ICS Engineering Inc.
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Voltage Drop in a Circuit

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.

Electrical Load Calculations for equipment in Alberta and BC
Voltage drops on these long circuits like this one on a dock in BC, will result in the far lights being dimmering than the front lights.

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.

What Can Go Wrong with Voltage Drop

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:

  • Motors can't start up - Appliances that require higher in-rush current levels may not start up as the voltage is below their minimum operating power level. Voltage drop needs to be taken into account when designing circuits for motors including MCA vs FLA parameters.
  • Intermittent operation - If an appliance does start up, it may shut down when it detects a noticeable voltage drop. This will usually happen to computers & gaming systems during 'that perfect high-scoring game'. Customers expect reliable power systems.
  • Inconsistent lighting levels - Street lighting design must take into account the gradual reduction in voltage level as a 5% drop in voltage can be noticeable on lighting levels.
  • Blown Fuses & Tripping Circuit Breakers - Remember that P=VI so if V is low, an appliance may increase its I to reach its desired power level. When multiple loads on a circuit increase their current consumption, it could cause the safety mechanism to trip.
  • Increased current consumption may cause motors and ballasts to overheat and reduce their operating life. Resistive loads such as heaters & incandescent lights will also have a reduced life span. Fluctuating voltage levels caused by other loads on the system may cause an annoying flicker.

Voltage Drop Recommendations

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.

Let's Design Things Better

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.