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If a 100A breaker panel has five 15A breakers and four 20A breakers could this overload the panel?

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The short answer is "no". A 100A panel will be protected by a 100A or smaller main circuit breaker or fuse, and if the sum of all the loads on the various branches exceeds the main breaker rating, it will trip and power will be disconnected long before any damage can occur to the panel.

for USA, Canada and countries running a 60 Hz power supply service. The short answer is that safe capacity in buildings is determined by a load calculation, not by adding up the nominal ratings of breakers. A quick check for signs of overload would be to check the temperature of your 100A main breaker. Use a $50 non-contact thermometer from a hardware or home improvement store and look for the main to be significantly hotter than the average of other breakers -- over 100F or more than 5-10F hotter than other breakers could be cause for concern.

On a 100A-rated electrical service, you have 19,200 W of continuous power capacity to stay within the 80% of rating load limit indicated by National Electric Code. You can calculate about 3W per square foot of living space for convenience receptacles and lighting, and you should also subtract the wattage of each piece of permanently-installed equipment (such as a garage door opener, electric range, 240V water heater, etc.).

Just like any resource, anything could be abused. You typically do not use all of your electrical circuits at the same time. A 100A panel will usually also be the equipment for disconnecting from the utility service, and have a MAIN BREAKER service-disconnect that will trip (eventually) when you have exceeded safe home capacity. Each individual 15 or 20 amp circuit could also have an 'overload' plugged into them. Just as long as you don't turn on every device at the same time, the smaller circuit breakers won't blow and everything is fine.
Clarification: A 100 amp electrical panel is so called because it is rated at 100 amps. The main breaker or fuse is intended to disconnect power if the load exceeds 100 amps on either of the 2 "legs" of the service. The load on the panel is not the cumulative RATING of the nameplate current limits of the individual breakers in the panel. It is the actual load on the breakers that adds up to the load.
A 15 amp breaker is rated at fifteen amps, so that it will shut off if the current goes ABOVE 15 amps. This is to protect the circuit conductors from damage.
You could have 20 circuits with 15 amp breakers on each one and only have a load of 5 amps on the whole panel [not 300]! Remember there is no current if there is no load [load allows current]!
Do you get a speeding ticket because your speedometer goes up to 120 mph if you are going 25?


As always, if you are in doubt about what to do, the best advice anyone should give you is to call a licensed electrician to advise what work is needed.

If you do this work yourself, always turn off the power
at the breaker box/fuse panel BEFORE you attempt to do any work AND
always use an electrician's test meter having metal-tipped probes
(not a simple proximity voltage indicator)
to insure the circuit is, in fact, de-energized.

Thanks for the feedback!

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