[MEP Center Engineering]

There is no fixed conversion ratio: size the aluminium cable from scratch with the same three checks as copper (current-carrying capacity after derating, voltage drop, short-circuit withstand). In practice aluminium ends up one to two standard sizes larger, roughly 1.4 to 1.7 times the copper cross-section.

Why aluminium is larger

• Resistivity at 20 °C: copper 0.0172, aluminium 0.0283 Ω·mm²/m, a ratio of about 1.64. For equal resistance (equal voltage drop) aluminium needs about 1.6 times the area.
• For equal current-carrying capacity the ratio is smaller, because a larger conductor dissipates heat better. Tabulated aluminium ampacity is about 78 % of copper of the same size.
• Short-circuit factor k (IEC 60364-5-54, XLPE): copper 143, aluminium 94, so the minimum size from S ≥ √(I²t)/k is about 1.5 times larger.

Worked example (IEC 60364-5-52, method E, XLPE multicore)

1. Design current: I_B = 100 000 / (√3 × 400 × 0.9) = 160.4 A. Protective device I_n = 160 A.
2. Derating: ambient 40 °C (0.91) × two circuits touching on perforated tray (0.88) = 0.80.
3. Required tabulated current: I_t ≥ 160 / 0.80 = 200 A.
4. From the method E table: copper 50 mm² is too small, copper 70 mm² passes; aluminium 70 mm² is too small, aluminium 95 mm² passes (about 182 A after derating, ≥ 160 A).
5. Voltage drop, ΔU = √3 × I_B × L × (R cos φ + X sin φ), with R at 90 °C and X ≈ 0.08 Ω/km:

Cable	R at 90 °C (Ω/km)	ΔU (V)	ΔU (%)
Cu 70 mm²	0.342	11.4	2.85
Al 95 mm²	0.410	13.5	3.37
Al 120 mm²	0.324	10.9	2.72

If your voltage-drop budget for this sub-main is 3 %, copper 70 mm² passes, aluminium 95 mm² fails and aluminium 120 mm² is the answer. Here voltage drop, not ampacity, governs the aluminium size. On long routes this is the usual result, and it is why a fixed ratio misleads.

Short-circuit and other checks

• Confirm S ≥ √(I²t)/k with the let-through energy of the actual MCCB, using k = 94 for aluminium XLPE.
• The protective conductor of an aluminium cable has a higher impedance, so recalculate fault-loop impedance and disconnection time.
• Aluminium is normally used only from 16 mm² upwards for fixed power circuits.
• Terminations: use bimetallic (Al/Cu) lugs or terminals rated for aluminium, contact grease, and the manufacturer’s torque. Re-torque checks belong in the commissioning plan.
• NEC practice: use AA-8000 series alloy conductors, take ampacity from the 75 °C column (NEC 110.14(C)), and make sure the equipment terminals are marked AL/CU.

Common mistakes

• Converting with a single “one size up” rule and skipping the voltage-drop check.
• Checking the cable against I_B and forgetting the I_n ≤ I_z rule for the breaker.
• Gland, lug and busbar connections sized for copper. A larger aluminium conductor may not fit the breaker terminals without spreaders. See Cable Gland Selection and Cable Tray Sizing: Fill, Spacing and Derating.

For deeper study: Electrical Power Systems Design course.

Check the edition of the code that applies to your project.

[MEP Center Engineering]

A large ratio of rated currents gives overload selectivity, not short-circuit selectivity. Above the MCCB’s instantaneous pickup both devices can trip, so total selectivity up to 6 kA can only be confirmed from the manufacturer’s tested selectivity tables (IEC 60947-2 Annex A) for that exact pair.

Step 1: Overload zone

Plot both time-current curves on the same sheet. With 160 A upstream and 32 A downstream the thermal curves are far apart, so overloads are selective. For gG fuses a 1.6 : 1 rating ratio is the classic rule. For breakers, compare curves with their tolerance bands; a ratio alone is not enough.

Step 2: Short-circuit zone

• C32 MCB magnetic trip: 5 to 10 × I_n = 160 to 320 A.
• MCCB magnetic set at 10 × I_n = 1 600 A, with a typical ±20 % tolerance, so it may trip from about 1 280 A.
• Without tested data, the selectivity limit is therefore only about 1.28 kA. That is far below the 6 kA available, so selectivity is partial.

Manufacturer tables often show a higher limit, because a current-limiting MCB clears the fault before the MCCB unlatches (energy selectivity). Such values are only valid for the tested combination. Different brands, or a different frame or trip unit, invalidate them.

Step 3: Ways to reach total selectivity

1. Choose an MCCB with an electronic trip unit and use a short-time delay, with the instantaneous threshold I_i set above the fault level at the sub-board. Then check the cable’s I²t withstand and the fault-loop disconnection time with the new setting.
2. Use a pair the manufacturer lists as “T” (total) up to the fault level at the sub-board.
3. Reduce the prospective fault at the sub-board, for example with a longer or smaller feeder, if the voltage drop allows.
4. Zone-selective interlocking is an option between ACBs and electronic MCCBs, not between an MCCB and an MCB.

Worked check (illustrative)

Fault level at the sub-board: 6 kA. MCCB 160 A, electronic, I_sd set to 8 × I_n = 1 280 A, short-time delay 0.1 s, instantaneous I_i above 6 kA. The C32 clears any fault above 320 A in less than 10 ms, well inside the MCCB’s 0.1 s delay. The pair is selective up to the instantaneous threshold, subject to the manufacturer’s table. Then confirm the feeder cable withstands the I²t passed in 0.1 s.

What to put on the drawings

• A note: “Selectivity between the upstream and downstream devices shall be total up to the prospective short-circuit current shown. Contractor to submit manufacturer selectivity tables for the proposed devices.”
• The fault level at each board, the trip settings (I_r, I_sd, t_sd, I_i) and the curve type.

Codes

IEC 60364-5-53 asks for selectivity where safety or continuity requires it, and for safety services it is essential. NEC 700.32 and 701.32 require selective coordination for emergency and legally required standby systems, and 620.62 for elevators. See also 10 Common Mistakes on Single-Line Diagrams.

For deeper study: Electrical Power Systems Design course.

Check the edition of the code that applies to your project.

[MEP Center Engineering]

The usual limits are counted from the origin of the installation, not from the final board: IEC 60364-5-52 (Annex G) recommends 3 % for lighting and 5 % for other uses when supplied from a public LV network, and 6 % and 8 % from a private transformer. At 60 m, 2.5 mm² fails the voltage-drop check and also the fault-loop check with a C16.

Step 1: Set the budget

Subtract the drop already used upstream. If the sub-main to this board uses 1 %, the lighting final circuit has 2 % left of a 3 % total. NEC treats this as an informational note (3 % for a branch circuit, 5 % for feeder plus branch), and many specifications make it mandatory.

Step 2: Calculate

Single-phase: ΔU = 2 × L × I × (R cos φ + X sin φ). For small cables at cos φ = 1, X can be ignored. Use R at the operating temperature (70 °C for PVC): R₇₀ = R₂₀ × (1 + 0.00393 × 50).

Cable	R at 70 °C (Ω/km)	ΔU at 10 A, 60 m (V)	ΔU (%)	Total with 1 % upstream
2.5 mm²	8.87	10.6	4.63	5.63 % (fails)
4 mm²	5.52	6.6	2.88	3.88 % (fails)
6 mm²	3.69	4.4	1.92	2.92 % (passes)

The circuit needs 6 mm², even though 2.5 mm² carries 10 A easily.

Step 3: Check fault-loop impedance (often forgotten)

Automatic disconnection in a TN system needs Z_s × I_a ≤ U₀. A C16 must trip magnetically at 10 × 16 = 160 A, so Z_s ≤ 230 / 160 = 1.44 Ω. Use conductor resistance at operating temperature, or apply a 0.8 factor to values at 20 °C, not both.

• 2.5 mm² with a 2.5 mm² protective conductor: loop R = 2 × 0.060 × 8.87 = 1.06 Ω. With Z_e = 0.40 Ω, Z_s = 1.46 Ω, which fails.
• 6 mm²: 0.44 + 0.40 = 0.84 Ω, which passes.
• Alternatives are a B16 (Z_s ≤ 2.88 Ω) or RCD protection. Outdoor circuits usually need a 30 mA RCD anyway; see RCD Types AC, A, F and B.

Practical points

• Use the real cable route length, including vertical drops and the loop through fittings, not the straight-line distance.
• For distributed loads (lamps spaced along the route) the drop is lower than with the whole load at the end. Calculate it segment by segment rather than halving it blindly.
• For motors, check the starting voltage drop as well (often limited to 10 to 15 % at the motor terminals during starting). See Motor Starting: DOL, Star-Delta, Soft Starter or VFD.
• LED drivers tolerate a wide voltage range, but emergency luminaires and control gear may not. Do not relax the limit on that basis.

Common mistakes

• Measuring 3 % from the final board instead of from the origin.
• Using resistance at 20 °C for voltage drop and then calling the result conservative.
• Upsizing phase conductors but leaving a small protective conductor, so Z_s still fails.

For deeper study: Electrical Power Systems Design course.

Check the edition of the code that applies to your project.

[MEP Center Engineering]

The 100 m is the channel: at most 90 m of fixed horizontal cable (permanent link) plus at most 10 m of patch and equipment cords in total, with up to four connections (ISO/IEC 11801-1, ANSI/TIA-568.2-D). An outlet 85 m away along the route is close to the limit once slack and vertical drops are added, so measure carefully.

Step 1: Check the length

• Route length to the outlet: 85 m.
• Add vertical drops, for example 2 × 3 m (down from the tray at the distributor and down to the outlet), plus a service loop of about 3 m at the rack. That gives about 94 m, which is over the 90 m permanent link.
• Options: move the outlet group to a consolidation point fed from a closer distributor, add a second floor distributor, or re-route. Do not rely on shorter patch cords to compensate. Standards allow longer work-area cords only with a reduced horizontal length, under defined rules.

Step 2: Size the pathway

Cat6A U/UTP outer diameter is typically 6 to 8.5 mm. Use the value from the datasheet. Example with 48 cables of 7.5 mm:

• Cross-section of one cable: π/4 × 7.5² = 44.2 mm². Total: 48 × 44.2 = 2 121 mm².
• At a 40 % design fill (TIA-569 for conduit with three or more cables; many designers also use 40 % for trays to allow growth), the required area is 2 121 / 0.40 = 5 302 mm².
• A 150 × 50 mm basket tray (7 500 mm²) is 28 % full, which leaves room for future cables.

Step 3: Bend radius, separation, PoE

• Minimum bend radius for installed U/UTP is 4 × the outer diameter, about 30 mm here. Specify pull-box and conduit-bend sizes to match.
• Separation from power: EN 50174-2 and TIA-569 give separation distances that depend on the cable type, the type of barrier and the power circuits. Show a metal divider or a separate tray, and crossings at 90°, rather than a generic distance on the drawings.
• PoE heats the bundle. With Type 3 and Type 4 PoE (up to 60 W and 90 W at the source), keep bundles small or use open baskets, and follow the cable manufacturer’s derating (ISO/IEC TS 29125, TIA TSB-184-A).

What to show on drawings

• Floor distributor locations with the 90 m radius checked along routes, not as a circle on plan.
• Tray sizes with the fill percentage, and the number of outlets per room.
• Labelling scheme and the test standard (permanent link test to Class E_A / Category 6A).

Common mistakes

• Measuring 90 m on plan and forgetting drops and slack.
• Filling trays to 100 % on day one.
• Running data and power in one unsegregated tray to save space.

See also Cable Tray Sizing: Fill, Spacing and Derating and MEP Drawing Standards: Layers, Sheet Numbers, Title Blocks.

For deeper study: MEP Center online certified courses.

Check the edition of the code that applies to your project.

[MEP Center Engineering]

Storage = total bitrate × recording time. The bitrate per camera depends on resolution, frame rate, codec and scene activity, so take it from the camera’s calculator for your settings. The resolution check is done with pixel density on the target (EN 62676-4), not with megapixels alone.

Step 1: Bitrate per camera

Typical values for 4 MP with H.265 at 15 fps are 2 to 6 Mbit/s. Busy, night-time or noisy scenes are at the upper end. This example uses 4 Mbit/s, constant bitrate, continuous recording.

Step 2: Total bitrate and storage

1. Total: 64 × 4 = 256 Mbit/s.
2. Per day: 256 / 8 = 32 MB/s × 86 400 s = 2 765 GB, about 2.76 TB/day.
3. 30 days: 2.76 × 30 = 82.9 TB of usable capacity.
4. Add about 10 to 15 % margin (scene changes, file system), giving roughly 92 to 95 TB usable.
5. RAID: with RAID 6, two disks hold parity. For example, 12 × 10 TB disks give 100 TB usable before formatting, which is about 91 TB as the operating system reports it (TB vs TiB). This is just below the target, so choose 12 × 12 TB or two arrays. Always state capacity as usable, after RAID.

Motion-based recording or smart codecs can cut storage by 30 to 70 % in quiet areas, but size continuously recorded cameras without that saving.

Step 3: Network

• Edge switch: 16 cameras × 4 Mbit/s = 64 Mbit/s, so a 1 Gbit/s uplink is plenty.
• Core to NVR: 256 Mbit/s recording plus live viewing and playback streams. Keep sustained load below about 50 % of the link, or use 10 Gbit/s if several viewing clients pull main streams.
• PoE budget per switch: sum the cameras’ maximum PoE class (not typical) plus IR and heater loads in winter.

Step 4: Is 4 MP enough? (EN 62676-4)

The DORI levels are defined by pixels per metre at the target: detect 25, observe 62.5, recognise 125, identify 250 px/m. A 4 MP camera has about 2 560 horizontal pixels, so for identification the field-of-view width at the target must be no more than 2 560 / 250 = 10.2 m. A 12 m wide gate needs two cameras or a higher resolution, and a lens chosen so the 250 px/m is reached at the actual target distance.

Common mistakes

• Using the camera’s minimum bitrate from a brochure for night scenes.
• Quoting raw disk capacity instead of usable capacity after RAID.
• Specifying megapixels without a pixel-density requirement for each camera view.
• Forgetting storage for the retention period required by the client’s policy or local data-protection rules.

For cable routes and racks see Cable Tray Sizing: Fill, Spacing and Derating; for backed-up recording, UPS Sizing: kVA, kW and Battery Autonomy in Practice.

For deeper study: MEP Center online certified courses.

Check the edition of the code that applies to your project.

[MEP Center Engineering]

Doors on an escape route must let people out at all times. Either the free-egress side has mechanical hardware that always opens (lever, EN 179 emergency exit device, EN 1125 panic bar), or an electrically held lock must be fail-safe and release on fire alarm, on loss of power and from a manual release device. A fail-secure lock without mechanical free egress is not acceptable on an escape door.

Two acceptable arrangements

1. Fail-secure strike or motor lock, with mechanical free egress. The lock controls entry only. From the escape side the door always opens with one action on the lever or panic hardware. Security is kept during power loss and egress is never affected. This is often the best option.
2. Fail-safe lock (magnetic lock or fail-safe strike) that locks the egress side as well. Then you need release on fire alarm activation, on power loss, and from a manual release (break-glass or push button) next to the door. In Europe EN 13637 covers electrically controlled exit systems. In the US, NFPA 101 (7.2.1.6) covers delayed-egress and sensor-release locks, typically with a release device mounted 1 015 to 1 220 mm (40 to 48 in) above the floor and within 1 525 mm (60 in) of the door. Delayed egress is limited to 15 s, or 30 s where the authority approves.

Wiring principles

• The fire alarm release must remove power from the lock directly, downstream of the power supply and its battery. A software command through the access controller is not enough.
• The manual release must act on the lock circuit by hardware, without depending on the controller.
• Release should be by zone, as set out in the cause and effect matrix. See How to Write a Fire Alarm Cause and Effect Matrix.

Worked example: power supply and battery

Six 24 V DC magnetic locks at 0.25 A each, plus the controller and readers at 0.5 A.

1. Load: 6 × 0.25 + 0.5 = 2.0 A. Choose a PSU of at least 2.5 A plus the battery charging current.
2. Security autonomy required: 4 h. Capacity: 2.0 × 4 = 8 Ah. Divide by 0.8 for ageing: 10 Ah.
3. Choose two 12 V 12 Ah batteries in series for 24 V.

The battery keeps doors secured during a mains failure. A fire alarm still drops the locks, because the release relay sits after the battery. If the client prefers doors to release on mains failure, no battery is needed for the locks, but say so in the specification.

Common mistakes

• Magnetic locks on escape doors with no manual release.
• Fire alarm release wired to the controller input only.
• Stair doors used by firefighters left locked on the stair side without a defined re-entry policy.
• No test of the release in the commissioning procedure.

Emergency lighting at these doors: Emergency Lighting: Quick Design Rules.

For deeper study: MEP Center online certified courses.

Check the edition of the code that applies to your project.

[Author]

Posts