Voltage drop describes how the supplied energy of a voltage source is reduced as electric current moves through the passive elements (elements that do not supply voltage) of an electrical circuit.
Whenever a current is flowing, the current will meet dynatron, which will impede its flow, the amount of voltage loss through the entirety of a circuit, or even a part is voltage drop. In low voltage lighting systems such as LED strip lights, voltage drops occur because the input voltage from the power supply gradually decreases over the length of the strip.
The longer the flexible led strip light is, the more resistance the current has to overcome; hence it loses voltage along the way. If the flexible LED strip light runs off a 12-volt power supply, then you have 12 volts going into the strip light system at the start but the other end will not have 12 volts due to the voltage drop.
Voltage drop is a function of wire length, wire thickness, and the total watts of power used by the flexible LED strip lights. A reduction of brightness and color accuracy in longer flexible LED strip lights is due to excessive voltage drop. The excessive voltage drop may result in unsatisfactory operation of, and damage to, electrical outlets and the flexible LED strip tape lights. The distance from the power source to the desired end distance of the flexible LED strip tape is called a run. Shorter and/or thicker wires will raise the brightness and color consistency to the strip lights’ full potential.
How much voltage drop is acceptable? A footnote (NEC 210-19 FPN No. 4) 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. In a 120 volt 15 ampere circuit, this means that there should be no more than a 6 volt drop (114 volts) at the furthest outlet when the circuit is fully loaded. It also means that the circuit has a resistance that does not exceed 0.4 ohms.
What are the consequences of “excess” voltage drop in a circuit? Excess voltage drop can cause the following conditions:
1. Low voltage to the equipment being powered, causing improper, erratic, or no operation – and damage to the equipment.
2. Poor efficiency and wasted energy.
3. 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.:
1. Is the high resistance connection in contact with a combustible material?
2. Is there air flow to dissipate the heat?
3. Is the area around the connection insulated, so that heat cannot escape.
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.
Some agencies arbitrarily set a maximum voltage drop criteria of 10% to be considered unacceptable and a hazard. The author believes that any voltage drop difference of >1% from an adjacent receptacle should be investigated, that any voltage drop difference of >2% from an adjacent receptacle should be considered a hazard, and that using a maximum voltage drop criteria of more than 8% (3% above the “efficiency” recommendation) is courting disaster. A 3% voltage drop (3.6 volts in a 120 volt circuit) at one connection with a 15 ampere flow develops 54 watts of heat – which can cause ignition under certain conditions.