Conflicting AHJ requirement regarding NEC 240.21 (C)

[clsarmiento]

I have a situation wherein a 50KVA, 240/120volt, single phase, 3-wire step down transformer is fed from a 480volt meter/service disconnect.
The secondary of this transformer feeds an automatic transfer switch (ATS), which in turn feeds a panelboard with 200amp MCB. The step down transformer, ATS and panelboard are all in the same line up and within sight of each other. Distances are the following: From service disconnect to ATS = 8feet; from ATS to panelboard = 5feet. One jurisdiction did not require overcurrent protection on the secondary side of the transformer ahead of the ATS; another jurisdiction is requiring an OCP ahead of the ATS in compliance with NEC 240.21 requirements. What is the correct interpretation of NEC 240.21 (C) on the foregoing situation? Thanks.

 

[jaggedben]

Well there's this:

Single-phase (other than 2-wire) and multiphase (other
than delta-delta, 3-wire) transformer secondary
conductors are not considered to be protected by the
primary overcurrent protective device.

...which makes me think the AHJ requiring the additional overcurrent protection is correct.

 

[Carultch]

Well there's this:



...which makes me think the AHJ requiring the additional overcurrent protection is correct.

The way to think about this, is whether fault currents are guaranteed to line up winding-to-winding, that you can have the primary OCPD protect the secondary conductors by proxy. The primary OCPD extends protection to the secondary, per the way current scales by the transformer voltage ratio. So on a single phase, 240V to 120V transformer, a 50A primary OCPD acts as a 100A secondary OCPD. The secondary is treated as an extension of the primary circuit, except at a different mix of current and voltage. This is ONLY the case when currents and fault currents line up directly across transformer winding pairs. Single phase 2-wire, and Delta-to-Delta 3-wire.

If you have a more complex situation, such as any WYE system, or any system with a center tap, the fault current on one phase of the secondary can go "unnoticed" by the primary OCPD, if it divides up multiple two of the phases of the primary.

 

[mopowr steve]

I have a situation wherein a 50KVA, 240/120volt, single phase, 3-wire step down transformer is fed from a 480volt meter/service disconnect.
The secondary of this transformer feeds an automatic transfer switch (ATS), which in turn feeds a panelboard with 200amp MCB. The step down transformer, ATS and panelboard are all in the same line up and within sight of each other. Distances are the following: From service disconnect to ATS = 8feet; from ATS to panelboard = 5feet.

Did you mean transformer to ATS is 8' ?

Would seem to me that you are within 25' (or within sight ) and that you could be correct as long as your panel has a main OCPD.
If not, if the ATS has a main I would not see a problem either.

 

[electrofelon]

Well there's this:



...which makes me think the AHJ requiring the additional overcurrent protection is correct.

But the secondary conductors may be protected as is, depending on if the installation meets 240.21(C). My interpretation is the question is whether the switch needs some sort of protection. I am not going to go through 404, but me recollection is there is no requirement that a switch be protected at its rating

 

[texie]

The way to think about this, is whether fault currents are guaranteed to line up winding-to-winding, that you can have the primary OCPD protect the secondary conductors by proxy. The primary OCPD extends protection to the secondary, per the way current scales by the transformer voltage ratio. So on a single phase, 240V to 120V transformer, a 50A primary OCPD acts as a 100A secondary OCPD. The secondary is treated as an extension of the primary circuit, except at a different mix of current and voltage. This is ONLY the case when currents and fault currents line up directly across transformer winding pairs. Single phase 2-wire, and Delta-to-Delta 3-wire.

If you have a more complex situation, such as any WYE system, or any system with a center tap, the fault current on one phase of the secondary can go "unnoticed" by the primary OCPD, if it divides up multiple two of the phases of the primary.

Yep, I think you summed it up well. I would add though that it is not just a fault that is an issue. You could severely overload an install such as this before the primary OCPD ever saw it. Hence the rule.

 

[Carultch]

Yep, I think you summed it up well. I would add though that it is not just a fault that is an issue. You could severely overload an install such as this before the primary OCPD ever saw it. Hence the rule.

Good point. I guess by "fault", I simply mean the general case of any issue that is supposed to cause the OCPD to trip.

 

[jaggedben]

But the secondary conductors may be protected as is, depending on if the installation meets 240.21(C).

The text I quoted is in 240.21(C)(1).

 

[electrofelon]

The text I quoted is in 240.21(C)(1).

But there are multiple ways to protect the secondary conductors, even if they cant be or are not protected by the primary OCPD. We dont have enough information to tell if the installation meets 240.21(C) 3,4, or 6. I take the transformer secondary conductor rules to be rather straightforward so thats why I was assuming the presence of a switch in the secondary conductors that was confusing things.

 

[jaggedben]

But there are multiple ways to protect the secondary conductors, even if they cant be or are not protected by the primary OCPD. We dont have enough information to tell if the installation meets 240.21(C) 3,4, or 6. I take the transformer secondary conductor rules to be rather straightforward so thats why I was assuming the presence of a switch in the secondary conductors that was confusing things.

Admittedly I've never dealt with AHJ enforcement (or lack thereof) regarding any of these provisions. I'm just reading the code (like they do :roll.

With that said, it's not immediately clear to me from the language why 240.21(C)(1) doesn't cover all such installations, while (2) through (6) covers particular instances which all seem to be mutually exclusive. Considering the nature of the OPs question, maybe the answer is that the section is open enough to interpretation that the AHJ requiring additional protection has a reasonable opinion.

 

[electrofelon]

Admittedly I've never dealt with AHJ enforcement (or lack thereof) regarding any of these provisions. I'm just reading the code (like they do :roll.

With that said, it's not immediately clear to me from the language why 240.21(C)(1) doesn't cover all such installations, while (2) through (6) covers particular instances which all seem to be mutually exclusive. Considering the nature of the OPs question, maybe the answer is that the section is open enough to interpretation that the AHJ requiring additional protection has a reasonable opinion.

240.21(C)(1) would apply IF you have the requisite type of transformer AND IF the primary protection, after "correcting" for the turns ratio, does not exceed the ampacity of the secondary conductors. If you dont meet all of the requirements to use 240.21(C)(1), then you need to provide protection for the secondary conductors per one of 240.21(C) 2 thru 6.

 

[Smart $]

240.21(C)(1) would apply IF you have the requisite type of transformer AND IF the primary protection, after "correcting" for the turns ratio, does not exceed the ampacity of the secondary conductors. If you dont meet all of the requirements to use 240.21(C)(1), then you need to provide protection for the secondary conductors per one of 240.21(C) 2 thru 6.

The secondary is 3-wire single phase, so the primary OCPD does not protect the secondary conductors under 240.21(C)(1).

In all cases of 240.21(C)(2) through (6), the first piece of equipment must be a OCPD or a disconnecting means with integral or immediately adjacent OCPD or grouped OCPD's in the case of 240.21(C)(3).

 

[Carultch]

The secondary is 3-wire single phase, so the primary OCPD does not protect the secondary conductors under 240.21(C)(1).

In all cases of 240.21(C)(2) through (6), the first piece of equipment must be a OCPD or a disconnecting means with integral or immediately adjacent OCPD or grouped OCPD's in the case of 240.21(C)(3).

I understand that the first piece of equipment needs to be an OCPD. Can one terminate 240.21(B) and 240.21(C) conductors in adapter terminals if required for the application, given that the full path is rated for the required ampacity?


One place you might see this, is in the outside unlimited length rules, where you'd need to step it up for voltage drop in excess of your equipment factory terminals. Then transition to a short piece of a smaller wire to fit the equipment. This often would be an in-line splice reducer, but in extreme cases it could be a multitap device in the event that you have to change the number of parallel sets to curtail voltage drop.

 

[Smart $]

I understand that the first piece of equipment needs to be an OCPD. Can one terminate 240.21(B) and 240.21(C) conductors in adapter terminals if required for the application, given that the full path is rated for the required ampacity?


One place you might see this, is in the outside unlimited length rules, where you'd need to step it up for voltage drop in excess of your equipment factory terminals. Then transition to a short piece of a smaller wire to fit the equipment. This often would be an in-line splice reducer, but in extreme cases it could be a multitap device in the event that you have to change the number of parallel sets to curtail voltage drop.

I would say yes... but best to check with your AHJ on this one.

 

[electrofelon]

The secondary is 3-wire single phase, so the primary OCPD does not protect the secondary conductors under 240.21(C)(1).

I concur. I wasnt saying that the OP situation could use this, I was just summarizing the general requrements

In all cases of 240.21(C)(2) through (6), the first piece of equipment must be a OCPD or a disconnecting means with integral or immediately adjacent OCPD or grouped OCPD's in the case of 240.21(C)(3).

I am not certain that I agree with that. I think a reasonable case can be made that it is permissible to put a switch in the transformer secondary conductors.

 

[Smart $]

...
I am not certain that I agree with that. I think a reasonable case can be made that it is permissible to put a switch in the transformer secondary conductors.

Yes, you can put in a switch... if it serves as a disconnecting means, for which the criteria is exemplified by:
240.21(C)(2)(1)(b)
240.21(C)(3) general regarding switchgear or switchboards (i.e. see Article 408)
240.21(C)(4)(3)

Neither 240.21(C)(5) nor 240.21(C)(8) have any, but I believe most AHJ will allow.

IMO, an ATS cannot serve as a disconnecting means because it does not provide a means to manually disconnect both sources.

 

[electrofelon]

Yes, you can put in a switch... if it serves as a disconnecting means, for which the criteria is exemplified by:
240.21(C)(2)(1)(b)
240.21(C)(3) general regarding switchgear or switchboards (i.e. see Article 408)
240.21(C)(4)(3)

Neither 240.21(C)(5) nor 240.21(C)(8) have any, but I believe most AHJ will allow.

IMO, an ATS cannot serve as a disconnecting means because it does not provide a means to manually disconnect both sources.

I dont see any reference to a disconnecting means, except in 240.21(C)(4)(3)

 

[Smart $]

I dont see any reference to a disconnecting means, except in 240.21(C)(4)(3)

It is implied in 240.21(C)(2)(1)(b) "the equipment containing an overcurrent device(s) supplied by the secondary conductors...", as in a fusible safety switch... otherwise called a disconnecting means in Code speak.

It is implied in 240.21(C)(3) by "For the supply of switchgear or switchboards...", which typically have a switch immediately ahead of its load OCPD.

Anyway, you're the one that said a switch is allowed. If you can't "see it" then you are incorrect... :slaphead:

 

[jaggedben]

240.21(C)(1) would apply IF you have the requisite type of transformer AND IF the primary protection, after "correcting" for the turns ratio, does not exceed the ampacity of the secondary conductors. If you dont meet all of the requirements to use 240.21(C)(1), then you need to provide protection for the secondary conductors per one of 240.21(C) 2 thru 6.

It seems to me you're just asserting the opposite of what I said, without offering any justification. I'm not saying you're wrong. But I would wager that the AHJ requiring the OP to have a secondary side OCPD would not accept assertion as argument.

 

[electrofelon]

It seems to me you're just asserting the opposite of what I said, without offering any justification. I'm not saying you're wrong. But I would wager that the AHJ requiring the OP to have a secondary side OCPD would not accept assertion as argument.

I was just trying to explain those sections. Perhaps I am not understanding what you are questioning about those sections. Maybe try it this way: All transformer secondary conductors need protection. There are some cases when the primary protection will also protect the secondary (C1). In installations where that is not the case, you have to meet one of C2 thru C6