sizing of breakers in an electrical distribution panel

[OGHENEKUME]

what are the basics of sizing the breakers in an electrical distribution panel. that takes care of current discrimination at the same time minimizes the tendency of over rating breakers and panel board sizes

 

[waross]

80% of continuous load.
250% of motor current.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

Conductors are sized for the load, breakers are sized to protect the conductors.

Unless it's a motor circuit with separate overload protection, in which case see #2 above (assuming an Inverse Time circuit breaker).


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[kply_231]

@waross: when you say 250% of motor current, are you referring to the overload relay settings on a motor branch circuit? Usually like @jraef mentioned, conductors are sized for atleast 125% of total load and breaker is sized to protect the conductor downstream of the breaker. This means the current rating of the breaker should be less than the maximum ampacity of the conductor, and not less than 125% of the total current drawn by the load (Breaker gets loaded only 80% this way). Also, note that a breaker can be loaded up to 100% of its rating if the breaker is approved for 100% loading. Another thing to watch for when sizing the conductor is the cable insulation temperature and the temperature that the cable terminals of other equipment can withstand. For eg., if the terminals of the circuit breaker cannot exceed more than 75 degree C, the cable ampacity for 75 degree C cable insulation is what must be used for selecting the cable.

 

[waross]

A conductor must have both over current protection and overload protection.
Over current protection or short circuit protection; This is the instantaneous trip setting. Typically 5 to 10 times the breaker rating.
Overload protection; An inverse time trip based on the rating of the beaker.
In a motor circuit the motor overload relay provides overload protection for the supply conductors.
Over current or short circuit protection is allowed to be 250% of the motor FLA to allow the motor to start.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LFAVianna]

Usually, it is necessary to take into account the coordination e selectivity of the panel. As mentioned above, the conductor must be specified to the nominal current of the load and the circuit-breaker will need to protect the condutor, according to its overload/damage curve. Whatever, the circuit-breaker usually will be placed into a MCC or a panel with another loads, which is desired to be selectivity. Depending of the load and panel size, there are a lot of types of curves to be used in the overload and magnetic units.
Also, the circuit-breaker must have capacity to open and eliminate faults and electrical arcs without damaging its main contacts, these is obtained choosing a circuit breaker with breaking capacity (in kA) higher than the short-circuit level of the panel.

 

[kply_231]

@waross,
are you referring to long time trip setting of a circuit breaker when you say - "inverse time trip"

 

[waross]

the breakers that I visualize in a panel do not have adjustable long time trip settings.
The setting is fixed at the breaker rating. The trip characteristics follow an inverse time curve. The higher the current the quicker the trip, until the instantaneous trip operates at 5 to 10 times rated current.

Bill
--------------------
"Why not the best?"
Jimmy Carter