Iran Nezam Electrical Design Exam common mistakes
Iran Nezam Electrical Design Exam · Understand the traps and how to avoid them.
Sizing the main incoming feeder and transformer from the arithmetic sum of all connected loads instead of applying a demand (coincidence) factor to each load group and then a diversity factor across groups, so the design current is 30–60% higher than needed.
Candidates memorize connected-load tables but never practise the two-stage reduction; the Iranian practice of tabulating 'installed kW' per panel encourages carrying the raw sum straight into the transformer calculation.
What to do instead
Build a load schedule with columns for connected load, demand factor per load type (lighting, sockets, motors, HVAC, elevators), and then apply a building-level diversity factor. Show the demand factor source (national code table or justified engineering assumption) next to each row and carry the reduced kVA - not kW - into transformer sizing at the specified power factor.
Computing voltage drop with only the resistance term (or with a rule-of-thumb mV/A/m figure taken from a foreign table) and ignoring reactance for large conductors and power-factor correction, then accepting a result that passes while the actual drop at the far panel exceeds the limit.
Most textbook examples use small cables where R dominates; candidates carry the same shortcut to 240 mm² feeders and motor circuits where X is comparable to R, and to low power factors where the reactive component matters.
What to do instead
Use the full expression ΔU = √3·I·L·(R·cosφ + X·sinφ) for three-phase and 2·I·L·(R·cosφ + X·sinφ) for single-phase, taking R and X at operating temperature from the cable manufacturer's data. State the assumed cosφ and conductor temperature explicitly, and check the drop from the origin of the installation (transformer LV terminals) cumulatively through each distribution level, not just the last cable.
Selecting protective device ratings by matching the cable ampacity only, without verifying that the device's breaking capacity exceeds the prospective short-circuit current at its installation point, and without checking the I²t let-through against the cable's withstand.
Candidates treat 'Ib ≤ In ≤ Iz' as the whole design check and forget that a panelboard fed close to a transformer may see 20–40 kA, far above a typical 6 kA or 10 kA MCB.
What to do instead
For every board, calculate the three-phase and single-phase (line-to-neutral) prospective fault currents from the transformer impedance plus cable impedance, then select devices with rated breaking capacity above the larger value. Where the fault level is high, use current-limiting devices or a cascading/back-up combination and verify the cable's short-circuit withstand with the adiabatic equation S ≥ √(I²t)/k using the correct k for the conductor and insulation.
Designing the earthing system as a single resistance value without distinguishing the functions: system earthing (neutral point), protective earthing of exposed conductive parts, equipotential bonding of metallic services, and lightning protection - and mixing TN-S, TN-C-S and TT assumptions within one installation.
The topic is compressed into 'measure the earth resistance' in most study material, so candidates never practise the disconnection-time check that makes the earthing arrangement valid.
What to do instead
State the earthing arrangement for each part of the installation and prove it: for TN systems verify that the loop impedance Zs satisfies Ia ≤ U0/Zs with the disconnection time from the relevant table; for TT systems verify RA·Ia ≤ 50 V and specify an RCD. Draw the bonding of water, gas, structural steel and telecommunications earth bar separately from the lightning down-conductor system, and specify the test links and disconnection points.
Performing the illuminance calculation by dividing total lumens by floor area (lumen method with a guessed utilization factor and a maintenance factor of 1.0) and then presenting the result as the maintained illuminance without a room index or a real luminaire photometric file.
The lumen method formula is easy to memorize and the utilization factor is the hardest input, so candidates silently drop it or invent 0.5; maintenance factor is forgotten entirely.
What to do instead
Compute the room index K = (L·W)/(Hm·(L+W)) with Hm the mounting height above the working plane, read the utilization factor from the chosen luminaire's photometric data for the actual reflectance combination, and apply a maintenance factor from the cleaning cycle and environment (typically 0.8 for clean offices, lower for industrial). Report maintained illuminance, uniformity (Emin/Eavg) and glare rating (UGR) against the target values for the task.
Treating the generator as an unlimited source and sizing it on kW alone, ignoring the motor starting kVA, the transient voltage dip, and the fact that the neutral/earthing arrangement of a generator set (separately derived source, switched neutral) changes the fault path.
Candidates size standby generators from the same load schedule as the transformer and never model starting; the earthing change is invisible unless the single-line diagram is drawn.
What to do instead
Size the set on both steady-state kVA at the specified power factor and on the largest motor start (or the sum of starts) with an acceptable transient dip (commonly ≤15–20% at the generator terminals); check the alternator's subtransient reactance X''d for the fault level it can deliver. On the single-line diagram show whether the neutral is switched, whether the star point is earthed separately, and how the earth-fault protection is arranged in both modes (mains and generator).
Placing smoke detectors on a grid by area coverage alone, without respecting the spacing-to-wall rule, the effect of ceiling height and shape, beam pockets, air movement from diffusers, or the need for detectors in ducts and voids - and without checking that the sound pressure level from alarm devices is adequate in every occupied space, including bedrooms and bathrooms.
Coverage area per detector is the one number candidates remember; the installation geometry rules and the audibility requirement are learned as separate facts and never applied to a plan.
What to do instead
Lay out detectors using both the maximum area per detector and the maximum distance from any point to the nearest detector, keeping detectors at least the specified distance from walls and air outlets. Treat each beamed bay as a separate compartment when beams exceed the depth threshold. Then verify alarm audibility at the bedhead (typically 75 dBA at the head, or 15 dBA above ambient) and specify visible alarm devices where the sound level or occupancy requires it.
Answering code questions from memory of a foreign standard (IEC 60364, BS 7671 or NEC) and quoting its table numbers or disconnection times as if they were the Iranian requirement, without distinguishing mandatory national regulation from advisory good practice.
Most available study material is imported; the national codes and the publisher's mandatory status are harder to find, so candidates default to what they memorized and lose marks on applicability.
What to do instead
For each topic, be able to state which document governs in Iran (the national building code electrical chapter, the national wiring regulations, utility connection requirements, and fire-code provisions) and what it requires, then note where an IEC or other standard is used only as a design reference. When a value is uncertain, say which document would be consulted rather than inventing a clause number.
Producing drawings and specifications that a contractor cannot build from: single-line diagrams without cable sizes, lengths, protection ratings and fault levels; panel schedules without spare ways and phase balance; no cable routing or tray sizing; specifications that name a product without technical characteristics or testing requirements.
Candidates treat documentation as a formality after the calculation and reproduce generic specification text, so the design intent never reaches the drawing.
What to do instead
For every feeder show origin, destination, cable type and size, length, installation method, protective device rating and curve, and the calculated voltage drop. Balance single-phase loads across phases in the panel schedule and reserve 20–25% spare capacity. In the specification, describe performance requirements (rating, IP degree, breaking capacity, standard complied with, type tests) rather than brand names, and add the inspection and test plan that the supervision stage will use.
Ignoring the supervision duties in the answer: no hold points, no inspection and test plan, no witnessing of insulation resistance, earth resistance, continuity, RCD trip tests or protection settings, and no procedure for reviewing contractor submittals or recording non-conformities.
Design candidates study calculations and treat the supervision part of the syllabus as administrative text, so they cannot describe what a resident engineer actually does on site.
What to do instead
Prepare a short inspection and test plan keyed to the installation stages (conduit and box installation before concreting, cable pulling, termination, energization), listing the test, the acceptance criterion and the record form. Know the standard tests: insulation resistance at the specified test voltage, earth electrode resistance, protective conductor continuity, RCD operating time and current, polarity, and protection relay settings against the coordination study - and state that energization proceeds only after these records are signed.