9.1 Voltage Drop & Continuous Load Calculations

Load calculations sit at the center of the PRC Registered Master Electrician (RME) exam because they are where design intent meets code compliance. Two ideas dominate this section: sizing conductors and overcurrent protective devices (OCPDs) for continuous loads, and controlling voltage drop (VD) so equipment actually receives the voltage it was designed to run on. Both concepts show up constantly in Philippine Electrical Code (PEC) Part 1 problems, and both are tested with numeric questions, not just definitions.

Continuous Loads and the 125% Rule

A continuous load is a load where the maximum current is expected to continue for 3 hours or more. Classic examples include store lighting that stays on for a full business day, signage, and certain fixed HVAC or process equipment that runs for extended, uninterrupted periods. Contrast this with a noncontinuous load — a load that cycles on and off, or that runs steadily for less than 3 hours, such as a garbage disposal or a portable power tool used intermittently.

The standard NEC/PEC-aligned sizing convention is this: for a continuous load, both the branch-circuit conductors and the OCPD (breaker or fuse) must be sized at no less than 125% of the continuous load current. If a circuit also has a noncontinuous portion, the general approach is 100% of the noncontinuous load plus 125% of the continuous load, then the conductor and device are selected to be no smaller than that combined figure.

Why 125%, Not 100%?

Ampacity tables are built around a conductor reaching a stable, safe operating temperature under load. A load that runs for only a few minutes never gets the conductor, its insulation, or its terminations to full steady-state temperature — there simply isn't time. A load that runs for 3 hours or more, however, drives the conductor and its terminations to their maximum sustained temperature. Standard circuit breakers and conductor terminations are only rated for continuous duty at 80% of their marked rating (equivalently, requiring the load to be no more than 80% of the device rating, which is mathematically the same as sizing the device at 125% of the load). The 25% margin is headroom against sustained heat buildup at the connection points — loose or undersized terminations are one of the most common causes of overheating and fire in the field, and this margin exists specifically to keep the terminals well within their thermal limits during that steady 3+ hour run.

Worked Example: Sizing for a 20A Continuous Load

Suppose a run of exterior signage draws a steady 20A and operates more than 3 hours per day (a continuous load).

The key move here is 20A x 1.25 = 25A — this is the minimum the conductor and OCPD must support. If the calculated minimum lands between standard device sizes (for example, a continuous load calculation yielding 27A, where the next standard breaker size is 30A), the rule is always to round up to the next standard size, never down. Undersizing a continuous-load circuit is a direct, testable code violation on the RME exam.

Voltage Drop: Why It Matters

Voltage drop (VD) is the reduction in voltage between the source (the panel or service) and the load, caused by the conductor's own resistance acting on the current flowing through it over the length of the run. Every conductor has some resistance per unit length; the longer the run and the higher the current, the more voltage is "lost" as heat in the conductor before it ever reaches the connected equipment.

Excessive voltage drop is not primarily a shock or fire hazard in the way an overloaded circuit is — it is a performance problem. Motors starving for voltage draw more current to produce the same torque, run hotter, and wear out faster. Lighting fed by an undersized, long circuit run dims and flickers. Electronic and control equipment can malfunction or reset under low voltage. None of this is inherently dangerous in the short term, but it degrades equipment life and performance, which is why voltage drop control is treated as design best practice rather than a hard prescriptive code violation with an automatic numeric threshold. There is no PEC article that says "voltage drop over 3% is an automatic failure" — instead, the code recommends staying within reasonable limits as good engineering practice, and the RME candidate is expected to know and apply those limits when sizing conductors for long runs.

Typical Best-Practice Targets

These are recommended design targets, not code-mandated maximums with an automatic violation attached — but a competent designer treats them as firm guidance, especially for long runs, motor circuits, and lighting.

The Voltage Drop Formula (Single-Phase, Simplified)

A widely used simplified approximation for single-phase voltage drop is:

VD ≈ 2 x K x I x L / CM

Where:

• VD = voltage drop (volts)
• K = resistivity constant of the conductor material (a fixed value for copper or aluminum, reflecting how much a conductor of that material resists current flow per unit of length and cross-sectional area)
• I = current in the conductor (amperes)
• L = one-way length of the circuit run (feet)
• CM = circular mil area of the conductor (a measure of the conductor's cross-sectional size — larger CM means less resistance)
• The factor of 2 accounts for the fact that current must travel out to the load and back through the return conductor — the round trip, not just the one-way distance

Worked Reasoning Example: Long Feeder Run

Consider a feeder that must run a long distance — say, from a main panel to a small outbuilding or a remote pump — where ampacity tables alone might technically allow a smaller conductor size for the load's current draw. Because the formula shows VD rising directly with both current (I) and length (L), and falling as circular mil area (CM) increases, a long run at a given current will always produce more voltage drop than a short run at the same current.

If a preliminary check shows the calculated voltage drop for a small, ampacity-adequate conductor exceeds the roughly 5% total target, the practical fix is to increase the conductor's cross-sectional area (a larger CM value) even though ampacity alone did not require it. A bigger conductor has proportionally lower resistance, which directly reduces the VD result in the formula. This is precisely why experienced designers often specify conductors one or two sizes larger than the ampacity minimum for long feeder runs — not because the smaller size would violate an ampacity rule, but because it would push voltage drop past the recommended design target, degrading motor and lighting performance at the far end of the run. On the RME exam, expect this concept tested as: "which of the following best reduces voltage drop on a long run?" — the correct reasoning path is always toward increasing conductor size (larger CM), since that is the only variable in the formula the designer can practically control without changing the load itself.