Does a commercial sub-panel require a main circuit breaker?

[Bbcathala]

Does the second section of a two section panelboard (panel and sub-panel) require a circuit breaker by NEC?
Here at my work (manufacturing facility) we have a 2 sections panel (3 phase 208/120V) connected using a lug connection. All the other sub-panels have their own main breaker.
I checked articles 215.3, 240.92 (A), 408.36, etc and I couldn't find where the NEC says is allowed to connect panelboards using lug connection.
Could you help me to find where the NEC talks about that?

Thank you

 

[don_resqcapt19]

The code does not require a main breaker for a panelboard where the panelboard is in the same building as the feeder breaker for that panel.

408.36 Overcurrent Protection.
In addition to the requirement of 408.30, a panelboard shall be protected by an overcurrent protective device having a rating not greater than that of the panelboard. This overcurrent protective device shall be located within or at any point on the supply side of the panelboard.

 

[LarryFine]

The main in the first section protects all, as if it was a single panel with twice the space.

 

[infinity]

Does the second section of a two section panelboard (panel and sub-panel) require a circuit breaker by NEC?
Here at my work (manufacturing facility) we have a 2 sections panel (3 phase 208/120V) connected using a lug connection. All the other sub-panels have their own main breaker.
I checked articles 215.3, 240.92 (A), 408.36, etc and I couldn't find where the NEC says is allowed to connect panelboards using lug connection.
Could you help me to find where the NEC talks about that?

Thank you

The NEC doesn't say much specifically about this installation. The feed through would need to meet a few parameters, the conductors would need to be at the minimum sized to the OCPD protecting the first panel and the feed through panel would need to be rated at the minimum for the size of the OCPD also.

 

[Bbcathala]

Thank you all for the quick clarification.
In my opinion the Code is a little vague about this matter.
First, I though the Code asked to protect the wires between the two sections as the same way as the feeders of the first section (from the transformer).

 

[infinity]

Thank you all for the quick clarification.
In my opinion the Code is a little vague about this matter.
First, I though the Code asked to protect the wires between the two sections as the same way as the feeders of the first section (from the transformer).

The panel is protected by the main circuit breaker and the feed through conductors are also protected by the main CB. For that reason no OCPD is required between the two panels. Since you mentioned the panel being fed from a transformer it's possible that the feed through conductors can actually be smaller than the conductors feeding the main from the transformer and it still can be code compliant.

For example a panel with a 400 amp main CB being fed from the transformer with 600 kcmil copper conductors then the feed through conductors could be 500 kcmil Cu.

 

[Greentagger]

The panel is protected by the main circuit breaker and the feed through conductors are also protected by the main CB. For that reason no OCPD is required between the two panels. Since you mentioned the panel being fed from a transformer it's possible that the feed through conductors can actually be smaller than the conductors feeding the main from the transformer and it still can be code compliant.

For example a panel with a 400 amp main CB being fed from the transformer with 600 kcmil copper conductors then the feed through conductors could be 500 kcmil Cu.

Help me understand why the feed thru can be smaller than the initial size of secondary conductors. Your example given 600, then fed thru with 500 kcmil. I see 240.21(C)(2)(1)(a)(b)? I’m probably missing something. Thanks in advance for the response.

 

[infinity]

Help me understand why the feed thru can be smaller than the initial size of secondary conductors. Your example given 600, then fed thru with 500 kcmil. I see 240.21(C)(2)(1)(a)(b)? I’m probably missing something. Thanks in advance for the response.

Sure, the secondary conductors from the transformer cannot utilize the next size up rule {240.21(C)} so in my example with a 400 amp OCPD in the panel the secondary conductors would need to be a minimum of 400 amps (600 kcmil=420 amps). On the load side of that 400 amp OCPD you can use the next size up rule so for 400 amps a 500 kcmil conductor (500 kcmil=380 amps) can be used via the next size up rule {240.4(B)}.

 

[ActionDave]

Sure, the secondary conductors from the transformer cannot utilize the next size up rule {240.21(C)} so in my example with a 400 amp OCPD in the panel the secondary conductors would need to be a minimum of 400 amps (600 kcmil=420 amps). On the load side of that 400 amp OCPD you can use the next size up rule so for 400 amps a 500 kcmil conductor (500 kcmil=380 amps) can be used via the next size up rule {240.4(B)}.

Yup. It's a handy little trick

 

[Greentagger]

Sure, the secondary conductors from the transformer cannot utilize the next size up rule {240.21(C)} so in my example with a 400 amp OCPD in the panel the secondary conductors would need to be a minimum of 400 amps (600 kcmil=420 amps). On the load side of that 400 amp OCPD you can use the next size up rule so for 400 amps a 500 kcmil conductor (500 kcmil=380 amps) can be used via the next size up rule {240.4(B)}.

Yes sir, appears all angles are covered. The feed thru are not secondary conductors after the overcurrent protection device. Thank you.

 

[electrofelon]

I panelboard NEVER requires a main breaker mounted integral to it, as Don noted in 408.36 But of course there are cases where you need one "somewhere", and its usually most economical and convenient to have a MB panelboard. I once compiled a list of all the reasons I could think of for needing a MB.

1. To comply with a disconnect rule for services and detached structures (also overload protection for the SEC).
2. To comply with 408.36
3. To comply with a tap rule or transformer secondary conductor rule in 240.21
4. To meet transformer protection required in 450.3 (typical for an SDS the MB serve triple duty and satisfy #2,3,4 in one shot).
5. To make a series rated combination with branch breakers.