Capacitors moved into the number batteries used to have.
In the 2024 edition, Article 320 was batteries and Article 360 was capacitors. In 2027 they swapped. A procedure that cites "Article 320" and means the battery room is now pointing at the capacitor article, and it will read as though it almost makes sense — which is what makes it dangerous.
This is not a deleted-article problem where the citation fails loudly. 320 and 360 are both live articles about stored-energy equipment. A battery procedure citing 320 lands on capacitors; a capacitor procedure citing 360 lands on batteries. Neither throws an obvious error. See the full map →
§ What 320 covers — and what it does not
320.1 covers the electrical safety-related requirements for safeguarding employees while working with capacitors that present an electrical hazard, other than electric double layer capacitors (EDLCs), supercapacitors, or hybrid supercapacitors.
EDLCs, supercapacitors and hybrid supercapacitors are carved out of 320 explicitly and covered by the new Article 370 instead. A UPS ride-through module built on supercapacitors answers to 370, not 320 — even though everyone on site calls it "the caps."
The Informational Note to 320.1 points to Informative Annex R for more information on working safely with capacitors.
§ Six thresholds, at 320.3
320.3 is titled Stored Energy Hazard Thresholds — and that title is the point. These are written in joules far more than in volts. Risk control measures must be selected and applied from the hierarchy of risk control where any threshold is exceeded.
| # | Hazard | Threshold |
|---|---|---|
| 1 | Contact thermal | Less than 100 V and > 100 J stored energy |
| 2 | Electric shock | ≥ 100 V and > 1.0 J stored energy |
| 3 | Electric shock | ≥ 400 V and > 0.25 J stored energy |
| 4 | Arc flash | ≥ 1.2 cal/cm² incident energy at the working distance |
| 5 | Hearing acoustic | > 100 J stored energy — regardless of voltage |
| 6 | Lung collapse acoustic | > 122 kJ stored energy when dI/dt > 105 A/s |
Read thresholds 2 and 3 together. As voltage goes up, the energy needed to create a shock hazard goes down — 1.0 joule at 100 V, but only 0.25 joule at 400 V. A capacitor that is trivially small in energy terms is still a shock hazard once the voltage is high enough.
§ The two hazards nobody has on a slide yet
Thresholds 5 and 6 are not electrical injuries. They are pressure injuries, and they are the reason this article deserves its own training time.
Hearing acoustic hazard — above 100 joules, regardless of voltage. The blast from a capacitor failure is loud enough to damage hearing on its own. Voltage does not appear in this threshold at all. 320.4(B)(3)(4)b requires hearing protection where stored energy exceeds 100 joules.
Lung collapse acoustic hazard — above 122 kJ, when dI/dt exceeds 105 amperes per second. This is a pressure wave capable of causing pneumothorax. It is a rate-of-change condition, not just an energy condition — the energy has to come out fast enough.
Almost every arc flash program in the field protects against exactly two things: burns and shock. Article 320 names hearing damage and lung collapse as thresholds requiring risk control. Hearing protection as arc-flash PPE is a genuinely new idea for most crews, and it is triggered by a number — 100 joules — that a lot of installed capacitor banks clear easily.
§ What 320.4 requires
Qualification and training. Employees who perform work on equipment with capacitors that exceed the 320.3 thresholds shall be qualified and trained in the specific hazards and controls. Unqualified persons who work on electrical equipment with capacitors shall be trained in and familiar with the safety-related work practices necessary for their safety.
Risk assessment. Follows the overall risk assessment procedures in Chapter 1; additional protective measures are selected per the hierarchy of risk control at 110.3(H)(3).
PPE determination. Capacitor voltage and stored energy for the exposure; thermal PPE above 100 J; electric shock PPE per 130.7 at or above 100 V; arc flash PPE per 130.7 above 1.2 cal/cm² (5 J/cm²); and hearing protection above 100 J.
320.4(B)(3)(1) is specific: an exposure shall be considered to exist when a conductor or circuit part that could potentially remain energized with hazardous stored energy is exposed. The test is whether the part could still hold energy — not whether anyone believes it has been discharged.
The dc link capacitors inside every UPS module and every VFD on the mechanical plant are Article 320 equipment. They hold energy after the machine is isolated, which is precisely the condition 320.4(B)(3)(1) describes.
Note the article boundary carefully: the UPS electronics are Article 330, the battery is Article 360, the capacitors are Article 320, and if the ride-through is supercapacitor based it is Article 370. One machine, four articles.
Hearing protection is the practical change. If your capacitor banks clear 100 joules — most do — the risk assessment has to reach a conclusion about hearing protection, and most existing data center PPE matrices do not have a row for it.
The full Chapter 3 map →
Batteries and capacitors swapped. Here is everything else that moved.
Article 310 — dc thresholds →
The common dc baseline underneath this article.
Article 330 — UPS and drives →
The electronics half of the same machine.
NFPA 70E 2024 → 2027 transition training
A focused session on the Chapter 3 rebuild — including the capacitor thresholds most programs have never assessed.
See the schedule Bring it onsite