protection of transformer secondary conductors

[winslowfam]

Hi...I have spent a good amount of time already trying to completely understand this portion of the code, but I still need a bit of clarification. I was asked by a client if the installation of a 208/120V panelboard by a contractor at his facility meets the code requirement for secondary conductor/panelboard protection. The panelboard is fed from a 480-208/120V transformer and is within 10 feet of this transformer...but it does not have a main breaker. The primary protection for the transformer is rated <=125% of the transformer FLA. The secondary conductors are properly sized. After much reading, I think these secondary conductors are allowed to be protected by the transformer primary overcurrent device, along with the panelboard (240.21(C )(2) and 408.36(B)).

Am I correct? Thanks for your help...

 

[Smart $]

Hi...I have spent a good amount of time already trying to completely understand this portion of the code, but I still need a bit of clarification. I was asked by a client if the installation of a 208/120V panelboard by a contractor at his facility meets the code requirement for secondary conductor/panelboard protection. The panelboard is fed from a 480-208/120V transformer and is within 10 feet of this transformer...but it does not have a main breaker. The primary protection for the transformer is rated <=125% of the transformer FLA. The secondary conductors are properly sized. After much reading, I think these secondary conductors are allowed to be protected by the transformer primary overcurrent device, along with the panelboard (240.21(C )(2) and 408.36(B)).

Am I correct? Thanks for your help...

No.

A 208Y/120V 3? 4W secondary cannot be protected by the primary OCPD [240.21(C)(1)].

 

[david luchini]

Hi...I have spent a good amount of time already trying to completely understand this portion of the code, but I still need a bit of clarification. I was asked by a client if the installation of a 208/120V panelboard by a contractor at his facility meets the code requirement for secondary conductor/panelboard protection. The panelboard is fed from a 480-208/120V transformer and is within 10 feet of this transformer...but it does not have a main breaker. The primary protection for the transformer is rated <=125% of the transformer FLA. The secondary conductors are properly sized. After much reading, I think these secondary conductors are allowed to be protected by the transformer primary overcurrent device, along with the panelboard (240.21(C )(2) and 408.36(B)).

Am I correct? Thanks for your help...

No, you cannot protect the secondary conductors (or the panelboard) via the primary OCPD on a 480-208/120V transformer.

 

[winslowfam]

240.21(C ) is a section that describes how secondary conductors can be installed without overcurrent protection on the secondary of a transformer. 240.21(C )(1) is only one of these senarios. 240.21(C )(2) describes another scenario where secondary conductors do not require protection on the secondary...if the secondary conductors are less than 10 feet in length and terminate on a device rated for those conductors. this scenario is what I think allows this installation to be code-compliant. or I might be wrong, too...

 

[david luchini]

240.21(C ) is a section that describes how secondary conductors can be installed without overcurrent protection on the secondary of a transformer. 240.21(C )(1) is only one of these senarios. 240.21(C )(2) describes another scenario where secondary conductors do not require protection on the secondary...if the secondary conductors are less than 10 feet in length and terminate on a device rated for those conductors. this scenario is what I think allows this installation to be code-compliant. or I might be wrong, too...

Except that you are trying to protect the secondary conductors with the primary OCPD, which is described in 240.21(C)(1). And the installation does not meet the requirements of 240.21(C)(1).

Also, regarding the panel, the exception in 408.36(B) allows the panelboard to be protected by the primary OCPD where that protection is in accordance with 240.21(C)(1) (not 240.21(C)(2).)

Also, 240.21(C)(2) does NOT describe a scenario where secondary conductor do not require protection, it describes a scenario where the conductors do not need protection at the point they receive their supply.

 

[winslowfam]

ok...like I said, it's a bit confusing to me. What is the point of 240.21(C )(2) that allows 10 feet of unprotected wiring? It says that it is allowed as long as these conductors are rated not less than the rating of the device (panelboard?) where they terminate? It says "rating of the device or the overcurrent device", so it seems that you are not required to terminate only on an overcurrent device within 10 feet. If the "device" is a panelboard, it isn't saying it must have a main breaker to protect the secondary conductors. For the record, we always include an overcurrent device on the secondary of a transformer in our designs...but, in this case, my answer to our client may force his contractor to make a change, which costs money, so I want to be sure the code requires it.

 

[david luchini]

ok...like I said, it's a bit confusing to me. What is the point of 240.21(C )(2) that allows 10 feet of unprotected wiring? It says that it is allowed as long as these conductors are rated not less than the rating of the device (panelboard?) where they terminate? It says "rating of the device or the overcurrent device", so it seems that you are not required to terminate only on an overcurrent device within 10 feet. If the "device" is a panelboard, it isn't saying it must have a main breaker to protect the secondary conductors. For the record, we always include an overcurrent device on the secondary of a transformer in our designs...but, in this case, my answer to our client may force his contractor to make a change, which costs money, so I want to be sure the code requires it.

You are correct that 240.21(C)(2) doesn't require that the conductors terminate directly at an OCPD, but the "device" must have OCPD(s) that protect the conductors.

The device may be a switchboard...say an 800A mlo switchboard with (4) 200A c/b's, to protect 800A conductors. If there were (5) 200A c/b's, then the conductors are not protected.

But for the case of the panelboard, 408.36 requires that the panelboard be protected on the supply side by an OCPD not greater than the rating of the panelboard. This can be a MCB, or it could be a separate OCPD ahead of the panelboard. Terminating directly onto a panelboard does not meet 408.36 in this case.

 

[winslowfam]

ok...it makes sense what how you described that. As far as 408.36(B) goes, are you saying that a main breaker is needed because a delta-wye 3-phase (multi-phase) transformer would not meet the requirements of the exception of 408.36(B), where it says a panelboard can be protected by the transformer primary overcurrent device (and not require secondary protection) if it meets the requirements of 240.21(C )(1)?...and, in this case, it doesn't meet the requirements of 2240.21(C )(1) because it is a multi-phase transformer and is not a delta-delta transformer?

 

[Smart $]

ok...like I said, it's a bit confusing to me. What is the point of 240.21(C )(2) that allows 10 feet of unprotected wiring? It says that it is allowed as long as these conductors are rated not less than the rating of the device (panelboard?) where they terminate? It says "rating of the device or the overcurrent device", so it seems that you are not required to terminate only on an overcurrent device within 10 feet. If the "device" is a panelboard, it isn't saying it must have a main breaker to protect the secondary conductors. For the record, we always include an overcurrent device on the secondary of a transformer in our designs...but, in this case, my answer to our client may force his contractor to make a change, which costs money, so I want to be sure the code requires it.

240.21(C)(2) is regarding conductors, not ocpd. The reason it states conditions for both ocpd and device is to cover instances where secondary ocpd is and is not required. Where the secondary is other than 2-wire 1? or 3-wire 3?, ocpd is required.

PS: An MLO panelboard is not a device, a single breaker or single load is a device. Also note that 240.21(C)(2)(2) & (2)(3) actually state "control" devices.

PPS: I agree it is confusing. It should say something like the sum of all device ratings shall not be greater than the ampacity of the secondary conductors... so you can use an MLO panelboard... but then how do you keep someone from adding another circuit later and going over.

 

[winslowfam]

Is it true what I described earlier, that 240.21(C ) is a section that provides 6 distinct scenarios that allow a set of conductors to be connected to the secondary of a transformer without protection at the secondary, and that each of these 6 scenarios are independent of each other? In other words, if the installation meets the requirements of any of the 6 scenarios, it is then allowed? It doesn't have to meet the requirements of all 6...just one? if that's true, I can ignore 240.21(C )(1) if the installation meets the requirements of (2)?

 

[Smart $]

Is it true what I described earlier, that 240.21(C ) is a section that provides 6 distinct scenarios that allow a set of conductors to be connected to the secondary of a transformer without protection at the secondary, and that each of these 6 scenarios are independent of each other? In other words, if the installation meets the requirements of any of the 6 scenarios, it is then allowed? It doesn't have to meet the requirements of all 6...just one? if that's true, I can ignore 240.21(C )(1) if the installation meets the requirements of (2)?

Well 240.21(C) general says "as specified in 240.21(C)(1) through (C)(6)." As such, they all apply as long as a conditional statement does not preclude it from consideration. Even if they were independent, I don't see how you can get around 408.36(B).

 

[charlie b]

In other words, if the installation meets the requirements of any of the 6 scenarios, it is then allowed? It doesn't have to meet the requirements of all 6...just one?

Yes that is correct.

If that's true, I can ignore 240.21(C )(1) if the installation meets the requirements of (2)?

Again, correct. And you do meet those requirements. However, that is not enough. Everything connected to a power distribution system must be protected against overcurrent. In this case, if you don?t have a main breaker in the panel, or have a breaker or fuse between the panel and the transformer, then you will have left the panel without the required overcurrent protection.

 

[winslowfam]

Since this installation seems to meet the requirements of 240.21(C )(2) and the code definition of a "device" seems to include a panelboard (Art. 100), the only thing I see that would prevent this installation from being code-compliant is that it fails to comply with 408.36(B) "exception". This exception does allow a panelboard to not have protection on the secondary of transformer if it meets the requirement of 240.21(C )(1)...which, in this case it doesn't, because our transformer is a multi-phase transformer and is not a delta-delta transformer.

 

[david luchini]

Since this installation seems to meet the requirements of 240.21(C )(2) and the code definition of a "device" seems to include a panelboard (Art. 100), the only thing I see that would prevent this installation from being code-compliant is that it fails to comply with 408.36(B) "exception". This exception does allow a panelboard to not have protection on the secondary of transformer if it meets the requirement of 240.21(C )(1)...which, in this case it doesn't, because our transformer is a multi-phase transformer and is not a delta-delta transformer.

Whether or not it meets the requirements of 240.21(C)(2) is not determinable. The "device" must provide overcurrent protection for the secondary conductors, at their ampacity. If the sum of the overcurrent devices in the panel exceed the ampacity of the conductors, then you do not meet the requirements of 240.21(C)(2).

240.21(C) doesn't remove the requirement that the secondary conductors be protected at their ampacity, it allows the protection to be at other than their point of supply. See 240.21 Location in Circuit.

 

[charlie b]

. . . but, in this case, my answer to our client may force his contractor to make a change, which costs money, so I want to be sure the code requires it.

And you have all you need, right here:

. . . the only thing I see that would prevent this installation from being code-compliant is that it fails to comply with 408.36(B) "exception."

 

[electrofelon]

Everything connected to a power distribution system must be protected against overcurrent.

can you provide a code reference for that?