles and switches.
Electrical Distribution Equipment Fires in U.S. Homes
1988-1992 Average
| Cause of Fire | No. Of Fires |
| Total Electrical Distribution System | 42,300 (100%) |
| Faulty Fixed Wiring | 15,400 (36%) |
| Switches, Receptacles, Outlets | 4,800 (11%) |
The results of an in-depth investigation of 149 residential fires caused by electrical distribution systems was summarized in an article by Smith & McCoskrie.
Of the fires occurring as a result of:
- Faulty fixed wiring - poor/loose splices, damaged connectors, improper installation and ground faults accounted for 94% of these fires.
- Receptacles and switches - loose/poor connections accounted for 59% of these fires.
- Lighting fixtures - loose or poor connections accounted for 37% of these fires.
Most of these faulty circuits and receptacles could have been previously identified as hazards with a 15-ampere load test, and many of these fires could have been easily prevented.
RECOMMENDATIONS
For power efficiency, the NEC standard of 5% maximum voltage drop is recommended, from a safety perspective, because wiring connections in some homes deteriorate with time (particularly in homes that use aluminum wiring for power circuits), and do-it-yourself modifications may be less than professional. Excess voltage drop is a concern because of the potential fire hazard at high resistance connections, particularly on circuits that power electric motors while occupants of the dwelling are asleep, e.g. air conditioners, refrigerators, furnace fans, exhaust fans, etc.
For the protection of your family and your home it only makes sense to insist that your Home Inspector has made the investment in training and equipment to perform Voltage Drop testing.
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