Maximum Voltage vs. Peak Voltage: Sean White on Energy Storage System Wire Sizing

When you’re sizing wire for an energy storage system, the number you need isn’t the highest voltage the system could ever produce. Instead, it’s a specific, defined value called maximum voltage. In the National Electrical Code Article 706.9, maximum voltage is the input and output voltage listed on the ESS nameplate or system listing. That is what determines your wire sizing.

That distinction matters because there are several ways a system’s actual voltage can run higher than that number, but none of those change what you use for sizing. Still, they’re worth understanding.

We sat down with Sean White, lead instructor for HeatSpring’s NABCEP prep courses, to walk through how maximum voltage is defined, why it isn’t the same as peak voltage, and how voltage and current interact when you’re sizing conductors for an energy storage system.

Want to build a deeper foundation in code and system sizing? Check out Sean’s popular NACBEP PV Installation Professional (PVIP) Certification Prep bundle!

Transcript

Let’s talk a little bit about code. I’m very deep into it right now. I’m writing a book called The Energy Storage NEC Book: Mastering Code with Comedy, and it’s got some illustrations. It’s fun, kind of like reading a comic book. But let’s get serious for a second: what do you use for determining the maximum voltage of an energy storage system?

That’s covered in the National Electrical Code, under Energy Storage Systems – Article 706, at least until it changes in the 2029 NEC. So, what do we call the maximum voltage?

Maximum voltage is not the highest voltage you could ever have. It’s the voltage you use for wire sizing. That distinction trips people up. There are voltages that run higher than maximum voltage, but maximum voltage specifically is what you use to size your wires. It’s the energy storage system’s input and output voltage listed on the nameplate, or in the system’s listing. You’ll find it in the instructions.

So what kind of voltage runs a little higher than maximum voltage? Here’s something interesting: think about the outlet in your wall. Most energy storage system outputs are actually alternating current – though there are some UL 9540-listed systems that are DC. For a while, they were all AC output.

Let’s use 120 volts as an example, even though most systems are actually 240 volts — people are just more familiar with 120 since that’s what comes out of a wall outlet. A 120-volt system’s peak voltage actually goes up to about 170 volts, because AC is a sine wave — it goes up and down, up and down — and 120 volts is the RMS value, or root mean square, not the peak.

Here’s why they use RMS: if you square a number, then take the square root, negative numbers become positive. That matters because if you just averaged an alternating current waveform directly, half the time it’s above zero and half the time it’s below zero. It would average out to zero, which would make it look like there’s no voltage at all, when there clearly is. So instead, RMS gives you the actual average magnitude away from zero. The true peak – the top of that sine wave, whether it’s generated by rotating machinery or by an inverter reproducing a sine wave – is higher still. It’s not something you need to worry about for maximum voltage, but it’s a good thing to understand.

Another way voltage can run higher is cold temperature on a PV array. If you look at the open-circuit voltage on a PV module’s nameplate – the label on the back of the module, or the datasheet, listed as Voc – that’s your open-circuit voltage. In a cold climate, your actual voltage will run higher than that rated number. Just something to keep in mind.

Here’s another thing to know about energy storage systems: if you have a battery and inverter connected across a voltage range, lower voltage means higher current – because voltage times current equals power, so if voltage drops, current has to increase to deliver the same power. This is written into the code, and it’s shown up on associate exams, so you need to know it: when voltage is lower, that’s what determines the higher current you need to account for when sizing your conductor.

Current can also run higher than your rated current for another reason. Rated current is your continuous current – something the system sustains for three hours or more. But a lot of off-grid systems have surge capability, where current can spike to double the continuous rating, sometimes for just a second. You don’t size your wires based on those short surges, because wire sizing is all about heat: a conductor heats up as current passes through it, but that takes time, so a one-second surge doesn’t raise the wire’s temperature the way sustained current does.

So there you go: the maximum voltage of an energy storage system is simply the input and output voltage listed on the ESS nameplate.