National Electrical Code Tips: Article 690 -- Solar Photovoltaic Systems,
Only about 15% to 20% of the work of installing a Solar Photovoltaic (PV)
System is electrical. But that electrical portion can easily result in disaster
if not done correctly. Thus, Article 690 provides requirements for that portion.
- You can bet that at some point a PV system will lose power. If that system
is interactive with the utility, the inverter must automatically de-energize its
output to the connected electrical production and distribution network.
[690.61]. It must stay disconnected until that network is restored. Among other
things, this prevents dangerous feedback into that network.
- The same thing applies to ac modules [690.61].
- While disconnected from the network, the PV system can function as a
stand-alone system to support the connected load [690.61].
- Also see Article 705, if you have an interconnected system.
Something many people don't realize is that as the battery systems of PV
systems get larger, they begin to resemble the battery systems in data centers.
People in the battery industry are very aware of this, as are the relevant NEC
CMP (Code Making Panel) members and their counterparts on the IEEE battery
committee. Consequently, the battery requirements for PV systems draw from a
rich depth of experience, practical knowledge, and empirical data.
- Storage batteries for PV systems present an additional safety challenge and
thus additional installation and maintenance requirements to meet. In some
cases, it makes sense not to provide battery storage because doing so isn't
worth the cost and complexity of meeting these requirements. But if the
installation is going to be something more than merely a supplemental source
then these costs and complexities are worth incurring and addressing. The
battery installation must meet the requirements of Article 480 plus the
additional requirements of Part VIII of Article 690.
- There is confusion about what a "battery" is. Many people think of the jar
in their car as a "battery" because that's what we call it. An arrangement of
such jars in a string for purposes of storing power as in a data center loss of
utility power protection system (or for a PV system) is actually a battery. Each
jar is typically a cell, and you string these together to make a battery.
The automotive "battery" is really a string of cells in a single package. So
don't visually confuse the power storage jars (cells) with being a battery,
unless you're using a nonstandard system wherein the "battery" really is a
battery rather than a cell designed to be part of a string that makes up a
- If you install the PV power source per 690.41, the interconnected battery
cells are considered grounded [690.71(A)].
- For dwellings, connect the storage batteries so they operate at 50V or less
[690.71(B)(1)]. The NEC says you can skip this requirement if live parts aren't
accessible during routine battery maintenance, but such a setup makes proper
battery maintenance impossible. Ignore the exception, because it does not
account for the realities of battery maintenance.
- If the available short-circuit current from the battery exceeds the
interrupting or withstand ratings of other equipment in that circuit, then you
must install a listed, current-limiting overcurrent device in each circuit
adjacent to the battery, making sure it complies with 690.16 [690.71(C)].
- Many installers now use (Valve Regulated Lead Acid (VRLA) batteries. The
battery manufacturers and other battery experts (look up BATTCON and read the
presented papers) look at VRLA as a suboptimal choice. They have largely been
pushed by APC and others with a legacy background in small point of use systems
that depend upon VRLA.
VRLA batteries have many significant disadvantages compared to their "flooded"
counterparts. But they have some advantages (some of which are only perceived
and not real) that tip the balance toward chooseing VRLA. For example, they do
produce less gas than flooded batteries and thus can be colocated (in small
numbers) with actual equipment instead of being located away from the equipment.
A perceived but not real advantage of VRLA is they are maintenance-free. They
are actually harder to maintain, and for this reason do not last as long as
properly maintained flooded batteries. This is why you rarely see VRLA used in
data center battery rooms.
For a residential PV system, the VRLA is often a good choice. One reason why is
the homeowner isn't going to maintain them anyhow and is unlikely to ever be
qualified to do so. With the VRLA, the "maintenance" includes cleaning the
terminals and connections, and replacing based on time in service or possibly
based on a condition such as internal resistance or terminal voltage.
That's a huge advantage, though it's costly when scaled to any size because it
inherently wastes battery life over what you can get from flooded batteries.
Another advantage is VRLAs exempt you from the nonconductive case requirement
that comes into play when you have more than twenty-four 2V cells connected in