How to Size Cables Under DEWA 2017: Appendix 5 Tables, +50°C Correction and Grouping Factors

What DEWA Appendix 5 Gives You: Tables 2 and 3

The DEWA Regulations for Electrical Installations 2017 size power cables through Appendix 5: tables that pair each copper cross-section with the maximum rating of the protective device (MCB or MCCB) allowed to guard it. Table 2 covers multi-core armoured PVC cables (PVC/SWA/PVC); Table 3 covers armoured XLPE cables. These two tables are the starting point of every feeder and sub-main calculation in Dubai.

The tables differ because the insulation does: XLPE runs at a 90°C conductor temperature against 70°C for PVC, so the same cross-section carries a higher protective rating under Table 3. The first decision is therefore not the size at all — it is which table your specified cable belongs to.

• Table 2 (PVC/SWA/PVC, copper): 2.5 mm² → 10/15 A · 4 mm² → 20 A · 6 mm² → 30 A · 10 mm² → 40 A · 16 mm² → 50 A · 25 mm² → 60 A.
• Table 2 continued: 35 mm² → 80 A · 50 mm² → 100 A · 70 mm² → 125 A · 95 mm² → 160 A · 120 mm² → 180 A · 150 mm² → 200 A.
• Table 2 continued: 185 mm² → 250 A · 240 mm² → 300 A · 300 mm² → 350 A · 400 mm² → 400 A.
• Table 3 (armoured XLPE, copper): 10 mm² → 50 A · 16 mm² → 60 A · 25 mm² → 80 A · 35 mm² → 100 A · 50 mm² → 125 A · 70 mm² → 160 A.
• Table 3 continued: 95 mm² → 200 A · 120 mm² → 225 A · 150 mm² → 250 A · 185 mm² → 300 A · 240 mm² → 350 A · 300 mm² → 400 A.

Dubai Is Not 30°C: The Temperature Correction Ca

Appendix 5 ratings assume reference conditions. The DEWA regulations themselves set the climate frame in §1.3: a design ambient temperature of up to 48°C, with a 37.8°C 24-hour mean. Plant rooms, rooftop containment and risers in July sit far above the 30°C base of the air temperature table — so a cable chosen straight from Appendix 5 is, in Dubai summer conditions, effectively oversized on paper and undersized in the shaft.

The correction factor Ca comes from BS 7671, which DEWA incorporates. For cables in ambient air (Table 4B1, base 30°C), 70°C PVC insulation keeps 0.87 of its rating at 40°C, 0.79 at 45°C and 0.71 at 50°C; 90°C XLPE keeps 0.91, 0.87 and 0.82 respectively, falling to 0.50 (PVC) and 0.71 (XLPE) at 60°C. Cables buried direct in the ground use Table 4B2 (base 20°C): at 50°C the factor is 0.63 for PVC and 0.76 for XLPE.

Grouping: The Cg Factor from BS 7671 Table 4C1

Cables that share a tray, conduit or trench heat each other. The grouping factor Cg from BS 7671 Table 4C1 (bunched in air, on a surface, embedded or enclosed) reduces the effective capacity as circuits accumulate: 0.80 for 2 circuits, 0.70 for 3, 0.65 for 4, 0.60 for 5, 0.57 for 6, 0.50 for 9, 0.45 for 12, 0.41 for 16 and 0.38 for 20 or more.

One detail inspectors catch more often than any wrong number: Table 4C1 counts circuits, not cores. A three-phase feeder is one circuit for grouping purposes — counting its three cores as three circuits triples the penalty and wastes copper.

Worked Example: A 100 A Feeder at 50°C

The combined rule is simple: required cable rating = load current ÷ (Ca × Cg). Only then do you open Appendix 5. Take a 100 A load on an XLPE armoured cable, installed in air at a 50°C ambient, sharing containment with two other circuits — three grouped circuits in total.

Ca = 0.82 (XLPE, 50°C, Table 4B1) and Cg = 0.70 (3 circuits, Table 4C1), so the cable must be rated for 100 ÷ (0.82 × 0.70) = 174 A after corrections. Now Table 3: 70 mm² allows a maximum 160 A protective device — not enough. The next size, 95 mm², allows 200 A and is the correct pick. Without the corrections, 50 mm² at 125 A would have looked sufficient on paper.

• State the load current Ib and the protective device rating.
• Pick Ca for the real ambient temperature and insulation type (4B1 air / 4B2 buried).
• Pick Cg for the number of grouped circuits — circuits, not cores.
• Divide Ib by (Ca × Cg), then choose from the Appendix 5 table that matches your cable.

Mistakes That Come Back as Review Comments

Cable sizing errors in DEWA submissions are rarely exotic. They are the same four shortcuts, and each is visible to a reviewer in seconds.

• Sizing straight from Appendix 5 at the 30°C reference while the cable runs through a 45–50°C plant room.
• Counting cores instead of circuits when applying Cg — a three-phase circuit is one circuit.
• Quoting Table 3 (XLPE) ratings for a cable schedule that actually specifies PVC/SWA/PVC, or the reverse.
• Applying the air table (4B1) to a buried run — direct-buried cables take Ca from Table 4B2 with its 20°C base.

Run the Numbers with the Clause Behind Them

The free VerixHQ cable calculator applies exactly these corrections: pick the insulation, installation method, ambient temperature and number of grouped circuits, and Ca and Cg are looked up from the same BS 7671 tables used in this guide.

And when the question is not arithmetic but interpretation — which table your cable construction belongs to, how the climate clause applies to your installation — SparkProof answers with the exact DEWA 2017 section cited, in English or Arabic, so you can defend the choice at review.

Frequently asked questions

Read also

• DEWA Load Schedule & Load Calculation Requirements
• DEWA Earthing Requirements Explained: The 1 Ω Rule, Earth Pits and ECC Continuity
• DEWA Regulations for Electrical Installations 2017: How to Use the Document (Section Map)