Buzzer and bell transformer

W

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
Electromatic said:
250.30(A)(4) Exception 3
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Thank you! At least for the 2020 NEC, it's 250.30(A)(5) Exception 3.

So that answers the OP's questions (1) and (2): for a voluntarily grounded Class 1, 2, or 3 transformer secondary of at most 1000VA, the GEC may be omitted per the above, and the system bonding jumper can just be sized as 250.30(A)(1) Exception 3, generally just the EGC of the circuit supplying the transformer.

As to question (3) on OCPD, there's nothing directly in Article 240 that lets you omit it, but 240.3 does refer you to Article 725, which can amend Article 240. I expect the allowance is in Article 725 somewhere, although it may only apply to Class 2 and Class 3 sources. Any pointers?

Cheers, Wayne
 
Besoeker3

Besoeker3

Senior Member
Location
UK
Occupation
Retired Electrical Engineer
Very much simpler for me - batteries. They have lasted four years so far.
 
tortuga

tortuga

Code Historian
Location
Oregon
Occupation
Electrical Design
wwhitney said:
Thank you! At least for the 2020 NEC, it's 250.30(A)(5) Exception 3.

So that answers the OP's questions (1) and (2): for a voluntarily grounded Class 1, 2, or 3 transformer secondary of at most 1000VA, the GEC may be omitted per the above, and the system bonding jumper can just be sized as 250.30(A)(1) Exception 3, generally just the EGC of the circuit supplying the transformer.

As to question (3) on OCPD, there's nothing directly in Article 240 that lets you omit it, but 240.3 does refer you to Article 725, which can amend Article 240. I expect the allowance is in Article 725 somewhere, although it may only apply to Class 2 and Class 3 sources. Any pointers?

Cheers, Wayne
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I typically run 18AWG for 24 volt stuff like that see Table 11(A) for Inherently Limited Power Source Overcurrent Protection is Not Required
otherwise for 24 volts a 4A fuse would suffice. Also 240.4(D)(1)
 
T

texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
As others have correctly noted, such as the OP's situation, the secondary of most Class 2 24 volt xformers do not need to be grounded. But it is worth noting that there are many cases where it is required such as HVAC equipment that is supplied at 480 volt, 480 volt pump control panels, etc. In these cases the code requires the 24 volt control secondary to be grounded. This does not necessarily mean they need a GES.
 
don_resqcapt19

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
wwhitney said:
But 250.20(A) allows it to be a grounded system, and the schematic posted shows a grounded secondary.

250.30(A)(1) Exception 3 then provides relief on the System Bonding Jumper size in this application. Does anything provide relief for a GEC connection for such an installation?

Cheers, Wayne
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details details details :D

250.30(A)(5) Exception 3 addresses the GEC.


However if you leave it an ungrounded system as most are, I don't see anything that says a grounding electrode system and grounding electrode conductor is not required.
 
tortuga

tortuga

Code Historian
Location
Oregon
Occupation
Electrical Design
don_resqcapt19 said:
However if you leave it an ungrounded system as most are, I don't see anything that says a grounding electrode system and grounding electrode conductor is not required.
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I hinted at that in post 17, 251.121 Exception permits a wire type branch circuit equipment ground to be the GEC.
 
don_resqcapt19

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
tortuga said:
I hinted at that in post 17, 251.121 Exception permits a wire type branch circuit equipment ground to be the GEC.
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but does not change the minimum permitted size of the GEC..EGC would have to be 8 AWG and follow all of the other rules for GECs.
 
W

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
don_resqcapt19 said:
However if you leave it an ungrounded system as most are, I don't see anything that says a grounding electrode system and grounding electrode conductor is not required.
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For ungrounded SDSs, 250.30(B)(1) says "A grounding electrode conductor . . . shall be used . . . as specified in 250.30(A)(5) or (6), as applicable." And 230.30(A)(5) has Exception 3 for "a system that supplies a Class 1, Class 2, or Class 3 circuit and is derived from a transformer rated not more than 1000 volt-amperes." So the Exception applies to ungrounded SDSs, by direct reference.

Cheers, Wayne
 
T

texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
don_resqcapt19 said:
details details details :D

250.30(A)(5) Exception 3 addresses the GEC.


However if you leave it an ungrounded system as most are, I don't see anything that says a grounding electrode system and grounding electrode conductor is not required.
Click to expand...
Well gee, you have raised an interesting question for sure. But yet we know that the typical ungrounded 24 volt control secondary never has a GES. Is this an oversight in the code that has never been caught? It seems odd that it is clear for grounded control circuits derived from an xformer but not for the more typical ungrounded control circuit xformer. Gonna need some research for sure.
 
L

letgomywago

Senior Member
Location
Washington state and Oregon coast
Occupation
residential electrician
texie said:
Well gee, you have raised an interesting question for sure. But yet we know that the typical ungrounded 24 volt control secondary never has a GES. Is this an oversight in the code that has never been caught? It seems odd that it is clear for grounded control circuits derived from an xformer but not for the more typical ungrounded control circuit xformer. Gonna need some research for sure.
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I think it has to do with it being a class 1,2, or 3 transformer but I'm not sure.
 
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