Bus rating and circuit breakers

[cppoly]

Is there a limit to the breaker sizing in a switchboard? For instance, a switchboard with a 1,000A bus has 2,000A in overcurrent protection. For an existing installation how do you know you are not overloading the bus when there's that much OCPD?

 

[Lectricbota]

Is there a limit to the breaker sizing in a switchboard? For instance, a switchboard with a 1,000A bus has 2,000A in overcurrent protection. For an existing installation how do you know you are not overloading the bus when there's that much OCPD?

Normally the switchboard itself would have an OCPD somewhere before it.

 

[kwired]

Is there a limit to the breaker sizing in a switchboard? For instance, a switchboard with a 1,000A bus has 2,000A in overcurrent protection. For an existing installation how do you know you are not overloading the bus when there's that much OCPD?

The main overcurrent device will protect from overloading.

If you are in the exceptions where you are allowed up to six service disconnecting means then it is possible to overload, but if you have done load calculations correctly that risk is not very high, unless more load is added without recalculating load on the service.

 

[cppoly]

For existing buildings, there aren't load calcs available. So if a new tenant is coming in, and you have two 400A circuit breakers available to you, how do you know if you can use the full 400A without actually overloading the board? I guess the primary would trip if you did but you wouldn't want that to happen. Is a good way to review past utility bills based off of peak values?

 

[Smart $]

For existing buildings, there aren't load calcs available.

Perhaps not the original load calc's... but nothing says you can't do a new one.

... Is a good way to review past utility bills based off of peak values?

The only acceptable methods are prescribed by Article 220. If you don't have the original or want to do a new one, you are then limited to 220.87.

 

[augie47]

Basically what Smart$ states, however, if the power co has peak load info that can be helpful in making your determination but keep in mind even their peak load info is over a particular time period and might not address a shorter term load.