I searched through the NEC code book and can't find any sections dedicated to the protection of power supplies (that supply 24VDC to pressure transmitters, speed sensors, etc.). I don't expect to find anything as detailed as Art. 430 for motor protection, but feel like there should be some section addressing this.

Now the main components of a power supply are a control transformer and a rectifier with capacitors to reduce ripple in the DC output voltage. So should I be looking at protection rules for those components individually? Or should I just go with the 125% rule? Also, do I need to account for inrush currents due to the presence of capacitors?

  • \$\begingroup\$ Is this within an industrial control panel? (Art. 409 territory, also UL 508) \$\endgroup\$ Commented Aug 1, 2018 at 1:17
  • \$\begingroup\$ It is. I see that table 409.3 mentions several categories of equipment including capacitors, but doesn't mention power supplies. So looks like I might just have to spec out the breaker for the power supply based on the components inside it. Thank you. \$\endgroup\$
    – vasiqshair
    Commented Aug 1, 2018 at 11:54
  • \$\begingroup\$ I would check UL 508 then, although a garden-variety UL listed power supply should be OK to run on an ordinary branch circuit (15A, appropriate voltage for the supply), as it will have a supplementary protector (fuse) inside it already. \$\endgroup\$ Commented Aug 2, 2018 at 1:00

1 Answer 1


We use CEC up here, so our code won't be the same, but of note is the fact that our code is split into Part 1 and Part 2. Part 1 pertains to an electrician's work mostly, standards for the installation of electrical equipment. Part 2 pertains to manufacturing and design standards for devices to be installed. If your question is along the lines of Part 1, the installation and wiring of discrete physical devices, it should be in your NEC code book somewhere. Looking at it from this perspective, you have to consider power factor of devices and capacitors separate from them.

We have a section on capacitors starting with

26-200 Capacitors Exempted

The requirements of Rules 26-202 to 26-222 shall not apply to capacitors that form component parts of factory assembled electrical equipment nor to surge protective capacitors.

For capacitors external to a device and not surge protection:

26-208 Conductor Size for Capacitors

(1) The ampacity of capacitor feeder circuits and branch circuits shall be not less than 135% of the rated current of the capacitor.

(2) Where a branch circuit supplies two or more capacitors, the overcurrent device protecting the conductors of the branch circuit shall be considered as protecting the taps made thereto to supply single capacitors, provided that

(a) the tap is not more than 7.5m long; and

(b) its conductors comply with Subrule(1) and also have an ampacity not less than one-third that of the branch circuit conductors from which they are supplied.

Code goes on to explain selection and rating of disconnecting means for capacitor feeders and branch circuits. I can provide more, but obviously it's the wrong code. Just hoping the terminology might help you. For us, section 26 is "Installation of Electrical Equipment".

If you're talking about a 1 piece AC to DC power supply that just happens to have capacitors in it, it is likely already power factor corrected to the degree it needs to be and branch circuit breakers can be sized based on it's regular AC ratings. The reason separate capacitors are added to motors in electrical is for power factor correction, and the power factor is the reason the conductors between those capacitors and the inductive load have to be oversized. Equipment with substantially low power factor should say so in it's documentation. I think with a power supply they probably take advantage of the necessary presence of inductors and capacitors to make sure they stay within .9 to 1 power factor range, and for this to be a substantial concern in the first place, the power supply would have to be drawing quite a considerable current. If this is a Part 1, question, bear in mind specifically that the breaker protects the wire, and the wire is sized according to the need of the circuit.

If you're asking more about a breaker to be part of that device, that would fall under Part 2 of our code and is likely similarly split up in your country. Hope this helps.


The golden rule for circuit breaker setpoint performance is that it should be set as low as practicable, this plays out to mean that you want to have it as low as it can be without causing nuisance tripping. With a known load this can be quite precise, and note that specialized fuses like slow blow types exist for the purpose of having a circuit interruptor with a lower setpoint, despite momentary transients that may exceed that. These can do things like taking the inductive current spike from a motor and not blowing, despite having a setpoint closer to the motor's much lower running current.

So option 1 if you have it and you don't need to be absolutely sure of relevant design codes first, is to experimentally measure the max load of your project, take into account any other factors, like variable wire length, etc and decide on a type and value from there. If the maximum load is empirically known, you can likely select a lower value.

If you have to guess, I would order one at most 10% higher than the device's rated current.

If this is for commercial purposes, keep hunting until you find the relevant code rule.

In electronic circuits like the power supply you have, wire(trace) size and circuit protection I believe would be measured, simulated or calculated.

  • \$\begingroup\$ How would you size a breaker for a power supply rated for 110V, 1.8A? The 125% rule and one size up yields a 3A breaker. Is that how you would size it? \$\endgroup\$
    – vasiqshair
    Commented Jul 31, 2018 at 22:36
  • \$\begingroup\$ Is the breaker a part of the device or no? Very important distinction. For a branch circuit breaker, you'd end up with a 15A panel breaker, for relays and specialized breakers, lower, but usually smaller breakers are part of a device. It's also important to make the distinction between work and a personal project. If you just need a rule of thumb for a personal project that you can physically experiment with things are a bit different. If it's for work though, it's a question of finding the relevant code for your region. \$\endgroup\$
    – K H
    Commented Jul 31, 2018 at 22:43
  • \$\begingroup\$ Agreed. The breaker is not part of a device and looks like this: ab.rockwellautomation.com/Circuit-and-Load-Protection/… The breaker is fed from the secondary of a transformer. Downstream from the breaker you have the AC-DC power supply which powers a pressure transmitter. \$\endgroup\$
    – vasiqshair
    Commented Jul 31, 2018 at 22:50
  • \$\begingroup\$ Ok up here those would be "lowest practicable" and you shouldn't have to derate for power factor but you should derate by 80% if they are not rated for continuous duty. If you can't measure your max load current, use the rated current of the device. That said, this sounds like you're doing it for work, so it's quite likely you do need to track down any relevant code rules. \$\endgroup\$
    – K H
    Commented Jul 31, 2018 at 23:06

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