Selection and Sizing of DC String Cables for Solar PV Rooftop Projects

Example – Sizing of solar DC string cables

Problem:

Let’s consider a project with a single-string configuration. This means that only one string (i.e. one positive conductor and one negative conductor) is coming from the PV array till inverter. We have an array of 10 modules connected in series in a string (Module Characteristics: Vmp = 37 V, Imp = 8 A, Isc = 8.5 A).

Overcurrent Protection Device – 15A

Properties of the cable available in the market: 4 sq mm: Resistance at 20 deg – 5.09 Ohm/km, Current Carrying Capacity (CCC) – 55 A

Distance from the last module to the Inverter is 30 m. Pl determine, whether the mentioned 4 sqmm DC cable be used for this project and what shall be the maximum Voltage Drop (%).

Solution:

Step 1: Current Carrying Capacity (CCC)

Required Current Carrying Capacity of cable for this project = Current rating of Overcurrent Protection device = 15A

Current Carrying Capacity of Available DC cable in the market (4sqmm) = 55A (To be determined from cable datasheet)

Since the CCC of available cable is more than the required CCC, we can select this 4 sqmm cable.

Step 2: Resistance for Total Length

• Resistance at 90 deg (Ohm / km) = 5.09 x [ 1 + 0.0040 (90 – 20) ] = 6.52 Ohm / km
• Total Length of Circuit = String cable length + Module leads
  • Each module comes with approx. 2 m module lead i.e. 1 m positive 1 m negative. Since we have 10 modules in series, hence total Module lead length = 2 x 10 = 20 m
  • Also from site we know that, Distance from Module to inverter is 30 m. Please note that circuit length should have length of both positive and negative conductor. Hence, string cable length will be 2 x 30 = 60 m.
  • Total Length of Circuit = 60 + 20 = 80 m or 0.08 km

Step 3: Vmp of Array

Vmp of Array = 10 x 37 = 370 V

Step 4: Voltage Drop (%)

Voltage Drop (%) = (8 x 0.521 x 100) / (370) = 1.12%

Since, the Voltage Drop of selected DC Cable(i.e. 4 sqmm) is less than 2% and the Current Carrying Capacity is also in range, we can select this 4 sqmm cable.

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