OT: Breaker Sizing For Control Panel

Lancie1

The 30 HP is to run a plactics trim grinder it is a pretty heavy duty application. The other motors will start in a sequence but it is just with contact intelocks no timers in the current circuit. We may use a PLC on it but we have not decided on that yet but if we do we will put time delays in.

The 30 HP is to run a plactics trim grinder it is a pretty heavy duty application. The other motors will start in a sequence but it is just with contact intelocks. . .

Click to expand...

From what I know now, I would go with a 100 Amp main breaker in the control panel, then fuses for each motor starter. Start the grinder first, with interlocks that allow the other motors to start sequentially.

There are exceptions in the NEC for "Group Installation" multiple-motor control panels where you can use one set of fuses (or breaker) if the motors are a related group and meet all the requirements of NEC 430.53(C). However, it will safer if you put fuses and motor overloads on each motor.

You can use the 125A bucket to feed the panel. If I remember correctly, that should be a #4AWG (don't have my Ugly's here). You can then put a 100A or even lower as the Main in the panel.

BUT.... There is a lot more to installing a power circuit than just getting the amp sized correctly.
You need to know the available fault current at the MCC and at the control panel to properly size the breaker. Just picking the cheapest 10K AIC breaker could easily get someone killed. The breaker needs to be sized above the available 3 phase short circuit current. You also need to take into consideration voltage drop from the MCC to the panel along with how the wiring will be installed.

As for sizing the breakers/fuses for each individual motor starter, you need to know the same thing. Most starters can only handle 5K of fault current. If there is more than that avaialble (and you need to know how much is available), then a set of current limiting fuses or a current limiting breaker is mandatory.

Also be aware that having a 125 Amp breaker feeding a 100 amp breaker does NOT guarantee co-ordination. In other words, a fault inside the panel could trip the MCC breaker before the panel main. You must perform a co-ordination study if that is important (since it is only feeding this panel, it is not important normally).

As a final step, you need to perform an Arc Flash study to determine what kind of incident energy is available and therefore what kind of PPE (cotton shirt, level 2 FR shirts, etc) is required to protect the life of anyone entering the cabinet.

Lastly, if you are interested in some advice, then I always install a lockable disconnect on the side of the panel before feeding the panel. For arc flash, it eliminates any voltage inside the panel when locking it out. I install all control transformer outside the panel to help with heat buildup. I've stopped using air conditioners in panels that house PLCs and simple control circuits, it isn't necessary since most can handle 140 degress F. If you will have controls that need to be accessed regularly, then consider 2 panels side by side (one for 480 VAC and 120 VAC and the other for the 24 VDC). This will go a long way to eliminating the arc flash concern.

Sorry to be long winded. I find many engineers and designers don't properly build a cabinet.

brucechase said:

Lastly, if you are interested in some advice, then I always install a lockable disconnect on the side of the panel before feeding the panel. For arc flash, it eliminates any voltage inside the panel when locking it out.

Click to expand...

brucechase said:

consider 2 panels side by side (one for 480 VAC and 120 VAC and the other for the 24 VDC). This will go a long way to eliminating the arc flash concern.

Click to expand...

brucechase said:

You can use the 125A bucket to feed the panel. If I remember correctly, that should be a #4AWG (don't have my Ugly's here). You can then put a 100A or even lower as the Main in the panel.

BUT.... There is a lot more to installing a power circuit than just getting the amp sized correctly.
You need to know the available fault current at the MCC and at the control panel to properly size the breaker. Just picking the cheapest 10K AIC breaker could easily get someone killed. The breaker needs to be sized above the available 3 phase short circuit current. You also need to take into consideration voltage drop from the MCC to the panel along with how the wiring will be installed.

As for sizing the breakers/fuses for each individual motor starter, you need to know the same thing. Most starters can only handle 5K of fault current. If there is more than that avaialble (and you need to know how much is available), then a set of current limiting fuses or a current limiting breaker is mandatory.

Also be aware that having a 125 Amp breaker feeding a 100 amp breaker does NOT guarantee co-ordination. In other words, a fault inside the panel could trip the MCC breaker before the panel main. You must perform a co-ordination study if that is important (since it is only feeding this panel, it is not important normally).

As a final step, you need to perform an Arc Flash study to determine what kind of incident energy is available and therefore what kind of PPE (cotton shirt, level 2 FR shirts, etc) is required to protect the life of anyone entering the cabinet.

Lastly, if you are interested in some advice, then I always install a lockable disconnect on the side of the panel before feeding the panel. For arc flash, it eliminates any voltage inside the panel when locking it out. I install all control transformer outside the panel to help with heat buildup. I've stopped using air conditioners in panels that house PLCs and simple control circuits, it isn't necessary since most can handle 140 degress F. If you will have controls that need to be accessed regularly, then consider 2 panels side by side (one for 480 VAC and 120 VAC and the other for the 24 VDC). This will go a long way to eliminating the arc flash concern.

Sorry to be long winded. I find many engineers and designers don't properly build a cabinet.

Click to expand...

BruceChase

Thank you for the tips. I will try to incorporate them all. As for the arc flash part this place is really not to that level. A few years back before I started here I think they had a outside firm come in to do a study and documentation but they only went to the MCC's and panel boards not to the machine control cabinet level.

From those drawings I have this information. The utility transformer is 24940 primary and 480 secondary and 2500.0 kva and it has z% is 5.600%. I don't really know what the z% is?

From the utility transformer to the switchgear is 60 feet of 500 awg/kcmil and qty /phase 12 and I assume that means 12 of these conductors for each phase?

From the switch gear breaker to the mcc the length is 100 feet of size 500 awg /kcmil qty/phase 2 and this switchgear breaker is 800.0 amps feding the mcc. The mcc does not have a main breaker it just connects to lugs but all buckets in that mcc are breakers and there is one 240/120 panel baord in the mcc.

As I said we are not to the machine level with this but if I am ging to buy something new I would like to get the correct parts. Using this info how would I calculate the interupt for the breaker?

Is the a way to oversize it and be safe? My run from the mcc to my new panel will be about 100 feet in rigid conduit.

On the drwing the utility transformer also shows this

Voltage 480v
SCA 3p 46768.9
X/R 3p 8.000

If the transformer is 2500 KVA what is this SCA 46768.9 number. Sorry if these are basic questiosn but I am just struggling trying to understand the are flash.

If I remember correctly, that should be a #4AWG. . .

Click to expand...

Is the a way to oversize it and be safe? My run from the mcc to my new panel will be about 100 feet in rigid conduit.

Click to expand...

Yes, but you do not need to oversize the wire. Use the conservative wire size recommended by NEC table 310.16, which would be #1 AWG 55-degree C wire, allowable ampacity 130 Amps for 3 current-carrying conductors in a raceway. Bruce's guess at #4 is too small. The next size down, #2, is rated for only 115 Amps, so would be too small for your 125 Amp breaker. You are allowed to de-rate the circuit breaker if it is not a 100% trip breaker (trips at 125 Amps instead of 80% of trip rating), but why take the risk? Your voltage drop for a 100-feet run of #1 AWG wire at 480 volts and a 100 Amp load would be 1.22%, so you will not need to worry about oversizing the wire.

You can spend a lot of time trying to calculate the short-circuit current, or you can read the AIC rating on the nameplate on the existing 125 Amp MCC breaker (probably in the range of 35 kA to 65 kA) and make sure that your new breaker has that rating, or larger. Also try to look up a trip curve for the old breaker, and make sure that your new 100 Amp control panel breaker will triap at all points on the current curve before the old 125 Amp breaker. However you will have to spend some time to find these trip curves for both breakers.

Ooops, of course Lancie1 is correct on the cable size. So much for memory.

The SCA number is the short circuit amps available. From what you've said, the utility provides almost 47KA. Running this through a quick calculator (that does NOT add in motor contribution), then at the switchgear you would have just under 45KA. At the MCC you would have about 33KA, and at the panel - assuming #1 AWG in conduit - you would have about 17KA.

NOTE - THESE NUMBERS ARE ROUGH ESTIMATES THAT DO NOT TAKE INTO CONSIDERATION MOTOR CONTRIBUTION OR THAT ALL LENGTHS ARE CORRECT.

If it were me, I would choose a 65KA breaker for the MCC and a 35KA breaker for the panel. You could do 65KA for both, but cable impedance would severely limit the current going to your panel. If you don't have a lot of large motors, then the motor contribution shouldn't be enough to increase the fault current much beyond the 17KA.

There are plenty of calculators out there to help do this. You need to understand what you are putting in for the numbers. I would suggest doing some reading on some of the websites about fault current calculations. There are some software packages that are available that your company should consider purchasing or hire a firm (like they did at the beginning) to help with some of this.

It is really sad that your company hasn't embraced arc flash safety.
Before you do too much, please read this article:

http://www.iaei.org/magazine/2012/03/arc-flash-a-survivors-perspective/

and watch these videos:

http://donniesaccident.com/

http://www.metacafe.com/watch/1555440/arc_flash_while_racking_a_breaker/


Then show them to your manager/boss and explain that you don't want to go home like that and neither should anyone else at your plant.

I am glad to hear that you want to do this right.

brucechase said:

Ooops, of course Lancie1 is correct on the cable size. So much for memory.

The SCA number is the short circuit amps available. From what you've said, the utility provides almost 47KA. Running this through a quick calculator (that does NOT add in motor contribution), then at the switchgear you would have just under 45KA. At the MCC you would have about 33KA, and at the panel - assuming #1 AWG in conduit - you would have about 17KA.

NOTE - THESE NUMBERS ARE ROUGH ESTIMATES THAT DO NOT TAKE INTO CONSIDERATION MOTOR CONTRIBUTION OR THAT ALL LENGTHS ARE CORRECT.

If it were me, I would choose a 65KA breaker for the MCC and a 35KA breaker for the panel. You could do 65KA for both, but cable impedance would severely limit the current going to your panel. If you don't have a lot of large motors, then the motor contribution shouldn't be enough to increase the fault current much beyond the 17KA.

There are plenty of calculators out there to help do this. You need to understand what you are putting in for the numbers. I would suggest doing some reading on some of the websites about fault current calculations. There are some software packages that are available that your company should consider purchasing or hire a firm (like they did at the beginning) to help with some of this.

It is really sad that your company hasn't embraced arc flash safety.
Before you do too much, please read this article:

http://www.iaei.org/magazine/2012/03/arc-flash-a-survivors-perspective/

and watch these videos:

http://donniesaccident.com/

http://www.metacafe.com/watch/1555440/arc_flash_while_racking_a_breaker/


Then show them to your manager/boss and explain that you don't want to go home like that and neither should anyone else at your plant.

I am glad to hear that you want to do this right.

Click to expand...

Can you elaborate on the part in red? I don't quite understand that. Do you mean that by both breakers being 65 KA that it will limit the current the circuit can handle for operational purposes? Or did you mean that 65ka for both is not necessary because the cable impedance will reduce the fault current to 17ka?

I went to a one day arc flash class when starting this job and I seem to remember that the control panel had to be rated and labeled the same as the device in the panel with the lowest KA rating?

Some of the panels here have the sccr labels also. Do you need sccr and arc flash labeling or one or the other?

Where does each one take precedence?

If it were me, I would choose a 65KA breaker for the MCC and a 35KA breaker for the panel.

Click to expand...

I understood that the 125 Amp MCC breaker is existing.

Also we already have a spare 125 amp breaker bucket in the MCC would this be okay to use?

Click to expand...

. . .all buckets in that mcc are breakers.

Click to expand...

My experience in replacing MCC breakers is that you can't afford to do it unless absolutely necessary. The MCC manufacturers assume that the present MCC owners are a captive audience and that they have a monopoly on replacement buckets. The price for replacement MCC devices is about 3 to 4 times more than a comparable new device in a new MCC! Usually it is more practical to find a used one, or simply limit the load to the exisiting breaker rating, and coordinate your downstream control-panel breaker with the exisiting MCC breaker.

These mcc units are about 15 years old but they are allen bradley and we can still get the buckets and breakers.

I don't know what the cost is but they have never had a issue with buying them so I can change it pretty easy. The 125 bucket is pretty new and was put in for a project that got canned I am told so it may be 65 ka.

Still getting them is not the problem. The question is whether your employer will stand still for highway robbery. The ones I worked for never would when they saw the quote for a replacement. I only remember a very few cases where the jacked-up price of a MCC replacement breaker or motor starter was allowed. Of course, if your company has money to burn or throw away (drilling equipment manufacturer?), then go for it!

I would either go out and read the label on the breaker (if the MCC is off) or dig out the contract on the MCC and look up the breaker specifications. It is best to know, rather than guess or assume or hope that it is rated for 65 kAmps interrupting current.

Circuit breaker have 3 main properties:
- Setting range in 80F1, 2.2-4A
- Electro magnetic tripping range, 50A (sometimes depends of In-Setting)
- Rated Short-Circuit Capacity ( Icn ):, 100kA

Main switches have also, see 41.5Q1:
- Nominal thermal current 125A
- Breaking capacity 640A (e.g. for motor start Current)
- Rated Short-Circuit Capacity ( Icn ):, 16kA (or nnkA/1s)