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Wiring, fuses and breakers for an array: the 1.25 and 1.56 rules

Solar PV physics · Advanced · about 15 minutes · part of Solar PV Systems

A PV conductor carries current for hours, and on an edge-of-cloud morning more than the datasheet’s Isc. So the circuit is rated at 125 % of Isc, and the conductor at 125 % of that again: 1.56 × Isc. With three or more strings in parallel each string gets its own fuse, because a fault in one is fed by all the others. The battery side has the opposite problem: a battery can push thousands of amps into a short, so the fuse sits within 450 mm of it and is never bigger than the cable it protects.

Wiring, fuses and breakers for an array: the 1.25 and 1.56 rules: the circuit on the simulator's bench

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How it works

Three strings of one 400 W module: Isc 10.3 A each, 30.9 A together. The combiner conductor must carry 1.56 × 30.9 = 48.2 A: 10 mm² PV cable (98 A) does; 4 mm² (55 A) does too, with less margin. Each string fuse is between 1.56 × 10.3 = 16.1 A and the module’s 15–20 A series-fuse rating: 20 A. The battery breaker is sized for the hybrid’s 160 A at 24 V and no bigger than the 4/0 cable’s 260 A.

Run the 🛡 Safety check: B2, B3 and B4 read these very numbers. Then take one string fuse out and run it again.

Try it on the bench

  1. Change the combiner cable to 2.5 mm² PV (select it): B2 fails at 1.25 × Isc.
  2. Set the battery breaker to 300 A: bigger than the cable, B4 fails: the cable would burn before it trips.
  3. Delete one string fuse: B3 fails for the three parallel strings.

Parts on the bench

In the course

Solar PV Systems, module “4 · Array configuration and the string window”.

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