45 KVA Transformer - Local Disconnect Rating

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scrub12

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I have a 70A C/B feeding a 60A-rated Non-Fused local disconnect switch for a 45 KVA Dry-Type Transformer rated at 54 FLA. Is the 60A rating of the local disconnect switch rating acceptable per NEC?
 
The "ampacity" of the switch is less than what is protecting it from overload (70A).

I would say 110.9, second sentence, "Equipment intended to interrupt current at other than fault levels shall have an interrupting rating at nominal circuit voltage at least equal to the current that must be interrupted."

A transformer can carry more than FLA and therefore could have a higher current than the disconnect's rating.
 
Thanks, All. It gets tricky with article 404.

404.13 (C) states Knife Switches rated less than 1,000V and under 1200A are considered "General Use Switches".

404.14 (A) (1) Alternating-Current General-Use Snap Switch: Resistive and inductive loads not exceeding the ampere rating of the switch at the voltage supplied.

In regards to 110.9, this part of the code is related to fault current so not sure it applies to this question. Yes, the 60A disconnect will be rated for for available fault current.
 
scrub12 said:
Thanks, All. It gets tricky with article 404.

404.13 (C) states Knife Switches rated less than 1,000V and under 1200A are considered "General Use Switches".

404.14 (A) (1) Alternating-Current General-Use Snap Switch: Resistive and inductive loads not exceeding the ampere rating of the switch at the voltage supplied.

In regards to 110.9, this part of the code is related to fault current so not sure it applies to this question. Yes, the 60A disconnect will be rated for for available fault current.
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IMO, 404 says nothing about protecting the switch at its rating with an OCPD.

Thinking about 110.9 some more, as long as your load calculation is 60 amps or less, I think you are good to go and there would be no violation
 
electrofelon said:
Thinking about 110.9 some more, as long as your load calculation is 60 amps or less, I think you are good to go and there would be no violation
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Ya... I was thinking about that also. 110.9 really only make sense if the load (on the high side) is greater than 60A.

I think it is odd that transformers don't have a minimum D/C rating.

I will say, that other sections, like motors, include verbiage that might require the D/C be rated for 115% of the motor current. Even through a transformer. (430.109). Since transformers and motors have similar inductive characteristics I would have assumed there would be a subsection within transformers that spoke to the necessary rating to interrupt or disconnect power to the line side.

I am of the personal opinion that installing a 60A disconnect with a 70A OCPD is an issue since the disconnect is not rated to operate at 70A continuously. The OCPD will not protect the switch from overload (and theoretically over temperature at terminals).

Now can I find a sections that says that? I don't know. You could always say it is an issue of 110.3(A)(5) & (7), 110.3(B), . But I hate quoting 110.3 for noncompliance issues.
 
Elect117 said:
I am of the personal opinion that installing a 60A disconnect with a 70A OCPD is an issue since the disconnect is not rated to operate at 70A continuously.
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Does that matter? In general a switch only needs to be rated for the load, not the circuit rating. E.g. it's fine to use a 15A snap switch on a 20A circuit. The reference in Article 404 was already given.

Cheers, Wayne
 
wwhitney said:
Does that matter? In general a switch only needs to be rated for the load, not the circuit rating. E.g. it's fine to use a 15A snap switch on a 20A circuit. The reference in Article 404 was already given.

Cheers, Wayne
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Before I answer your question, I should say that 404.14(A) is for snap switches and I don't know if a 60A handle switch is considered a snap switch. I always thought of light switches as snap switches because they snap between poles. Turning off a normal handle switch doesn't snap.

"
404.14 Rating and Use of Switches.
Switches shall be listed and used within their ratings. Switches of the types covered in 404.14(A) through (E) shall be limited to the control of loads as specified accordingly. Switches used to control cord-and-plug-connected loads shall be limited as covered in 404.14(F). "

A through D are snap switches. E is for dimmers or occupancy type switches and F is for cord and plug. So arguably only the first sentence of 404.14 would apply to a general use switch, "Switches shall be listed and used within their ratings.". Which is why I quoted 110.3(A)(5), & (7), 110.3(B).
~~~~~~~~

To your point, when you consider OCPD, wire sizing, disconnect rating, etc. for a transformer you can size things based on the load and not FLA. It raises a good point though. Does a switch need to be protected against overload?

That is why I quoted the sections 110.3 & 110.9. Saying that their ratings should not be exceeded. But a transformer's rating is more vague and so I defaulted to the rating of the OCPD and conductor. No necessarily the load (since there is no load in the OP, just FLA). If you just consider a transformer of 50kVA that only feeds a single light bulb, you could use a 15A switch. But I doubt it would survive the inrush. lol
 
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