Is it necessary to size small circuit panel mounted breakers to electronics per their FLA draw?

[bdn2004]

In the design of an electronic PLC cabinet there is to be a number of small circuit breakers. The circuit breakers will feed the I/O - solenoid valves, limits etc,power supplies, HMI screens, etc.
I noticed in a "go by" design they sized these circuit breakers at all different ratings: 2A , 4A, 10A etc apparantely to the load they feed.

The feeding circuit breakers into our panel are 20Amp 120VAC. My thought is to put in a row of 1P 10A circuit breakers. I don't see where this is a Code violation or something as it will protect the SIS wiring in the panel.

I just wondered if there is any advantage to sizing these per the load?

 

[itsmoked]

Well the point is often to limit the fault current to the specific loads in hopes of limiting damage during an event. Say the output of a power supply may short and there is 'hope' a correctly sized breaker will provide a shred of protection to the supply. The reality is it's probably not going to help. Fuses probably hold more protection is those situations.

Are you suggesting just using a bunch of breakers for individual load disconnection and no protection?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Fuses and breakers usually protect the system upstream of the devices.
Once a coil, solenoid or other device fails, it is too late. It's gone. The fuse or breaker is to protect the supply wiring.
Using the NEC as an example as the NEC probably does not apply to your panel; with some exceptions, breakers are generally sized at 125% of the rated load current. However, it is not required to use breakers smaller than 15 Amps.
In your case, select a breaker rating that will protect your panel wiring to the devices and protect your power supply.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[bdn2004]

one circuit breaker feeds a series of outputs 10 120vac solenoid for example.another feeds the ac to the dc power supply to the Ethernet switch. It's a means of disconnect as I see it. Sometimes these things get "substituted" or changed out, or perhaps a voltage swing? ... Reminds me once that someone told me to get my outdoor light bulbs 135v because the voltage is higher at night from the utility. It just seems an unnessary hassle to try to calculate out the current of present and unknown future load. And sizing so tight could set it up for a nuisance trip. Doesn't sound good "no protection" - but does it really? We don't seem to worry about it in our house. I like the other answer better!

 

[bdn2004]

And btw all the individual outputs to the solenoids are fused at 2 amp. These are the upstream device I'm speaking of. So it's cb - 16 fused blocks, cb - dc power supply, etc

 

[jraef]

First off, your comments lead me to believe you are in North America, so my response is geared that way.

From a "code" standpoint, the circuit breaker on a branch circuit is there mainly to protect the CONDUCTORS. So if you have 20A breakers and you run #12 conductors, the code is satisfied. The smallest field wiring you can run is 14ga, which would be protected by a 15A CB, so below that, the code no longer cares.

That doesn't mean that 20A or 15A breaker will protect the DEVICE, so that becomes a separate issue. You can use fuses, or you can use what are called "Supplemental Circuit Protectors", the little DIN rail mounted breakers you were probably referring to because they are the ones that come in all of those small ratings like 2A, 4A, 10A etc. Those are fine, and give you closer protection to the actual load, but they are also not listed as being a "Branch Over Current Protection Device" (BOCPD). So regardless of what size supplemental breaker you use, it needs to have another listed BOCPD somewhere ahead of it, either fuses or another breaker listed for that. On the other hand you can usually find a smaller size fuse that IS listed as a BOCPD (when used with the correct holder), eliminating that extra step.

Recently the distinction has become less clear in that a FEW of the people selling the little DIN rail breakers have taken them to UL to be listed AS a BOCPD by themselves, even at ratings below 15A. Generally they don't come in the same array of sizes and options as the Supplementary versions, but may suffice. You can distinguish those by looking for the info that it is "UL489" listed, because UL489 means it WILL be listed as a BOCPD. The UL number for the Supplementary Protectors is UL1077, so those are the ones that need another UL489 breaker ahead of them.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[Mbrooke]

I agree, but keep in mind 125% is for a continuous load. Anything running under 3 hours or not specifically listed in the NEC as continuous (car charger, storage type water heater, ect) can and is permitted to load a breaker to 100% of its listed handle rating.

 

[itsmoked]

Did I read somewhere you cannot use a UL1077 breaker in front of any motor period? Or does the same rule in that a UL489 somewhere ahead allows it.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

You can use them on motors, in fact before UL made the 1077 category just for them, they were allowed as UL508 listed manual motor starters for a while. But they did/do still need a 489 breaker or BOCP listed fuse ahead of them somewhere, which generally makes them pointless for single motors. Groups though, that's where they can make sense. For example one 15A 1P 489 listed breaker feeding 10 1A rated 1077 listed breakers for little fractional motors, perfectly OK.

Those come with different magnetic trip characteristics however, denoted by letters, usually A through D, and some of those are unsuitable for motors because they will nuisance trip on startup. That may be what was behind what you heard.

"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[itsmoked]

Thanks Jeff.

Often building a panel that is typically fed from a 15A breaker in a panel somewhere I like to put 'disconnect' breakers in a control panel to, say, protect a reagent pump that draws maybe an amp, so I add a little supplemental protection in the form of a 1077 breaker that might be 2A with a D curve. Seemed I then read something saying that was a no-no so that was my question. Thanks again.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

It's only a no-no if there is no 489 listed breaker ahead of it.

I've also seen it be an issue if someone was trying to use the lower rating of that breaker to be able to use smaller wire in the field (to try to get around the other rules / restriction on using smaller than 14 ga in the field). An inspector will reject it and people misinterpret the problem, thinking it was the issue of the breaker listing.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington