RME Board Exam common mistakes
RME Board Exam · Understand the traps and how to avoid them.
Computes motor branch current with I = P/V or hp × 746/V and ignores power factor and efficiency, then uses that current to select conductors and overcurrent protection.
The DC/unity-power-factor formula is overlearned, and nameplate mechanical output (hp or kW) is confused with electrical input kVA.
What to do instead
Convert hp to watts, then use I = P_out/(η × PF × V) for single-phase or I = P_out/(√3 × η × PF × V_L) for three-phase; when the nameplate gives full-load amperes, use that value for branch-circuit sizing.
Reads conductor ampacity directly from the ampacity table and uses it without ambient-temperature correction or conductor-count adjustment.
The table value is treated as the final answer because correction factors are easy to overlook when the question only gives load current.
What to do instead
Identify design ambient temperature and the number of current-carrying conductors, apply the correction and adjustment factors, then use the lower adjusted ampacity for load and overcurrent-device checks.
Sizes the overcurrent device to the calculated load current but never checks that the selected conductor is actually protected by that device.
Attention stays on the load calculation, and the principle that the overcurrent device protects the conductor is forgotten.
What to do instead
After derating, select a conductor with ampacity at least equal to the load requirement, then choose an overcurrent device rating not greater than the conductor ampacity and not less than the load, applying only permitted next-size-up rules.
Adds continuous loads at 100% when sizing branch-circuit conductors and overcurrent protection.
Continuous versus noncontinuous load classification is missed, or the 125% factor is treated as a demand factor rather than a sizing requirement.
What to do instead
Identify loads expected to run for 3 hours or more, multiply only the continuous portion by 1.25, add the noncontinuous portion, and then size conductors and overcurrent protection; keep this factor separate from demand factors.
Uses the neutral/grounded conductor as an equipment grounding conductor or bonds the neutral to ground inside a subpanel.
The terms grounded conductor, grounding conductor, and bonding jumper sound similar, and panel wiring is copied without tracing the grounding path.
What to do instead
Keep the grounded conductor and equipment grounding conductor separate; install the main bonding jumper only at the service; isolate the neutral bar in subpanels and bond enclosures through the equipment grounding conductor.
Counts only current-carrying conductors for box fill or applies the 40% conduit-fill limit to a single conductor.
The 40% figure is memorized without its condition, and box-fill allowances for devices, clamps, and grounds are overlooked.
What to do instead
For box fill, count each conductor, device, clamp, and grounding conductor using the applicable volume allowances; for conduit fill, use 53% for one conductor, 31% for two, and 40% for more than two conductors.
Connects an ammeter in parallel with a load or a voltmeter in series with a load during circuit testing.
Both instruments are meters and their symbols are similar, so the required series/parallel connection is confused under time pressure.
What to do instead
Connect an ammeter in series with the branch and a voltmeter in parallel with the component; select the correct multimeter mode and range, and verify the connection before energizing.
Uses P = √3 × V_L × I_L × PF but substitutes phase voltage or phase current for line values, or drops the √3 for a three-phase load.
Wye and delta relationships are not sorted out, and line versus phase quantities are treated as interchangeable.
What to do instead
For balanced three-phase power use line values with √3; for wye, V_L = √3 V_ph and I_L = I_ph; for delta, I_L = √3 I_ph and V_L = V_ph; if unsure, compute per-phase power and multiply by three.
Confuses motor overload protection with branch-circuit short-circuit and ground-fault protection and sizes both devices the same.
Both are called protection, but overload protection protects the motor from sustained excess current while branch-circuit protection clears short circuits and ground faults.
What to do instead
Size overload devices to the motor nameplate or code motor tables so they allow normal starting current; size branch-circuit short-circuit/ground-fault protection for conductor protection with the permitted motor allowances; check each rule separately.
Computes voltage drop using one-way conductor length for a single-phase circuit or mixes mm² and circular mils in the resistance formula.
The voltage-drop formula is memorized without the return-path length factor, and conductor area units are converted incorrectly.
What to do instead
For single-phase use the total conductor length (2 × one-way length); for three-phase use √3 × one-way length; keep units consistent by converting mm² to circular mils (1 mm² ≈ 1973 cmil) or by using resistivity in mm²·Ω/m.