Remember in engineering school, learning about circuit analysis and how to calculate voltage drops in a circuit? Well, this is one time when those text book examples are applicable in real life.
Street lighting, parking lot, sports field and outdoor lighting are long parallel circuits, just like the classic school book examples. In those examples, the wire was assumed to be perfect with no resistance but in real life, we have to design our electrical circuits to take impedance into account.
Welcome to the real world of electrical consulting. When designing these circuits, voltage drop calculations are required to ensure that the end of the circuit has enough power to drive the final load.
Voltage drop in a circuit is the gradual reduction in voltage along a passive wire that has inherent impedance (for AC) or resistance (for DC). It happens only when current flows in the circuit.
What this means is, for instance, a 120V panel is feeding a 100 foot circuit, the voltage at the receptacle could be 116V. A 3.3% drop in voltage (4 V / 120 V) will be seen even though there are no other loads on the circuit!
The problem only gets worse as more loads are connected on the circuit.
As the length of the circuit increases or as the current increases, so does the voltage drop! If the voltage level drops too much, say to 110V, then:
The Canadian Electrical Code 2012 recommends that the branch circuits have a 3% voltage drop and the entire feeder circuit less than 5%. Most electrical appliances can withstand these levels and usually have a working voltage level tolerance as low as 8.3% to 110 V.
The Canadian Electric Code is only a recommendation but as industrial control systems engineers, we try to limit of voltage drop to 3% (pending cost, application and logistics of course). At 3%, the difference in expected power supply is unnoticeable.
At 3% we are future-proofing the circuit to accommodate larger loads and lower power factors. Leaving margin for future loads will ensure our clients get a reliable power system as expected.