Supplimentary Circut Breaker Sizing

theColonel26

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How do you guys size supplementary breakers? Do you do it based on wire ampacity rating or based on what the max load of a circuit may be?


For example, if you have an 18AWG wire you may size the breaker for 7.5 Amps based on wire ampacity, but if it the devices it is supplying have a max load of 2 Amps you could size it for 2 Amps.


So how do you guys do it?


Yes, I know derating needs to be taken into account for wire rating, but that is not what I am asking about.
 
I size them according to the load.
Just think about if there is a malfunction in the device you are protecting, you would want the breaker/fuse to react as soon as possible.
Lots of times you run across devices that will have 14-16awg ran to them, but only have a load of 2-3 amps, I would not want 7-10 amps of wiggle room on that...
Here's a good example a machine programming port/receptacle. 14 awg ran to it 120Vac. GFCI breaker of 2 amps. (because only laptops should be plugged into it, not the operators 10 amp cooling fan or mag drill. Lol.
 
not the operators 10 amp cooling fan or mag drill. Lol.


Or the 55 gallon vacuums that was plugged into the programmer port, with the PLC wired to the same breaker. The line shut down as soon as the vacuum was switched on.



Now they are separate.
 
For a standard load you would size the breaker or fuse ate 125% up to the limits of the conductor in the case of a 2 amp load then the correct fuse would be 2.5 but that not everything you have to consider the type of load the amps available from the source and many other factors.
One thing I was taught a long time ago you can't protect the load you only isolate the damage as much as possible to the first faulted device
 
Originally posted by GaryS:

One thing I was taught a long time ago you can't protect the load you only isolate the damage as much as possible to the first faulted device

I absolutely agree with this. I have been in this gig for 30 years and can count on one hand the number of times a load device has survived tripping/blowing its overcurrent protection.

First of all, in the US you aren't supposed to use a supplemental protector to protect a wire. You need branch circuit protection for that. You can use supplemental protectors to help with coordination if you want but the wire size needs to stay large enough to handle the branch circuit current. Also, if you think you can keep a device alive with lower values of circuit protection you can try that as well.

After my branch circuit protection I size any other OCPD based on load. But I only do that in hopes of providing some degree of coordination.

Keith
 
For 480VAC circuits, I just use the Bussman SPD - since 99% of the time I am doing motor protection.

When it comes to supplimental protection on the 24VDC or 120VAC controls side, I typically protect the source voltage to it's max allowable (example: 5A power supply gets a 5A fuse)

For loads on PLC cards, I typically fuse to the max allowable rate.

Then, for individual protection, it depends entirely on device load. For instance, a 1A solenoid * 1.25 = 1.25A fuse/breaker.

When it comes to wire sizing, the important thing is to make sure the load won't catch the wire on fire. I don't see any issues whatsoever over-sizing as long as your circuit protective device is sized accordingly.

Also - any time it is possible, check the manufacturer's data sheet for fusing recommendations.
 
I installed a receptacle/port unit beside a panel for laptop use and powered it off the panel's 120 vac. Rated for only 5 amps. An operator plugged a floor buffer into it and they couldn't understand why the machine suddenly came to a stop when he started cleaning the floor. In spite of my caution label.
 
Or the 55 gallon vacuums that was plugged into the programmer port, with the PLC wired to the same breaker. The line shut down as soon as the vacuum was switched on.



Now they are separate.

Ah yes 120V PLC on the same breaker. That's interesting.
 
I absolutely agree with this. I have been in this gig for 30 years and can count on one hand the number of times a load device has survived tripping/blowing its overcurrent protection.

First of all, in the US you aren't supposed to use a supplemental protector to protect a wire. You need branch circuit protection for that. You can use supplemental protectors to help with coordination if you want but the wire size needs to stay large enough to handle the branch circuit current. Also, if you think you can keep a device alive with lower values of circuit protection you can try that as well.

After my branch circuit protection I size any other OCPD based on load. But I only do that in hopes of providing some degree of coordination.

Keith
Can you please give me a Code reference for this?


So you are saying if I have a 15A UL489 breaker on my incoming power and have a 14AWG off of that, that if I have a UL1077 3A breaker to futher distribute power after that that I have to have 14AWG after the 3A? WHAT??????



Would this also apply to a 20A 24V Power Supply?


Again code reference??????
 
You get two get out of jail free cards...sort of. NFPA 79 has several exemptions and adjustments for conductors that stay inside a control enclosure. You can kind of use this to your advantage. Also, control circuits are generally exempt from the branch circuit OCPD ruling. Once you are in the control circuit supplemental protectors are generally fine.

It all comes down to the definition of a branch circuit. I have always operated under the definition that a branch circuit is all the conductors and components from the branch circuit OCPD up to and including the load. I don't have a copy of the NEC with me (I'm on the road) but NFPA 79 7.2.1.3 states:

Supplementary overcurrent protective devices shall
not be used as a substitute for branch-circuit overcurrent protective
devices.

That NFPA 79 line was added to harmonize NFPA 79 with the NEC statement on supplementary protectors. So, if you accept the definition of a branch circuit from above and you accept NFPA 79 7.2.1.3 then it is inferred that only OCPD rated for branch circuit protection can be used to define the current in a branch circuit. UL1077 protectors are not rated as branch circuit OCPD.

Keep in mind that the branch circuit definitions only apply to power circuits. So your 24VDC stuff would all be exempt.

Keith
 
Also - if you are doing 24V controls - just do yourself a favor and either use class 2 power supplies or a circuit protective device that achieves class 2 ratings after a larger supply (such as Murrelektronik mico or Siemens selectivity modules)

With a class 2 power supply, you can even use non-UL components in your control circuits. Most devices specify to use a class 2 supply (such as HMI, power supplies, light curtains, etc)

You still have to size the wire correctly, but these devices make life pretty easy on the design front.
 
You get two get out of jail free cards...sort of. NFPA 79 has several exemptions and adjustments for conductors that stay inside a control enclosure. You can kind of use this to your advantage. Also, control circuits are generally exempt from the branch circuit OCPD ruling. Once you are in the control circuit supplemental protectors are generally fine.

It all comes down to the definition of a branch circuit. I have always operated under the definition that a branch circuit is all the conductors and components from the branch circuit OCPD up to and including the load. I don't have a copy of the NEC with me (I'm on the road) but NFPA 79 7.2.1.3 states:

Supplementary overcurrent protective devices shall
not be used as a substitute for branch-circuit overcurrent protective
devices.

That NFPA 79 line was added to harmonize NFPA 79 with the NEC statement on supplementary protectors. So, if you accept the definition of a branch circuit from above and you accept NFPA 79 7.2.1.3 then it is inferred that only OCPD rated for branch circuit protection can be used to define the current in a branch circuit. UL1077 protectors are not rated as branch circuit OCPD.

Keep in mind that the branch circuit definitions only apply to power circuits. So your 24VDC stuff would all be exempt.

Keith
What is the actual definition of a load? I'm running 6x 24V servos off of my 24VDC 20A Controls power supply. :D So is that controls or a load??????????? o_O Seems kind of gray to me.

Also - if you are doing 24V controls - just do yourself a favor and either use class 2 power supplies or a circuit protective device that achieves class 2 ratings after a larger supply (such as Murrelektronik mico or Siemens selectivity modules)

With a class 2 power supply, you can even use non-UL components in your control circuits. Most devices specify to use a class 2 supply (such as HMI, power supplies, light curtains, etc)

You still have to size the wire correctly, but these devices make life pretty easy on the design front.
We do, we almost exclusively use Phoenix Contact power supplies which are Class 2, or Omron (when dictated by customer).
 

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