Mixing Up TANSFORMER SECONDARY PROTECTION with downstream MAIN BREAKER in PANELBOARD

[Natfuelbilll]

So when does secondary transformer protection occur with a downstream panelboard Main Breaker? ???????????????????

 

[Strathead]

When it's location and size comply with 240.21.

 

[winnie]

You have 2 things you need to protect: the transformer secondary itself, and the transformer secondary conductors.

AFAIK a downstream breaker will always be considered to protect the transformer secondary (presuming suitable primary protection as well).

But the transformer secondary _conductors_ are essentially 'tap' conductors, and thus you have to meet the length rules so that the downstream breaker is considered to protect them as well.

-Jon

 

[Natfuelbilll]

When it's location and size comply with 240.21.

This is secondary conductor protection.

 

[Natfuelbilll]

AFAIK a downstream breaker will always be considered to protect the transformer secondary (presuming suitable primary protection as well).

That's the subject and what I'm trying to check. Is there a Code section that confirms this understanding?

Thank you

 

[david luchini]

So when does secondary transformer protection occur with a downstream panelboard Main Breaker? ???????????????????

When the Main Breaker is sized per the "Secondary Protection" column of Table 450.3(B).

 

[Strathead]

This is secondary conductor protection.

What are you asking then? The only other component on the secondary side is the windings and they don't get overcurrent protection. The only other required protection is primary protection.

 

[don_resqcapt19]

There is no location requirement for the secondary protection of the transformer that may be required by 450.3, as overload protection can be placed anywhere in the circuit.
The location requirement is for the protection of the secondary conductors as spelled out in 240.21(C). Where secondary protection of the transformer is required, it is almost always provided by the OCPD that protects the secondary conductors.

 

[Natfuelbilll]

What has had me befuddeled is the Exhibits 450.4 and Exhibits 450.5 in the NECHB (2014). Especially 450.5 where is shows an OCP fuse outside the panelboard.....

Thank you for all your responses!!!

 

[kwired]

What has had me befuddeled is the Exhibits 450.4 and Exhibits 450.5 in the NECHB (2014). Especially 450.5 where is shows an OCP fuse outside the panelboard.....

Thank you for all your responses!!!

Can you post those? some of us don't have that handbook.

There is the transformer secondary protection rules and there is a need to protect panelboard. Can do both with one device, as long as the length of secondary conductors doesn't exceed 240.21 requirements. Once you are past an overcurrent device the conductors are either a feeder or a branch circuit and becomes art 210, 215 or 225 applications.

 

[Natfuelbilll]

 

[david luchini]

The secondary transformer protection can be in the panelboard, or external to the panelboard. That's your choice.

 

[kwired]

View attachment 2555833

Keep in mind per NEC terminology the "panelboard" is the assembly of bus bars and overcurrent devices, the enclosure it is all installed inside of is a "cabinet". That there is just a one line diagram showing the order of items with little other details. Might have been better had the author drawn bus bar and ocpd's instead of a cabinet though. But also keep in mind the handbook is not an official NFPA interpretation of the NEC content just that of those that authored it - says this somewhere right in the front matter of the book as well.

 

[Natfuelbilll]

Thanks!