DIY Manual

24 | P a g e 10-543-1 REV A
DIY Manual
Shown are 3 parallel strings, each at 158Voc and 9.8A. String 1 suffered a failure with two PV
panels, the string voltage dropped to 79V, and reverse current from Strings 2 and 3 now flows
into String 1. Each module has a maximum current rating of 15A; as you can see, String 1 now
has 19.6A flowing across it, thus exceeding its max limit. Over time this excess current will cause
the wire in String 1 to heat up, and eventually lead to the insulation of the wire breaking down
and internal damage to the panels.
In the above diagram, we added three 15A breakers to each parallel string. How we selected 15A
will be discussed later on. Below describes the sequence of events:
#1 – Two panels fail in the top string, voltage bias changes, current reverses.
#2 – The current from the other two strings adds to 19.6A, and flows into the top string
(reverse-current flow).
#3 – The top string’s 15A breaker sees 19.6A and trips open, thus protecting the wire in
the top string from passing current higher than what it is rated to safely pass.
This is why you want string breakers – to protect from a reverse-current situation.
Over-Current Protection
A wire of a certain size can carry a certain amount of current, measured in amps. For example, a
#10 AWG wire can safely carry 30A. If a circuit can supply more current than the wire can safely
79Voc, 19.6A
(Isc)
158 Voc, 9.8A
158 Voc, 9.8A
15A breakers
#1
#2
#3
19.6A
Use string breakers when combining 3 or more parallel strings! NEC 690.9