Panelboard current rating 2

[jthjr]

If I have a 400 amp panelboard, what is the highest combination of breakers I can possibly have? Is there a formula? Would I be able to have say 5 100 amp breakers in a 400 amp panel?

 

[alehman]

There's no NEC rule like that. Sizing is based on the loads, not rating of circuit breakers. In the US, some cities have wierd rules like that, but those are fairly rare. Per NEC, the continuous load on a circuit beraker can't exceed 80% of the breaker rating.

 

[jbartos]

Comment on the original posting marked ///\\
If I have a 400 amp panelboard,
///This size or frame of panelboard is overlapping switchboard size. There is a difference between the panelboard and switchboard.\\ what is the highest combination of breakers I can possibly have?
///There is not really limit theoretically. Practically, there are often as many as needed by the downstream power distribution.\\ Is there a formula?
///The number of the circuit breakers increases as their frame goes down, e.g. down to 100A frame\\ Would I be able to have say 5 100 amp breakers in a 400 amp panel?
///Yes, especially if they have 100A frame, i.e. 100AF.\\\

 

[busbar]

jthjr, that is the basic idea behind load diversity. It is fairly common that the sum of ratings in branch breakers in a panelboard to be greater than the main bus rating.

 

[meihm]

Your hypothetical question is incomplete:

1. If the 400-amp panel has 400-amp protection ahead of it, the panel can have any combination of circuit breakers totaling up to 4200 amps (42 branch circuits, each at 100 amps). (This assumes the panel is one typical 42 circuit branch panel with 100 amp maximum branch circuit breaker frame sizes.)

(The 42-circuit limitation is for "mixed load" panels serving lighting and other loads. If the panel does not have any lighting loads, there is no limitation on the number of circuits (and hence circuit breakers) in any given "panel". Functionally however, panels are manufactured with a maximum of 42 circuits. Panel boards with greater than 42 circuits are comprised of one or more normal panels connected in series or parallel. In this instance, with a number of parallel panels all connected electrically as a "single panel", there is theoretically no limit to the number of panels, and circuits.)

2. If the panel is to provide overcurrent protection where a main disconnect is required, AND where the main disconnect is provided by using the six disconnect rule, then there IS a total sum of the handle rating limit.

For example: a separately derived system requires a 400 amp overcurrent protection on the secondary side; a 400 amp main lugs only (no single main overcurrent device installed ahead of the panel)panel is used. Up to a maximum of six handles (6 separate circuit breakers)can be used as long as the sum of their ratings does not exceed 400 amps total; e.g., 5 circuit breakers at 20 amps, 1 circuit breaker at 300 amps.

3. You would have received a lot more answers to this question had it been posted under the "IEEE (electrical) Code Issues" forum.

 

[jbartos]

Comment on the previous posting: The practical limit of 100A circuit breakers will be given by the load on each 100A circuit breaker. Considering that a circuit breaker is not loaded more than 80% of its rating (which is highly recommended and complying with NEC,
e.g. visit

http://www.ecmweb.com/ar/electric_sizing_circuit_breaker/

for: Circuit Breaker Sizing),
then 5 x 100A x 0.8 = 400A which is the main circuit breaker nominal rating. However, the main 400A circuit breaker should not be loaded more than 0.8 x 400A = 320A. This would leave 4 x 100A x 0.8 = 320A or four 100A circuit breaker limit rather than five 100A circuit breaker limit.

 

[meihm]

With respect to jbartos and in regards to his response to my response: The hypothetical says 400 amp overcurrent protection.

Your commentary on the derating of circuit breakers is partially correct (for long continuous loads), is incorrect for "100% rated" circuit breakers, and is hardly germane.

I illustrated an example where THERE are limitations on the "sum of the handles".

 

[alehman]

meihm's examples are correct. The second applies for two cases I can think of-

1. secondary overcurrent protection for a transformer, where the sum of the breaker ratigns must not exceed the rating required for a single device.
2. services, where the sum of the breaker ratings must be adequate to carry the total load as calculated per Article 220.

 

[uno]

Please check the application of the panelboard. While going through a Siemens online training on Panelboards I found a following caution.

" For a panelboard to be considered for lighting and appliance branch circuit more than 10% of the overcurrent devices must be rated for 30A or less"

Any guesses as to where it might have come from?

Checkout the above website at:

http://www.sea.siemens.com/step/templates/lesson.mason?pan:1:3:5

 

[jbartos]

Comment: The main posting did not post whether or not the 400A Panelboard has a 400A main breaker. Many panelboards do have main circuit breaker. I assumed that the panelboard had 400A main circuit breaker. Such circuit breaker would be used for the upstream transformer secondary protection.
Please, notice that 100% rated circuit breakers still may be derated because of ambient temperature, altitude, harmonic currents, unbalanced currents, etc.
Visit

http://www.sea.siemens.com/speedfax04/sec06/06-54_55.pdf

for 80% and 100% rated circuit breakers. 100% rated circuit breakers are more expensive and not every circuit breaker manufacturer offers them.

 

[alehman]

reply to uno: The National Electrical Code states "A lighting and appliance branch-circuit panelboard is one having more than 10% of its overcurrent devices protecting lighting and appliance branch circuits. A lighting and appliance branch circuit is a branch circuit that has a connection to the neutral of the panelboard and thathas overcurrent protection of 30 amperes or less."

Further, "A power panelboard is one having 10% or fewer of its overcurrent devices protecting lighting and appliance branch circuits."

Lighting and appliance branch circuit panelboards are required to have dedicated overcurrent protection. Power panelboards are not.

 

[jbartos]

Suggestion: Perhaps a little definition of the discussed equipment from NEC could be helpful:

Panelboard. A single panel or group of panel units designed for assembly in the form of a single panel, including buses and automatic overcurrent devices, and equipped with or without switches for the control of light, heat, or power circuits; designed to be placed in a cabinet or cutout box placed in or against a wall, partition, or other support; and accessible only from the front.