circuit breaker for 400HP motor

[tbeek]

I am reviewing a vendor drawing and I think they undersized the circuit breaker for this project.
The CB is the 460Vac feeder CB and is 600 Amp.
It feeds the following:
1) RV Soft Starter for a 400HP 3P ac motor, FLA=432A, LR=6.28x, 1.15 SF load = 498 A, NEC 430.250=477 A
2) 30 Amp CB for a 15HP motor, 21 FLA
3) 3 Amp CB for a 0.75 HP motor, 1.6 FLA
4) 2 Amp CB for a 0.5 HP motor, 0.9 FLA
5) 6 Amp CB for a 3 KW resistive heater, 3.8 A
6) 4 Amp CB for a 480V/120V transformer that feeds a 200W heater and 150W of lighting

Am I right to tell them that the 600 A CB is too small?

 

[Jraef]

I am reviewing a vendor drawing and I think they undersized the circuit breaker for this project.
The CB is the 460Vac feeder CB and is 600 Amp.
It feeds the following:
1) RV Soft Starter for a 400HP 3P ac motor, FLA=432A, LR=6.28x, 1.15 SF load = 498 A, NEC 430.250=477 A
2) 30 Amp CB for a 15HP motor, 21 FLA
3) 3 Amp CB for a 0.75 HP motor, 1.6 FLA
4) 2 Amp CB for a 0.5 HP motor, 0.9 FLA
5) 6 Amp CB for a 3 KW resistive heater, 3.8 A
6) 4 Amp CB for a 480V/120V transformer that feeds a 200W heater and 150W of lighting

Am I right to tell them that the 600 A CB is too small?

Depends. Is the entire control panel assembly UL listed by them? If it is, you are done. In other words if it was designed and listed as a complete system, you do not need to apply NEC rules to the interior components. And by the way, the rules for protective devices are all for MAXIMUM sizes by the way, not minimums.

I happen to think that this is too small and will nuisance trip, but there is no rule against that. However there are rules regarding MINIMUM sizes of conductors. That might be worth pursuing given that in a lot of cases people go backward and size conductors based on breaker sizes instead of sizing conductors to the load and then PROTECTING them with the right breaker.

Also, given the 3A, 2A, 6A and 4A breaker sizes, those are likely those little "MCB" (Miniature Circuit Breakers) devices that are an off-shoot of the IEC breakers. Although you can now get UL489 listed versions of those from most suppliers, people are often unaware that those breakers have really low SCCR ratings, usually 5kA, never more than 10kA. So those will become the limiting devices in the overall SCCR rating you can have on that panel, as is now required by NEC Article 409 for a control panel. That means you will only be allowed to connect that panel to a power system that is capable of delivering no more than 10kA in available fault current. Good luck with that on a transformer big enough to handle a 400HP motor!

 

[tbeek]

No, the entire assembly is not UL listed by them.
The 30, 2, 3, 6, & 4 Amp CB's are all MMCCB (Miniature Molded Case Circuit Breakers UL489)
The 600 Amp circuit breaker has a 65 kA RMS fault rating UL/CSA.

 

[petersonra]

it is probably adequate considering they are using a soft start.

maybe the cost of the next size breaker made a difference.

I would still prefer a higher amp Cb.

 

[Jraef]

No, the entire assembly is not UL listed by them.
The 30, 2, 3, 6, & 4 Amp CB's are all MMCCB (Miniature Molded Case Circuit Breakers UL489)
The 600 Amp circuit breaker has a 65 kA RMS fault rating UL/CSA.

Ok, but like I said, those MCBs are NOT going to be rated any higher than 10kA and that is going to bring the entire panel SCCR rating down to that level, so take that into consideration. It doesn't matter that the RVSS breaker is 65kAIC, it's the lowest rated component in an untested unlisted panel that counts.

Good luck.

 

[jim dungar]

The biggest problem with protecting motors is the inrush current.
Things will probably work if the 600A breaker has an electronic trip with adjustable Short time and Instantaneous functions when paired with the soft start.

The next issue is the long time current, but most motors do not run at full loading, so marginal installations often get into trouble when they have one that does.

Jraef has already pointed out SCCR issues.
Sections 110.9 and 110.10 have been in the NEC for many decades, but these are still two of the most common 'violations' I find when doing power system assessments, even in new installations.

 

[Jraef]

Just thought of something else WRT those MCBs.

You have to make sure that your supply source is 480/277V Wye, those MCBs are NEVER rated to be used on anything but a Wye system, i.e. no more than 277V to ground. This is what is called a "slash rating" and is addressed in the NEC in 240.85. Chances are it's OK from that aspect if it's a new service, but you cannot ASSume.

I bring all this up because you are charged with reviewing this design, but the fact that the panel builder is NOT a UL panel shop and has used devices that are generally unacceptable for applications like this indicates that they do NOT know what they are doing, or they don't care. That should be a big red flag for you to tread carefully and review thoroughly before putting your name on it.

 

[tbeek]

Yes, the MMCB's are 480Y/277 VAC, with only 10 kA SC Interupt rating

 

[Jraef]

Yes, the MMCB's are 480Y/277 VAC, with only 10 kA SC Interupt rating

And is your power supply 480Y/277? If you have a 480V delta supply, you are done, it's a no-go. If it is Y, then the you still must deal with the AFC/SCCR issue. If you have only 10kA at the terminals of the panel, then you are fine on those issues. If you have more than 10kA then you must bring it down to 10kA or less, change the components to where everything is above whatever AFC you have, or have the panel engineered and listed at that higher level.

That would then leave you only with the conductor size question and the possibility of nuisance tripping on that 600A main breaker. Looking back on some old data sheets from when I built soft starters at a factory, we used 600A TM breakers on 400HP starters. But for us there were no other loads on that breaker, JUST the soft starter and CPT. You have roughly another 30A. If those other loads will only start AFTER the main motor is done ramping that should be fine. If everything starts at the same time it may be problematic.

 

[tbeek]

For the 400 HP motor they have chosen to run 4/0 conductors, two per phase.
Minimum ampacity required is 477 FLC (NEC-2008 Table 430.250)*1.25 = 596 Amps

The conductors are marked MTW and THW copper, 105 ?C.
They are in free air inside a NEMA 3R ventilated enclosure located outside.

Table 310.17 says 360 Amps for copper THW 75?C
0.82 correction factor for ambient 41-45 ?C = 295 A
Times 2 in parallel = 590 Amps

My concerns:
1) 590 Amps is only 124% of 477 (i.e. Not to code)
2) 210.19(A)(1) says "...before the application of any adjustment or correction factors." Should the ambient correction of 0.82 not be applied? In which case 2*360=720 > 596
3) Can anyone tell me the ampacity of 4/0 conductors for MTW (based on Table 13.5.1 in NFPA 79-2007) per 310.15(A)(1) FPN No. 2
4) The lower ampacity (for either THW or MTW) applies, per 310.15(A)(2), correct?
5) Table 310.17 ambient correction factors: are they for the average ambient temp, or the maximum ambient design temp?
6) Is there a deration for two cables next to each other?
7) 110.14(C)(1)(b)(2). Does this mean that even though the conductors are rated 105 ?C (which would allow them to carry more current) The code does not let you take credit for that. i.e. The code limits you to the 75 ?C ampacity listed in tables 310.16 -310.19

 

[tbeek]

For the 400 HP motor they have chosen to run 4/0 conductors, two per phase.

What I meant to say was "For the conductors between the 600 A Circuit breaker and the soft starter, and from the soft starter to the bypass contactor and back, they have chosen to run 4/0 conductors, two per phase."
The conductors from the soft starter to the 400 HP motor are not in their scope of supply.

 

[tbeek]

Another concern I had was that although there are times were the motor will start and run continously for 6-8 hours, usually it will be starting and stopping frequently. As many as 10 times per hour. Do they need to use Table 430.22(E) to size the conductors?

 

[Jraef]

Another concern I had was that although there are times were the motor will start and run continously for 6-8 hours, usually it will be starting and stopping frequently. As many as 10 times per hour. Do they need to use Table 430.22(E) to size the conductors?

Chances are that unless that motor is seriously de-rated, i.e. it is a 250HP load and a 400HP motor, 10 starts/hour is grossly exceeding the rating of that motor. Most large (above NEMA*) motors like that are good for 2-3 Starts/Hr, some as low as 1.5 or even 1. You had better check that out as well.

*NEMA MG-10 design guidelines for AC induction motors stops at 250HP, above that everyone is on their own. But a 250HP 1800RPM motor is rated for 3.7 starts/hour. A 200HP version would be 4/hr and a 150HP is 4.5/hr, so assuming a trend of fewer Starts/Hr as HP increases, there is virtually no likelihood that a 400HP is 10/hr. Probably 3 or less.

 

[tbeek]

Thanks for the general insight on the motor starts vs HP.
Correct, the motor would only be allowed 2 cold or 1 hot starts per hour, at maximum inertia.
In this case, it is ok though. The motor was designed with a copper bar rotor, and was specifically ok'ed for 12 equally spaced starts per hour with the given startup inertia (of only ~1% of max inertia). The soft starter is limited to 10 equally spaced starts per hour.

 

[Jraef]

...The soft starter is limited to 10 equally spaced starts per hour.

Doubtful. I understand that's what THEY may have said to you, but very few are really capable of that much abuse. It's more likely a salesman's promise to avoid losing the order. The general design criterea for soft starters is to assume that the soft starter will never have to take more abuse than the motor can take, i.e. the same NEMA MG-10 design guide.

What brand? What Current Limit setting? What's the Start Time? I used to work for a soft starter mfr, one of my job duties was competitive analysis. We tested the (bogus*) claims of amlost every brand out there, certainly all of the major names and many of the also-ran/bit players.

*Everyone in that industry lies, some just more than others...

By the way, ~ 1% of max. inertia? Seems like someone did factor in a de-rate then. Or is it because it's an unloaded start? You never said what kind of machine it is.

 

[tbeek]

It is an unloaded, low pressure, start of a recipricating compressor. When at full load, the discharge pressure of the recip is ~5000 psig. So there is a significant savings in power.

If at the max inertia that the motor can handle, the 100% voltage start time, with a load torque proportional to the square of the speed, would have been 31 seconds. Safe stall time 52 sec cold, 41 sec hot. Locked rotor amps are 628%.

With the actual unloaded start inertia, the start time with the soft starter limited to 400% is 2.5 seconds. (When limited to 300% the start time is 6.5 seconds.)

Back on the issue of conductor ampacity.
The conductor manufacturer's website lists 454 Amps for 105 ?C dry. With a footnote: The 105?C cable ampacities are adapted from ICEA P-54-440/NEMA WC51-86(R1991).
Can you use manufacturer's data, or do you need to use the table in NEC?

 

[Jraef]

...
Back on the issue of conductor ampacity.
The conductor manufacturer's website lists 454 Amps for 105 ?C dry. With a footnote: The 105?C cable ampacities are adapted from ICEA P-54-440/NEMA WC51-86(R1991).
Can you use manufacturer's data, or do you need to use the table in NEC?

There are slight differences between UL and the NEC. But if your panel is not UL listed, you are left with the NEC rules. It makes no difference what the mfr says (claims).

 

[tbeek]

I had them change the conductors. I would have been ok with the 4/0 if they would have used a wire from the 90 ?C column. They could not find a supplier of THW-2, so they went to 250 kcmil THW instead.

 

[weressl]

Chances are that unless that motor is seriously de-rated, i.e. it is a 250HP load and a 400HP motor, 10 starts/hour is grossly exceeding the rating of that motor. Most large (above NEMA*) motors like that are good for 2-3 Starts/Hr, some as low as 1.5 or even 1. You had better check that out as well.

*NEMA MG-10 design guidelines for AC induction motors stops at 250HP, above that everyone is on their own. But a 250HP 1800RPM motor is rated for 3.7 starts/hour. A 200HP version would be 4/hr and a 150HP is 4.5/hr, so assuming a trend of fewer Starts/Hr as HP increases, there is virtually no likelihood that a 400HP is 10/hr. Probably 3 or less.

RTD's with extended function MPR will take out the guesswork from the max number of safe starts, but 10 does not seem extensive.