Solar Install: Do I Need Fuses?

Do I need fuses on my PV system? Seems like a simple question. Getting the answer was anything but simple. The Energy Trust inspector, Gary, has a rule-o-thumb for fuses based on the number of strings in parallel: 2 strings don't, 3 strings might, and 4 strings do. I have 4 strings, so the rule of thumb says that I do need fuses. However, there are no fuses on my system. Hmmm..

I am using the term fuse; it would be more correct to say open circuit protection device (OCPD) which includes circuit breakers. Let me remind you, that I am not an electrician or PV installer. I am just a guy that had solar panels installed, so I am going to use the term fuse.

Let's start by discussing why you would need fuses on a PV system in the first place. The reason is to protect from over-currents and reverse currents. The two things that need protect are the wiring and the solar panels. If you have batteries as part of your system, this can be another current source. I will not cover that case.

The first case is a the simpler one to explain. The panels have a short circuit current rating (Isc). If the wiring can not carry the worst case current load, there must be something to prevent meltdown. The better answer is to make sure that the wires (even at high temp) can carry whatever your system could deliver. Oversizing the wiring has the additional benefit of reducing loss and improving efficiency. It may seem silly to spend thousands of dollars on a PV and then save $50 on the wiring when it reduces the system efficiency by 2-3%. Yet this is just what many contractors do.

The second reason for fuses is to protect the strings from each other. Electricity follows the path of least resistance, normally this is through the inverter. However, if one string is not providing current and the others are, that path of least resistance may be through the non-producing solar panels. This can happen when one string is in the shade and the others are in sunlight. Solar panels have some built in tolerance for this reverse current flow, it is called the Module Series Fuse Rating. If the max current the other string(s) could back flow through the shaded modules is less than this tolerance value then no damage will occur to your panels and you do not need fuses.

This webpage has the specifications and equations for computing the currents.

Now you know the reasons and have access to the equations. So why does this system not require fuses?

Data for this system:

   Number of Strings (n) = 4
   Number of Strings in a Combiner Box Group (Ncb)= 2
   Module Isc = 3.83 A
   Module Series Fuse Rating = 15 

Let's crunch the numbers for the over current and reverse current. First the wiring over current calculation. In the combiner box, the 4 PV strings are grouped into 2 runs. This reduces the number of wires from the inverter to the roof. These wires carry the DC power the panels produce.

   Max current = 1.56 * Ncb * Isc 

for this system that is:

   Max current = 1.56 * 2 * 3.83 A = 11.95 A 

Now we know the max load. Can the conductor wires take it? The DC wiring is #10 "MC connector cables" which are USE-2 or RHW-2 wires, sunlight resistant, rated for 90 degrees celsius, rated for 50 amps. Plenty of ampacity, even when derated for temp.

Now to calculate the reverse current:

This 4kW system falls into the "Modern, small utility-interactive inverters" category of the whitepaper. The I fault equation is then:

   I fault = (n-1) * 1.25 * Isc 

for my system:

   I fault = (4-1) * 1.25 * 3.83 = 14.36 A 

This 14.36A result is less than the 15 Amp Module Series Fuse Rating so, if my assumptions and math are correct, no OCPDs are needed.

So there you have it. No fuses needed. If you have any comments or corrections, please post a comment. I would appreciate any 3rd party confirmation.

So why did the rule-o-thumb fail in this case? Older panels and most panels on the market today (2007) have a Isc value that is about twice as high as the the high-efficient panels in this system. Thin film and new high efficient panels both have lower Isc than traditional crystalline silicon. As these two new types of panels are used proliferate into the market, this rule of thumb (4 strings always need fuses) will hold less true.