Under sizing a breaker for CNC equipment

[I^2R]

Hi,
Have a customer adding four 5-axis CNC machines to their facility. Nameplate on the equipment is 100A at 480V. They have existing spare 70A & 80A breakers that they are adamant they want us to use as the new 100A breakers are $600+ each. Conductors will be sized to the FLA of the nameplate.
From experience, we know that the machines never draw more than 40A as the parts the customer is in the business of making barely tax the machine (they already have a dozen of more).
I have no fear that the breakers will trip, or that there is any safety concern, but don't want to have a code violation. Worst case scenario is a nuisance trip (and possibly crashing a tool).

What specific code article would this be a violation of? Most everything motor related talks about sizing the conductors to the load and the OCP being sized to protect the conductor, not the possible FLA. I would like to be able to stand on a solid code reference should I refuse to do this.

Thanks.

 

[ActionDave]

There is no code section that says "using a smaller breaker on larger wire is okay". The code is permissive and as long as the conductors are adequately protected your fine. I would do it.

There is a small chance you would run afoul of 250.122(B) but that should be easy to ignore or overcome.

 

[iwire]

In my opinion as far as the code you must size the circuit based on the labeling not what the customer is sure will be fine.

220.14(A) Specific Appliances or Loads. An outlet for a specific appliance or other load not covered in 220.14(B) through (L) shall be calculated based on the ampere rating of the appliance or load served.

 

[ptonsparky]

Install the properly sized wires for the machine FLA and as long as the breaker lugs are large enough walk away. Send the bill and if you are worried include note that the breaker used was at customers request.

 

[ActionDave]

In my opinion as far as the code you must size the circuit based on the labeling not what the customer is sure will be fine.

Art. 220 is about branch circuit calculations. The OP said he is sizing the conductors based on the nameplate FLA.

 

[I^2R]

Install the properly sized wires for the machine FLA and as long as the breaker lugs are large enough walk away. Send the bill and if you are worried include note that the breaker used was at customers request.

This is pretty much the path we are going to take.

 

[david luchini]

Art. 220 is about branch circuit calculations. The OP said he is sizing the conductors based on the nameplate FLA.

I think Bob was pointing out that a smaller breaker would violate 210.20(A) based on the load calculated in 220.14(A).

 

[I^2R]

I think Bob was pointing out that a smaller breaker would violate 210.20(A) based on the load calculated in 220.14(A).

My feeling is that the NEC does a poor job of dealing with (doesn't deal with) "load factors" in industrial applications. There is a lot of equipment that does not operate anywhere near its nameplate, depending on it's usage, but the NEC provides no way for us to take that into account. I'm not so much talking about sizing a single branch circuit , but for sizing feeders and services, etc. We have a lot of customers with vastly over sized distribution and services that were sized by following the NEC.

 

[ActionDave]

I think Bob was pointing out that a smaller breaker would violate 210.20(A) based on the load calculated in 220.14(A).

Is a CNC machine a continuous load?

Continuous Load. A load where the maximum current is
expected to continue for 3 hours or more.

 

[iwire]

Is a CNC machine a continuous load?

Not relevant, we gave to provide a breaker at least as large as the FLA

 

[ActionDave]

Not relevant, we gave to provide a breaker at least as large as the FLA

I see in 210.19 that the conductors have to be sized for the max load, but I'm looking all over 210.20 and I'm not seeing where I have to use the biggest breaker allowed.

 

[I^2R]

I see in 210.19 that the conductors have to be sized for the max load, but I'm looking all over 210.20 and I'm not seeing where I have to use the biggest breaker allowed.

This was the reason for the OP. Easy for me to see where the code directs you to provide the proper sized conductor for a given FLA, and where it requires a MAXIMUM ocp for a given conductor, but I'm missing the language about under-sizing the OCP.

Considering the NFPA is safety focused, an under-sized OCP is not really a hazard, just a nuisance.

 

[iwire]

I see in 210.19 that the conductors have to be sized for the max load, but I'm looking all over 210.20 and I'm not seeing where I have to use the biggest breaker allowed.

You have to use a breaker that meets or exceeds the circuit load and when the equipment is labeled with the current it is that current that we must use.

Is it your view we could use a 15 amp breaker for this load and tell the customer 'good luck'? :huh:

 

[iwire]

This was the reason for the OP. Easy for me to see where the code directs you to provide the proper sized conductor for a given FLA, and where it requires a MAXIMUM ocp for a given conductor, but I'm missing the language about under-sizing the OCP.

How about this?

ARTICLE 670
Industrial Machinery

670.1 Scope. This article covers the definition of, the
nameplate data for, and the size and overcurrent protection
of supply conductors to industrial machinery.

670.4 Supply Conductors and Overcurrent Protection.
(A) Size. The size of the supply conductor shal1 be such as
to have an ampacity not less than 125 percent of the fullload
current rating of all resistance heating loads plus
125 percent of the full-load current rating of the highest
rated motor plus the sum of the full-load current ratings of
all other connected motors and apparatus, based on their
duty cycle, that may be in operation at the same time.


(C) Overcurrent Protection. Where furnished as part of
the machine, overcurrent protection for each supply circuit
shall consist of a single circuit breaker or set of fuses, the
machine shall bear the marking required in 670.3, and the
supply conductors shall be considered either as feeders or
as taps as covered by 240.21.

The rating or setting of the overcurrent protective device
for the circuit supplying the machine shall not be
greater than the sum of the largest rating or setting of the
branch-circuit short-circuit and ground-fault protective device
provided with the machine, plus 125 percent of the
full--load current rating of all resistance heating loads, plus
the sum of the full-load currents of all other motors and
apparatus that could be in operation at the same time.
Exception: Where one or more instantaneous trip circuit
breakers or motor short-circuit protectors are lIsedfor motor
branch-circuit short-circuit and ground-fault protection
as permitted by 430.52(C), the procedllre specified in
670.4(C) for determining the maximum rating of the protective
device for the circllit supplying the machine shall
apply ~vith the following provision: For the PllllJose of the
calculation, each instantalleous trip circuit breaker or motor
short-circuit protector shall be assumed to have a rating
not exceeding the I1wxinmm percentage of motor fitllload
current permitted by Table 430.52 for the type of
machine supply circuit protective device ernployed.


Where no branch-circuit short-circuit and ground-fault
protective device is provided with the machine, the rating
or setting of the overcurrent protective device shall be
based on 430.52 and 430.53, as applicable.

Considering the NFPA is safety focused, an under-sized OCP is not really a hazard, just a nuisance.

Yet still for fixed in place loads the NEC does not allow an 'undersized' OCP.

 

[wwhitney]

Yet still for fixed in place loads the NEC does not allow an 'undersized' OCP.

The article you quoted does not actually say that. It talks only about the minimum ampacity of the conductors, not the minimum OCP.

Cheers, Wayne

 

[iwire]

The article you quoted does not actually say that. It talks only about the minimum ampacity of the conductors, not the minimum OCP.

Cheers, Wayne

Slow down my friend.

 

[I^2R]

How about this?


670.4 Supply Conductors and Overcurrent Protection.
(A) Size. The size of the supply conductor shal1 be such as
to have an ampacity not less than 125 percent of the fullload <snip>


The rating or setting of the overcurrent protective device
for the circuit supplying the machine shall not be
greater than the sum of the largest rating or setting of the
branch-circuit short-circuit and ground-fault protective device
provided with the machine, plus 125 <snip>

Don't get me wrong, I don't think it's a great idea myself. Customer just wants me to show him why they cant use the existing breakers per the NEC, and accept the consequence of a potential nuisance trip.

I still don"t see where anything you referenced prohibits an undersized OCP on the external circuit feeding the machine.

 

[wwhitney]

Slow down my friend.

Did my eyes deceive me, or did you edit your post to include more of Article 670 between my first reading it and the above post?

Anyway, I don't see anything in 670.4(C) or in a quick skimming of 430.52 and 430.53 that gives minimum OCP values, just maximum OCP values. If you find a section that specifies a minimum OCP value, could you post that section directly?

Cheers, Wayne

 

[iwire]

Don't get me wrong, I don't think it's a great idea myself. Customer just wants me to show him why they cant use the existing breakers per the NEC, and accept the consequence of a potential nuisance trip.

I still don"t see where anything you referenced prohibits an undersized OCP on the external circuit feeding the machine.

Did you see this?

Where no branch-circuit short-circuit and ground-fault
protective device is provided with the machine, the rating
or setting of the overcurrent protective device shall be
based on 430.52 and 430.53, as applicable.

Do what you want but to me it is a violation.

 

[iwire]

Did my eyes deceive me, or did you edit your post to include more of Article 670 between my first reading it and the above post?

Yes, I was still working on my post when you responded. That was why I suggested you slow down.

Anyway, I don't see anything in 670.4(C) or in a quick skimming of 430.52 and 430.53 that gives minimum OCP values, just maximum OCP values. If you find a section that specifies a minimum OCP value, could you post that section directly?

Cheers, Wayne

Nah, I will accept defeat over searching.

Not something I would do, at the least is likely a 110.3(B) violation.