About the DCDB Sizing
DC Distribution Board (DCDB) sizing determines the rating of busbars, fuses, surge protection devices (SPDs), and DC isolators at the point where multiple AJB outputs are aggregated before feeding the solar inverter's DC input terminals.
How to calculate it
- 1Total DCDB input current = sum of all AJB output currents feeding this DCDB.
- 2Each AJB input to the DCDB gets its own DC fuse (rated 1.25× that AJB's output current) and surge protection device (Type II SPD, 1,000 V DC, 20–40 kA).
- 3DCDB output busbar: rated for total current × 1.25 safety factor.
- 4DC main isolator (on DCDB output): rated for total current and system voltage (typically 1,000 V DC or 1,500 V DC).
- 5Enclosure: IP42 minimum for inverter room; IP54 if outdoors.
Worked example
Three AJBs each outputting 120 A feed one DCDB. Total DCDB input = 360 A. Each AJB input fuse: 150 A. Output busbar: 360 A copper. DC isolator: 400 A, 1,000 V DC. Three Type II SPDs at 40 kA. Enclosure: IP42, wall-mounted in inverter room.
What you can calculate
- Main MCCB
- DC busbar
- Type 1+2 SPD
- IP65 enclosure
Frequently asked questions
Why is surge protection (SPD) important in a DCDB?
Solar plants are large outdoor electrical systems highly susceptible to lightning-induced surges — either directly from a nearby strike or from indirect surges conducted along the DC string cables. Without SPDs, a single lightning event can destroy inverter input stages (costing ₹2–10 L for a central inverter). Type II SPDs (20–40 kA) in the DCDB clamp transient voltages before they reach the inverter.
What DC voltage rating should DCDB components have?
Standard utility and C&I plants use 1,000 V DC systems — DCDB components must be rated for 1,000 V DC. High-voltage (1,500 V DC) systems are increasingly used for utility-scale plants above 1 MW — they reduce current, cable cost, and losses. Mixing 1,000 V and 1,500 V components is dangerous; ensure the entire DC side (panels, combiners, inverter, cables) is rated for the chosen system voltage.
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