Tap motor circuits without Dedicated Short Circuit Protection 2

[7carisfast]

We have a vendor who is supplying an integrated motor control cabinet on a piece of equipment. The motor is a 480V, 250HP motor. Down stream of the main CB, the vendor is tapping the power cables between the CB and the 250HP motor contactors to supply 2 small fan motors, a 3HP and a 7.5HP. All motors have dedicated O/Ls, but the smaller 3HP and 7.5HP motors do not have dedicated CBs and are relying on the fault protection of the main CB. I am not seeing how this is allowed under NEC Section 450.53. Can anyone offer their insight? I would prefer to have each small fan motor to have a dedicated CB, but the vendor is then implying there will delivery delays and cost implications. Both fans are required to be running for the main motor to run.

 

[7carisfast]

https://files.engineering.com/getfi...4-8afb-4a2d-a12e-599d5fa85533&file=Screen.jpg

 

[FacEngrPE]

What is FR1 and FR2?

 

[che12345]

Dear Mr. 7carisfast (Electrical)
"...Down stream of the main CB, the vendor is tapping the power cables between the CB and the 250HP motor contactors to supply 2 small fan motors, a 3HP and a 7.5HP. All motors have dedicated O/Ls, but the smaller 3HP and 7.5HP motors do not have dedicated CBs and are relying on the fault protection of the main CB.... vendor is then implying there will delivery delays and cost implications...."
1. I presumed that the (conductor size) supply to 2 small fan motors are [far too small] to be protected by the CB feeding the 250hp motor. Therefore, a suitable (lower) current rated [dedicated] MCB suitable for the small conductors is [mandatary], even though the motors are protected by O/Ls.
2. "... vendor is then implying there will delivery delays and cost implications..." This is commercial issue not technical. Put it in writing of your observation to the attention of the installation inspector and the commercial management. Let them make the final decision. Hands off, case closed.
Che Kuan Yau (Singapore)

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[djs]

Check out the NEC tap rules, in this probably 240.21(B)(1). For a discussion please see this.

 

[che12345]

Dear Mr. 7carisfast (Electrical)
Further to my post dated 5th instant; please look into the following:
1. a) Why the 250hp motor is (not shown) with [thermal overload protection]?
b) Refer to the recent proposal pertaining to the [SD connection] from (latest published catalogues).
2. Why all three motors are (not shown) being earthed?

@ Mr. FacEngrPE
FR1 and FR2 are [thermal overload] protection relays. Could be a typo error? Perhaps TR instead of FR.
Che Kuan Yau (Singapore)

 

[xnuke]

Are the motors also integrated with the equipment? If so, wouldn't NFPA 79 Standard for Electrical Machinery apply and not the NEC? I'm not familiar enough with NFPA 79, but it might have different requirements for group motor protection than the NEC.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[FacEngrPE]

I agree that

delivery delays and cost implication

is not an appropriate reason to accept a device that does not conform to required safety codes. Either it needs to ship compliant, or you will need to modify it on receipt to fix what is non compliant.

If the vendor is claiming he is compliant, he should be able to explain the method of compliance.

 

[jraef]

You have a couple of potential problems here. IF this was built to UL508A standards (which are only slightly different from the NEC rules), the size of the Main OCPD (QF or 3P4L, assuming they are fuses) is important, as is the size of the conductors of the main circuit. Your drawing does not give us enough information to make a call though.

As an "scientific wild ass guess", 250HP @ 480V would IMPLY that the conductors to the main circuit would need to be rated for NO LESS THAN 375.5A (302A for 250HP, x 1.25), so I'm going to guess 500kCMIL (380A). Per UL508A tap rules, the tap conductors must be NO LESS THAN 1/3 that size, so rated for 126A, making their MINIMUM size #1 AWG. In addition, the taps cannot be less than 1/10th the size of the Main OCPD, which I imagine is likely 500A? If so, or even if it is 600A or 800A, the previous rule for the 1/3 of the main conductors overrules that.

The NEC tap rules do not have the 1/3 of the main branch conductor rule, so it COULD be smaller, as in 50A rated if the Main OCPD is 500A, making them at BEST, #8 AWG as a minimum size.

In the unlikely even that they did use such large conductors for the small tap circuits, the panel as built would violate BOTH the UL 508A rules AND the NEC rules for tap conductors unless they added OCPDs for those taps in the form of fuses, breakers or MPCBs (which is what I would have used). So to FacEngPE's point, if "FR1 and FR2" are indeed MPCBs (Motor Protection Circuit Breakers, aka IEC Style Manual Motor Starters), then you are good to go. If they are standard Overload Relays, you are not, maybe for more reasons than this alone*.

*SCCR (Short Circuit Current Rating) could be a serious issue here too. Article 409 of the NEC requires that this panel have an SCCR listing on the front that is equal to or greater than the Available Fault Current of the system. You can get an SCCR via the UL508A listing process, but without that, you end up with a "courtesy" rating of just 5,000A Short Circuit Current. In an industrial facility using 480V, capable of feeding a 250HP motor like this, it is all but GUARANTEED that the AFC is going to be greater than 5kA and you will not be able to legally connect this panel. If they did get an SCCR listing, it will call for QF or 3P4L to be SPECIFIC devices, not just "whatever you can find". QF would have to be specific down to the manufacturer and model number of Circuit Breaker without the slightest deviation. If "3P4L" represents fuses, you can use equivalent CLASSES of fuse from different manufacturers, but the Current Limiting aspects must be the same or better than what the components were listed with.


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