Outdoor transformer Secondary conductor protection

[EErook]

I have an outdoor 480V switchboard feeding a outdoor 225kVA 480V Delta / 208Y120V transformer. I plan on feeding three 225A main breaker outdoor panels with 3 sets of 4/0 THHN secondary conductors. One panel will be on the same house keeping pad as the transformer and the other two panels will be on the other side of the property about 150 ft. away (still outdoors).

The transformer primary breaker will be 400A which allows me to forgo transformer secondary protection according to NEC table 450.3(B). Since this installation is all outdoors, will I need additional OCPDs for the secondary conductors? Reading NEC 240.21(C)(4) makes me think the main breakers in the panels is enough to protect the 3 sets of secondary conductors. Thanks in advance.

 

[augie47]

If I did my math correctly your primary OCP would have to be a 350 amp breaker (a "standard" size)

 

[Carultch]

The transformer primary breaker will be 400A which allows me to forgo transformer secondary protection according to NEC table 450.3(B). Since this installation is all outdoors, will I need additional OCPDs for the secondary conductors? Reading NEC 240.21(C)(4) makes me think the main breakers in the panels is enough to protect the 3 sets of secondary conductors. Thanks in advance.

The requirement for secondary OCPD is a function of the topology of the transformer, which is an independent issue from whether the installation is inside or outside. If there is a wye topology on either side or center-tapped secondary, or both, then you need secondary OCPD. It is only the topology combinations where the primary current is guaranteed to directly line-up across the winding pairs to the corresponding secondary current, that your primary OCPD can indirectly protect the secondary conductors. Wyes and center-taps can allow an overload on the secondary side that would be in the "blindspot" of the primary OCPD.

When the installation meets the requirement in 240.21(C) for an outdoor installation, you still need secondary OCPD. The way an outside installation helps you, is that you get an unlimited length of the secondary conductors before you connect to them.

 

[EErook]

If I did my math correctly your primary OCP would have to be a 350 amp breaker (a "standard" size)

Thanks for the heads up

 

[EErook]

The requirement for secondary OCPD is a function of the topology of the transformer, which is an independent issue from whether the installation is inside or outside. If there is a wye topology on either side or center-tapped secondary, or both, then you need secondary OCPD. It is only the topology combinations where the primary current is guaranteed to directly line-up across the winding pairs to the corresponding secondary current, that your primary OCPD can indirectly protect the secondary conductors. Wyes and center-taps can allow an overload on the secondary side that would be in the "blindspot" of the primary OCPD.

When the installation meets the requirement in 240.21(C) for an outdoor installation, you still need secondary OCPD. The way an outside installation helps you, is that you get an unlimited length of the secondary conductors before you connect to them.

I see. I’m just getting tripped up the three sets of secondary conductors each protected by a OCPD. Rereading 240.21(C)(2) says “The conductors terminate at a single circuit breaker or a single set of fuses that limit the load to the ampacity of the conductors. This single overcurrent device shall be permitted to supply any number of additional overcurrent devices on its load side.”
Can one argue that since each set of conductors terminate at a single OCPD it is compliant by code?

 

[augie47]

Can one argue that since each set of conductors terminate at a single OCPD it is compliant by code?


IMO, that is correct

 

[EErook]

Can one argue that since each set of conductors terminate at a single OCPD it is compliant by code?


IMO, that is correct

Thank you!

 

[david luchini]

Can one argue that since each set of conductors terminate at a single OCPD it is compliant by code?

Your installation would be compliant if, as Gus mentioned, you changed the primary OCPD to 350A so that the transformer would fall under "primary only protection."

If the primary OCPD was 400A, the transformer would need "primary and secondary protection" per 450.3(B), meaning the three secondary OCPDs would have to be grouped in one location.