NEC Batteries Solar Part 1

Welcome to the Battery, Solar and Interconnection Aspects of the National Electric Code Class! This is a continuation of a previous National Electric Code review.  In that course, we cover code as it relates to onsite energy systems.  In this class, we’ll discuss the portions of electric code specific to batteries, solar, and interconnection, to better understand design principles, with a focus on articles 480, 690, 705, and 706. 

This class cannot legally provide verbatim sections of code or full tables for the viewer. I highly recommend you purchase a copy of the National Electric Code, as it is a resource you will turn to again and again.  

Let’s dive right in!  Start with Section 480 with the definition of battery cells, which are the base unit of the battery . Electrolyte is the fluid between the cathode and anode. Terminals protrude from the cathode and anode, out of the battery, which attach to the battery inverter cables.

Helpful hint: Lead acid is at 2 volts while lithium ion is at 3.6 volts to 3.8 volts. There are different types of both lead acid batteries, and lithium battery chemistries. For example, lithium cobalt has a slightly higher cell voltage (3.6 to 3.8 volts), than lithium iron phosphate. Therefore, when the cells are wired to create a 48 volt battery, the lithium iron phosphate will have a slightly lower operating voltage for the same number of cells than lithium cobalt. 

Section 480 contains the general battery requirements which is then expanded on later in section 706 for higher voltage battery systems. In general, the battery NEC rules are fairly basic and expand as grid-tied batteries become more popular.  Let’s start with the general requirements and then get to the more advanced sections towards the end of the program.

Storage batteries and battery management equipment must be listed, but lead acid batteries do not. Basically, this implies that as the market moves towards lithium-ion batteries, we will see an increased focus on listing requirements. There’s more discussion on that to come. 

Here is a call-out for overcurrent protection not being required on prime movers for 60 volts or less. Prime movers are using the battery to start ignition or digital controls circuits that start up other systems, like an electric start generator. 

Ambiguous writing brings uncertainty into 48V battery bank design. While much of code only applied to higher voltage batteries, why would prime mover circuits be specifically called out here, if the intent of code was to only regulate batteries over the 60V DC threshold? This gives inspectors wide berth to interpret battery systems, subjecting any grid-tied battery system to the code requirements of higher voltage battery banks. 

A potential scenario: An inspector feels like there should be something in a battery circuit like overcurrent protection, even though it isn’t specifically called for within the code itself.   The fact that the battery is under 60V DC might not convince the inspector. We would like to see clarification in future volumes. At any rate, an electrician might find the current national electric code lacking for batteries. Remember:  code establishes a minimum threshold for safety. It is always okay to add additional fuses or disconnects in line with your battery system if you believe they are warranted. 

With regard to the specific lack of overcurrent protection or disconnect requirements for batteries under 60V, I recommend using listed batteries with built-in disconnects and overcurrent protection systems. Then, if  you’re skipping additional disconnects or overcurrent protection, I would refer an inspector to earlier code on feeder and power distribution taps, which do not require such items if made within 20’ of an electrical service. That is to say, if your batteries are within 5’ of the inverter and under 60V,  there’s no code-driven reason to include external disconnects and overcurrent protection.

Having overcurrent protection and disconnects on electrical power supply circuits is a good idea. However, many consumer-grade lithium batteries already have these features built in so adding external over current protection can be redundant.

Basically, there needs to be disconnects to allow higher voltage batteries to be divided into lower voltage batteries for servicing (where serviceable). Still, with the right planning, adding overcurrent protection is not a bad idea, even when skipping an additional disconnect, so that if a fuse does happen to blow, it doesn’t require getting inside the battery pack to service. 

With batteries, where external disconnect requirements come into play are if the batteries are far away from the battery inverters. If there is a wall that divides them, being in adjacent rooms or where the batteries are outside the building and the inverters are inside, where the cojoining cables go through a wall, it needs a disconnect. Item B says that if the disconnecting means is activated via remote control, and where those controls are not within sight of the battery system, the local disconnecting means needs to be locked in the open position to avoid someone remotely shutting it on the service panel.

So generally, common sense disconnecting says you need a disconnect if the two related items are not within sight of each other, and code generally treats that as within 25 feet. The disconnecting means shall be legibly marked with nominal voltage, current rating, and other items you’d expect to be called out on a listed component. Here’s a battery made by Fortress Power with a built-in 250 amp DC breaker built right into the battery pack, which should meet the code requirements of strict AHJs without the need of an additional external switch or overcurrent protection. 

Battery chemistries with a corrosive electrolyte the structure that supports the battery shall be resistant to deteriorating action. Batteries are heavy. When building a battery room, you put them on a shelf on a rack. That shelf or battery rack is usually metal. Obviously, that’s very conductive. I remember one of my first battery projects. We were picking them up with our on-site lift equipment by attaching some metal chains to the metal casings of the batteries. The battery terminals are just sticking out right there and so we had to tape the chains with some rubber resistive tape to move them! Otherwise, we’d get arcing between the battery terminals and the metal chain that could continue to conduct into the metal lift equipment and potentially electrocute the driver!

So, metal racks are not good for batteries without some insulative properties!  Concrete is considered to be conductive if it gets wet, so in damp basements, etc keep this in mind.  Likewise, you could have some voltage creep out of a

battery, and if it is on a metal rack potentially you can have the rack be energized and that be a safety hazard, so use rubber mats if you’re putting the batteries directly onto a concrete slab.

Here is a battery that is designed for floor mounting and it has an insulated hard plastic base so that it can sit right onto that concrete slab. Similarly, you would put a rubber mat on your metal battery rack or maybe an anodized metal is more resistant.  Painting the metal rack alone is not considered insulated battery rack.

Let’s talk about ventilation for a minute. There are sealed batteries and unsealed batteries. Unsealed batteries, like traditional flooded lead acid, produce gas as they’re being charged or discharged and that gas needs to be vented to the outside.  The question becomes, “Does a sealed battery also need to be ventilated?” 

You could say a great advantage of having a sealed battery is that you don’t need that ventilation. But, what happens when a sealed battery becomes unsealed? Let’s say the building catches on fire. The fire makes it to the battery room and starts burning up the casing of the batteries and it heats the battery to the point where it too catches on fire.

In that case, the sealed bat

tery becomes an unsealed battery. and so has been one um industrial battery fire and

(you have time here because I condensed, do you want to explain this fire?)

uh i believe it was arizona’s aps where uh it was actually the battery itself as far i’m not sure the investigation is complete yet but not not many suspects in this fire to begin wit the battery goes through some internal failure it catches on fire and all of a sudden it’s leaking gas into the room and for whatever reason and i don’t know the real details of this so just hypothetically speaking,

If your ventilation is not on an emergency or backup power circuit ,and so your ventilation isn’t working, when the fire is going on you can get gas accumulation inside that room and that’s exactly what happened in this APS fire.

The batteries caught fire, the room was not ventilating, the gas accumulated inside the battery container, and when the fire department showed up as soon as they opened the door the room exploded, sending several firemen to the hospital.  So, what we’re going to see is that as the sealed batteries get larger in size the ventilation requirements become mandatory even with a sealed battery. The takeaway from this is that you can’t just assume the battery room does not need to be ventilated simply because you have a sealed battery.

We will discuss these ventilation requirements in more detail in other parts of fire code, not national electric code. So, we see national electric code referencing the fire code for ventilation of  larger battery banks.

I don’t think you’re wrong on where the right location is for the battery. The best location for a battery is inside, in air conditioning, in a dust free environment.  Just like other solar electronics, you’ll get much better life and performance  that way.  Dust is as bad as temperature or anything else as far as I am concerned.

The main thing is live parts operating at 50 to 1000 volts nominal and this is squarely within the realm of home battery banks and  even  yeah i guess lead acid has 48 volt nominal

The lithium ion industry will refer to any battery bank near 48 volts as a 48 volt battery bank but on the spec sheet you’ll see a 48V battery resting voltage is typically above 51 volts. 

When you walk into a battery room and see exposed terminals, unprotected cables, and metal busbars outside of traditional distribution panel enclosures, the customer might ask if they touch those terminals, am i going to shock myself, and the answer is yes.

Either the batteries are bought individually and then wired together, or the next step up is a manufactured system where all the exposed parts are protected, or it’s something in between the two where the installer uses the battery manufacturer’s product and the inverter manufacturer’s product and you still have to wire them together.

That means you might go and buy the cheapest battery on the market and put it on a shelf in your garage, but if has exposed terminals you’re going to run into more design constraints than if those terminals are enclosed in a case provided by the manufacturer.

When needing to protect the user from exposed parts, that does not necessarily mean a locked room that can only be accessed by qualified persons.  It means you have to have a locked area, or a barrier, or a locked cage inside a garage to prevent people from getting into area where the batteries are. So per code, battery systems can be in a garage, even when on a shelf using different equipment from different manufacturers, if the proper barriers are considered.

Just as common is to buy a higher end manufacturer assembled system where the provided hardware protects already protected.?? Other suggestions are to elevate the batteries above the floor. Eight feet is the typical height to steer clear of mixed use areas. In the harshest circumstance, an inspector is entitled to ask for 30” from the equipment be unreserved space, such as where a vehicle would not be allowed to park.

These are standard work area rules found at the beginning of NEC. Less than that, a tough inspector may require this dedicate space regardless of how listed or protected the equipment is. Then you have to treat it like a locked cage.  So basically, if you want to put your batteries in the garage and you’re buying uh kind of just the battery and not something more top shelf you’re going to have to cage it off.

Do it yourselfers be warned: you can’t just buy a homemade battery with exposed terminals and keep it on a shelf.  You’re liable for that if you’re going to build your own battery pack and keep it under 50 volts and even just playing around with it with DIY, you need to secure the immediate location and make sure that secured area is a dedicated space. 

 You need to have a 1” clearance between the cell container and any wall. We’re going to get into differences for a completely enclosed battery inverter system like the Tesla Powerwall.  The Powerwall has its own battery and inverter completely enclosed in one shell that can be mounted directly to the wall. 

A battery that’s more like a car battery, which is just a big block with exposed terminals coming out of it, and you are allowed to build your own battery assembly. It’s a process that’s been going on for years and years, pre-Tesla era, and there needs to be a 1” clearance between the cell container and the wall. 

So, you’ll see battery mounting brackets that will separate the wall from the battery when you are wall mounting your batteries or if you’re floor mounting the batteries, you can’t just butt them up against the wall.  They’re not permitted contact with that adjacent wall. Even the shelf itself will have this clearance requirement and we’ll reference that a little bit later. 

There. the terminal clearances how much vertical space you need between the battery and the shelf above it. That is something that’s driven by the manufacturer spec so that’s not a clearly defined vertical clearance code. If you are putting your batteries in a battery room, you want that door to open outward rather than inward.  

Once your batteries get over specific voltage, and we’ll see this in the battery section more advanced battery section the doors need to have panic hardware so that you can’t get locked in to a battery room, even though it needs to be a walkable door.  Gas piping is not permitted in dedicated battery rooms! Keep that in mind with your electric water heaters.  

Another one that is a little it should seem obvious, but illumination is required in your battery rooms. I did a project where we put the batteries outside the building but the home was on a hillside, and we had a little alcove where we could put the batteries, tucking them up against the building foundation.

 Well, that little dedicated space if it’s not just a box that you’re standing outside of and then working into, if it’s an actual battery room, that battery room needs lighting. So, if you’re building a battery room adjacent to the building, it can’t just be four walls and a ceiling.  It needs to be a little bit more professional.  

All of your working clearance requirements apply that you see in the earlier sections of code, such as vertical spacing and clearance behind you. Basically, in a dedicated battery room, you need to be able to fully stand up in that room. That’s 6.5’ of vertical clearance and anywhere from 3-4’ behind you before you get to the next wall. Battery rooms have to be rather sizable and professional. If you don’t have enough space for a battery room, build a battery box instead.  Don’t build something that’s halfway a room, and don’t build a battery box that is so large that it’s really a room. 

In this adjacent section to 480 with batteries we have capacitors. In most national electric code in your battery circuits, when you’re sizing your cable you’re looking at the ampacity of the battery circuit and then you’re adding 25 additional headroom for that. There’s differences between continuous output that’s running full-time and intermittent output that is maybe running for under three hours or less.

In the beginning parts of code, we see either 100% of the intermittent output, the discontinuous output or an additional 25 for continuous load with capacitors it’s a special thing where they say the conductors have to be 35% rather than 25% and so as a building engineer with solar and batteries becoming more prominent, you may find yourself needing to take a master electrician exam.

For instance, you take all the sizing considerations.  Typically, you’re oversizing your conductors by 25% when it comes to electrical design. There are special exceptions, and solar is one of them.  We’ll talk about why a little bit later.  Capacitors are another one of them. Capacitors get a 35% oversizing and I believe that’s just because capacitors can be so rapidly discharged.  

So, that’s pretty much it for Section 480. It’s a very short section and requirements are being migrated towards the more specialty sections at the back of the book. So, the general rules sections end at Chapter Four and then we get to Chapter Five, we start to get into specialty requirements. Chapters One through Four are to do with services, feeders, wires and cables themselves, then general appliances like air conditioners, lights and dishwashers, etc 

Chapter Five  and onward starts to get into specialty applications, special ocurrences which isn’t within the scope of this class.

With lighting fixtures in a battery room, it’s not a terrible idea to have the lights be battery powered on their own, such as LED light strips glued to the ceiling. That can be a cheap and pragmatic way to add lights to where they don’t currently exist. For a higher end build, having the lights on a dedicated backup circuit is not a bad idea. 

Before we get to Article 690, let’s look at what we’re skipping over. Chapter Five give deals with special locations like boat dock. Chapter Six gets into special equipment like electric vehicle charging. For electric vehicles there are ventilation requirements for the garage based on the amperage ratings of the electric vehicle chargers.

Finally, we are at Article Section 690 for solar systems. Definitions start, and the most confusing definition is functional grounding.

A functionally grounded system is one that’s connected to ground. It’s confusing because it is an umbrella term which refers to grounded and ungrounded DC-side overcurrent protection systems, which have traditionally been connected to ground on one end. This practice results in a blind spot within the ground fault detection system, and the solution was to not attach either the positive or negative end of the DC circuit directly to ground, but instead to fuse both sides of the circuit and leave the circuit “floating”, meaning no direct reference to ground. 

But a floating system still has an equipment ground conductor and has its metal parts connected to ground, independent of the actual DC positive and negative circuit. So both the floating style DC circuit design, and the traditional style where one of the two circuit ends were positively attached to ground, are considered functionally grounded systems. 

Inspectors were concerned about “ungrounding” the DC circuit, and this term was added to assure inspectors that the grounding of metal parts wasn’t being thrown out by changing up the ground fault detection circuit. This blind spot was only a problem for commercial-sized arrays up at the combiner box – we don’t have time in class to cover this issue any further, but Google the Bakersfield or Dietz-Watson fire to find out more information.  Maybe add more here?

Well as a final note on those fires, it wasn’t just a blind spot in the inverter ground fault detection circuit that was at issue – the actual fault that burnt the buildings down was due to a failure to consider thermal expansion of the conduit up on the rooftop. Long exposed conduit runs on the rooftop can pull apart if expansion joints are properly considered, even if the conduit was otherwise properly installed at the beginning of the project.

 Thermal expansion would cause the conduit to pull out of the connectors, and then the sharp conduit edges would bite into the wires contained inside them. When the wires hit the grounded conduit, normally, a ground fault error would occur. Due to the inverter blind spot, the ground fault went undetected, and commercial inverters stopped connecting the DC conductor directly to ground, isolating the ground fault detection, as a result. 

Pro Tip: It’s really not confusing if you accept that all metal parts of a solar array need to be grounded, regardless of whether the inverter is a “grounded inverter” or a “floating” inverter. They’re both functionally grounded. 

The generating capacity of the system, according to National Electric Code, is the inverter output in kilowatts. This is a common question on interconnection agreements. If you’re asked on a permit docent what is the system size is, and it does not specifically ask for the DC size of the solar array, the correct answer is to provide the AC output size of the inverter.

System voltage is another interesting issue. Most believe that residential homes must use a 600V solar array, with commercial being 1000V and utility being 1500V or more. But the 600V residential restriction only applies to systems on or inside the building. Let’s say an off-grid cabin is in the middle of the woods, with an adjacent field 1000’ away from the cabin.

In other words, the solar array is ground mounted and far away from the house. What do utilities do to transmit electricity over long distances is to step up to extremely high voltage because as voltage increases, amperage decreases. Voltage is more sensitive to maxim limits – a 600 volts component works fine at 600V but can pop at just a little bit more, like the bursting of a just a little too full water balloon. Voltage has more to do with the toughness of the cable jacket rather than the thickness of the cable itself. 

Whereas amperage is a slower burn, having more to do with the thickness of the cable itself. If amperage limits are exceeded, the device might not fail immediately. It might run hot and start to smolder before catching on fire. Bringing the topic back home, by doubling the voltage, the amperage is cut in half, and so the required cable thickness is cut in half, and so less material is required to transmit electricity over a very long distance. This is why the power grid uses high voltage for transmission and distribution, with distribution circuits exceeding 10k volts and transmission circuits exceeding 100k volts. 

So if the solar array is really, really far away from the point of use, it isn’t a terrible thought to consider moving to a 1000V platform and three phase electricity, because this equipment is commonly available for commercial applications and code only constrains its use when literally on or inside a 1-2 family dwelling. Commercial buildings prefer 1000V products because of those savings and they don’t have the 600 volt residential constraint. Standard solar cable and most solar panels, by the way, are rated for 1500 volts.