Panelboard Loading

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cmiller

Member
Recently I was asked what the maximum allowable panel loading allowed by the NEC was.

My immediate response was that the panel should not be loaded past 80 percent of its rating. I was then asked to produce the reference.

In a search of the 2002 NEC Handbook, on page 272 under ?Should Neutral Conductors Be Oversized,? I found this sentence:

?According to 384-16(C) of the 1993 NEC (for the 2002 NEC, refer to 408.16), the total continuous load on any overcurrent device located in a panelboard should not exceed 80 percent of its rating (the exception being assemblies listed for continuous operation at 100 percent of its rating)."

In a continued search of 2002 NEC, Article 408.16, I could find no statement indicating that ??in a panelboard should not exceed 80 percent of its rating?.?

Did I miss something here?
Or is this a misprint or a paragraph that got omitted :confused: during a revision?
Or is this information been placed elsewhere in the NEC? If so where?
Or is this requirement no long applicable?

I did find in the 1996 NEC Article 384-16(C):
?Continuous Load. The total load on any overcurrent device located in a panelboard shall not exceed 80 percent of its rating where, in normal operation, the load will continue for three hours or more.?

I look forward to your responses.

Thanks

Carl
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
Re: Panelboard Loading

The panelboard bus bars are rated to carry 100% of their current, the same as any other conductor (wire).

However, the NEC requires us to size conductors at "125%" of the calculated load. And the vast vast majority of OCPDs are also sized based on a maximum 80% (125% of load) loading.
 

cmiller

Member
Re: Panelboard Loading

Jimj,

I appreciate your reply. I was thinking along the same lines that you wrote "... the NEC requires us to size conductors at "125%" of the calculated load. And the vast vast majority of OCPDs are also sized based on a maximum 80% (125% of load) loading.

However, I was hoping to find the "...total continuous load on any overcurrent device located in a panelboard should not exceed 80 percent of its rating..." clearly documented.

Thank you for the verification.

Carl
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
Re: Panelboard Loading

Carl,

You want something the NEC does not have (i.e. like conduit fill shall not exceed 40%).

The NEC requires us to size the OCPD based on load, not the load based on OCPD.

The NEC is not a design manual therefore you have the ability to "work the formulas backwards" to determine the possible panel loading. But you must then check your calculations to make sure the panel OCPD meets the 125% sizing requirement.
 

bwyllie

Senior Member
Location
MA
Re: Panelboard Loading

The maximum loading of a panelboard is the rating of that panelboard. look under the section on panelboards.
 

bennie

Esteemed Member
Re: Panelboard Loading

bwyllie, is correct. Actually all electrical equipment can be loaded to the rated current.

It is only when the load is on for more than 3 hours that the multipliers come in.
 
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