12AWG and 30A circuit breakers

[shansenbkx]

Looking for guidance from you and NEC that would help validate our position on an installation where we needed to install 30A breakers in a remote controlled power distribution panel utilizing AWG12 wire. With 20A breakers in place, we experienced multiple 'nuisance tripping' when the main panel was energized. The load is an electronic video display, with multiple power supplies, transformers, and LED modules. The tripping is attributed to very brief inrush current. The full load on any single circuit ranges from 8.6A to 11.4A. The service is 277/480 3 phase, and there are 150 circuits in the power panel.

Our nominal and even max loads are well within the load calculations for running on 12AWG wire, but the spike in current at startup in the first 5 or less milliseconds creates the need for the inrush capability of the 30A breaker. In terms of risk, if a short circuit condition were to take place, the spike in amps would still trip a 30A breaker, and in the case of a thermal or overload condition, the power supplies would go into shutdown. So there is little risk with this setup.

Note on Riser diagram "30A circuit breakers installed to account for inrush of power supplies at 277V to avoid nuisance tripping."

This load for our display installation is not a motor, but we liken the situation to motor overcurrent circuit protection. Circuit breakers for motor wiring may be upsized by up to 250% to account for inrush. It is possible to have 12AWG connected to a 60 amp breaker and still be code compliant. We understand the main reason why it is allowable is because motors are required to have thermal overload protection in addition to the circuit overcurrent protection. Similar to power supply wiring, if a short circuit occurs in the motor wiring, the spike in amps will still trip the upsized circuit breaker. And similar to how the power supplies in our display will shut down in the case of a thermal or overload condition, the overload protection on the motor will shut the motor down (not the circuit breaker.

We believe the case for allowing 12AWG with a 30A breaker in this specific situation is made stronger by reviewing conductor ampacities from the tables in NEC from 310.15 (2017 edition). The ampacity of 12 AWG THHN/THWN conductors in free air (which is what they are internal to our electronic display) is 40A.

Other examples of carve outs in the NEC that strengthen the case would be:

· Next higher standard overcurrent protection device rating allowed above the ampacity of the conductors being protected per 240.4 (B)
· A case could be made for using 12 AWG with a 30A breaker according to the Tap rules in 240.21.
· Carve outs for fixture wiring used with listed appliances in 240.5 B is also relevant in terms of safe wiring practices where smaller than typical wiring is allowed to be tapped to a larger circuit breaker than is allowed per 240.4 D 1-7.
· Table 240.4(G) is an entire list of applications where smaller conductors may be permitted.
· 310.15 (C) is another carve out where under engineering supervision conductor ampacities are allowed to be calculated differently.

We would appreciate your insight on how we can more clearly reference the NEC to assure the customer this installation is appropriate and safe. The customer is only thinking that 12AWG wire can connect to 20A breakers - no exceptions. We believe this to be an exception, but are looking to provide additional backup from NEC.

Thank you.

 

[jap]

Looking for guidance from you and NEC that would help validate our position on an installation where we needed to install 30A breakers in a remote controlled power distribution panel utilizing AWG12 wire. With 20A breakers in place, we experienced multiple 'nuisance tripping' when the main panel was energized. The load is an electronic video display, with multiple power supplies, transformers, and LED modules. The tripping is attributed to very brief inrush current. The full load on any single circuit ranges from 8.6A to 11.4A. The service is 277/480 3 phase, and there are 150 circuits in the power panel.

Our nominal and even max loads are well within the load calculations for running on 12AWG wire, but the spike in current at startup in the first 5 or less milliseconds creates the need for the inrush capability of the 30A breaker. In terms of risk, if a short circuit condition were to take place, the spike in amps would still trip a 30A breaker, and in the case of a thermal or overload condition, the power supplies would go into shutdown. So there is little risk with this setup.

Note on Riser diagram "30A circuit breakers installed to account for inrush of power supplies at 277V to avoid nuisance tripping."

This load for our display installation is not a motor, but we liken the situation to motor overcurrent circuit protection. Circuit breakers for motor wiring may be upsized by up to 250% to account for inrush. It is possible to have 12AWG connected to a 60 amp breaker and still be code compliant. We understand the main reason why it is allowable is because motors are required to have thermal overload protection in addition to the circuit overcurrent protection. Similar to power supply wiring, if a short circuit occurs in the motor wiring, the spike in amps will still trip the upsized circuit breaker. And similar to how the power supplies in our display will shut down in the case of a thermal or overload condition, the overload protection on the motor will shut the motor down (not the circuit breaker.

We believe the case for allowing 12AWG with a 30A breaker in this specific situation is made stronger by reviewing conductor ampacities from the tables in NEC from 310.15 (2017 edition). The ampacity of 12 AWG THHN/THWN conductors in free air (which is what they are internal to our electronic display) is 40A.

Other examples of carve outs in the NEC that strengthen the case would be:

· Next higher standard overcurrent protection device rating allowed above the ampacity of the conductors being protected per 240.4 (B)
· A case could be made for using 12 AWG with a 30A breaker according to the Tap rules in 240.21.
· Carve outs for fixture wiring used with listed appliances in 240.5 B is also relevant in terms of safe wiring practices where smaller than typical wiring is allowed to be tapped to a larger circuit breaker than is allowed per 240.4 D 1-7.
· Table 240.4(G) is an entire list of applications where smaller conductors may be permitted.
· 310.15 (C) is another carve out where under engineering supervision conductor ampacities are allowed to be calculated differently.

We would appreciate your insight on how we can more clearly reference the NEC to assure the customer this installation is appropriate and safe. The customer is only thinking that 12AWG wire can connect to 20A breakers - no exceptions. We believe this to be an exception, but are looking to provide additional backup from NEC.

Thank you.

Yes there are exceptions for smaller wire to be connected to a larger OCPD, but, my thought would be this is not one of them.

JAP>

 

[don_resqcapt19]

Sorry, it will get a big red tag from me...you can't make up your own rules, even under engineering supervision...if you need to use a 30 amp breaker, you need to use 30 amp conductors unless there is a specific provision in the NEC to permit otherwise.
Note that the ampacities in the Tables do not change the requirement for small conductors in 240.4(D)(5).

 

[LarryFine]

Note on Riser diagram "30A circuit breakers installed to account for inrush of power supplies at 277V to avoid nuisance tripping."

Welcome to the forum.

In my opinion, that note is the only chance you have, IF it's approved by the plan-reviewing authority.

As the other have said, however, this is not one of the conditions that allow greater OCP and/or lesser ampacity.

The code-compliant solution would be to reduce the load on each circuit by running more circuits.

Added: Another possibility is to stagger the start-ups to spread out the inrush current.

 

[don_resqcapt19]

Not all breaker brands have the same instantaneous trip points, maybe a different breaker with a higher instantaneous trip point would solve the problem.

 

[texie]

Not all breaker brands have the same instantaneous trip points, maybe a different breaker with a higher instantaneous trip point would solve the problem.

I'm thinking the same thing. Square D makes a QO single pole breaker in the 15 and 20 amp sizes for high inrush loads. Not sure if they have that for 277 volt though.

 

[tom baker]

I used a HID breaker for mercury vapor lights that have a high inrush, 20 A ckt. Solved the tripping when MV lights turned on.
150 circuits? Sounds like a very large sign.
And as the others said, this is not a application that an exception can be made

 

[kwired]

Sounds like you possibly building something that can or should be listed as a unit instead of being constructed to NEC requirements? If you did it that way NEC would only apply to the main power supply and not to the internal components. Can't tell you what rules might be to get it listed, but you might be able to do some of the things you were asking about.

 

[kwired]

I used a HID breaker for mercury vapor lights that have a high inrush, 20 A ckt. Solved the tripping when MV lights turned on.
150 circuits? Sounds like a very large sign.
And as the others said, this is not a application that an exception can be made

Kind of what I was thinking - probably needs to be listed instead of assembled to code like I was mentioning before.

 

[tortuga]

Sounds like you possibly building something that can or should be listed as a unit instead of being constructed to NEC requirements?

Article 409 Industrial control are not required to be listed per the plain jane NEC rules. OSHA and other AHJ rules often require them to be listed though.

 

[kwired]

Article 409 Industrial control are not required to be listed per the plain jane NEC rules. OSHA and other AHJ rules often require them to be listed though.

409 maybe gives you a few specific requirements, but if not covered in 409 then the rest of NEC applies to whatever you are constructing.

Having UL508 certification would let you build to the 508 standards which won't always match up to NEC.

Also was brought up that this sort of sounds like a sign being built, signs pretty much need to be listed also (600.3) but whatever is in listing standard for signs may not always match up with NEC when it comes to how you can make up items within the sign. NEC mostly just covers the supply to the sign and nothing within.

 

[gene6]

UL 508A is useless other than in the lower 48. The rest of the world no longer uses it. Plus the color coding in UL 508A is garbage.

 

[don_resqcapt19]

Sounds like you possibly building something that can or should be listed as a unit instead of being constructed to NEC requirements? If you did it that way NEC would only apply to the main power supply and not to the internal components. Can't tell you what rules might be to get it listed, but you might be able to do some of the things you were asking about.

But it sounds to me like the question is about the power supply to the equipment that would be covered by the NEC and not by the listing.

 

[tortuga]

The full load on any single circuit ranges from 8.6A to 11.4A. The service is 277/480 3 phase, and there are 150 circuits in the power panel.

Let me get this right, there are 150 277 Volt circuits in a single panel that can draw up to 11.4 amps each? For a video display?

 

[shansenbkx]

Let me get this right, there are 150 277 Volt circuits in a single panel that can draw up to 11.4 amps each? For a video display?

Correct. it is a large, LED Video display - something you would see at a NFL stadium or major College stadium. 55' x 120'.

 

[tortuga]

Wozers so what kinda panel is that? I was imagining 4 panels in a group or something.

 

[Strathead]

Correct. it is a large, LED Video display - something you would see at a NFL stadium or major College stadium. 55' x 120'.

What is the big deal with using #10 wire?

 

[winnie]

NEC won't permit 12 ga wire in a 30A breaker in this situation, and I bet with 150 circuits originating in a panel there will be derating issues as well.

Not to mention some 150*11*277*PF of heat being generated by the entire display.

My questions:
Assuming that the NEC applies in this situation, could a 30A breaker be combined with a 20A fuse to provide 20A circuit protection with sufficient inrush capacity?
Can different drivers with lower inrush be used?
What happens on the main when someone powers this system on?

Jon

 

[Tulsa Electrician]

I would like to see what type of breakers are in the panel
B,C,D sorry no A choice. If a test choose C, maybe even D.
I think it's 5 to 10 times on inrush.
Hope there is some kind of load control for start up.

At eight time all at once. Wow
Each breaker could see 88 amp for a few cycles.
150x88= 13,200
I'm Following this one.

 

[synchro]

Perhaps a reactor could be used to limit the inrush current. And if the resulting voltage drop is not acceptable, then a contactor bypassing the reactor could be activated when a sufficient time has elapsed after the system is energized to allow the load current to stabilize.