A circuit breaker that trips open intermittently may be indicative of a circuit overload or an intermittent short circuits. Two reasons a circuit breaker trips open are circuit overloads and short circuits. Circuit breakers, like the older, Edison-based fuses, are designed to protect electrical writing against circuit overloads and short circuits.
Short circuits occur when a hot wire comes into contact with the circuit's neutral conductors or when it comes into contact with some grounded surface. Short circuits are relatively easy to diagnose because the circuit breaker will not hold after resetting until the short circuit is located and repaired. Circuit overloads, on the other hand, are not so easy to diagnose because, depending on the amount of the overload current, the circuit may hold after being reset until the heat building up from the overload current causes the thermally operated circuit breaker to trip open once again.
A third possible cause for a circuit breaker to trip open is that the circuit breaker itself has become defective. Circuit breakers becoming defective is not a common problem with residential wiring systems, but it can happen. There might have been a manufacturing defect with the circuit breaker that the QC inspections did not catch. This kind of problem usually makes itself known relatively soon after the CB is newly installed. Another cause for a CB to become defective is a power surge that causes so much heat in such a short period of time that the bi-metal strip in the CB is weakened or damaged in some other way.
How much circuit current constitutes a circuit overload?
Residential 120-volt branch circuits are either protected by circuit breakers rated at 15-amperes or 20-amperes but that does not mean that these circuits can be loaded down at constant 15 or 20-amperes current. The reason for that is that most circuit breakers are designed to only carry a continuous current of up to 80 percent of their rated current. In other words, a circuit protected by a 15-ampere CB has a load rating of 1,440-Watts or 12-Amperes. A circuit protected by a 20-Ampere CB should be loaded to 1,920-Watts or 16-Amperes. These maximum safe loads are not only required by the CB's design, they are requirements of the National Electric Code (NEC). There is one exception to this rule, some circuit breakers are designed to carry 100-percent of their rated current and those CB's are so marked on their cases.
Troubleshooting and Isolating circuit overloads.
If you do not already own a clamping ammeter, now is the time to procure. There are many quality clamping meters on the market today at very reasonable prices but one of the best sources of tools for the do-it-yourself person is Harbor Freight. You could spend hundreds of dollars for a clamping meter but the Cen-Tech 7—Function, clamp—on multimeter sold by Harbor Freifht for less than $30 will be all the meter the average do-it-yourself person will ever need for residential electrical work. This meter tests resistance and AC and DC voltage without splicing wires or puncturing insulation. The meters 17-test ranges include
- DC Voltage—4 ranges 200mV/20V/200V/1000V
- AC CurrentAC Voltage—2 ranges 200V/750V—3 ranges 20A/200A/1000A
- Resistance—5 Ranges 200Ω/2kΩ/20kΩ/200kΩ/2MΩ
- Diode Test
- And a Continuity Test.
The actual product/operating manual is available in pdf format here.
Safety First
One of the very first rules that every do-it-yourself person is taught when doing an electrical project is to always turn the electricity off first by switching the circuit breaker to the off position. But, as James Thurber (1894-1961), the writer and cartoonist once said, “There is no exception to the rule that every rule has an exception.” This is one of those exceptions for electrical safety rules. When performing an electrical circuit load test, just as when performing circuit voltage tests, the power must be left on.
Performing the load test.
I do not believe in reinventing thw wheel. I have already published a step by step guide called “How to Tell If You Have a Faulty Breaker in Your Breaker Box” on e-How.
Circuit Overloaded
If the circuit is actually overloaded, the meter indicates a load of more than 12-Amperes on a 15-Ampere circuit or a 16-Ampere load on a 20-Ampere circuit, you will need to reduce the load by moving some of the plugged in loadsto a different branch circuit.
Replacing a defective circuit breaker.
If the load current is 80 percent or less of the circuit breaker rated amperes and the breaker continues to trip open, you will need to replace the circuit breaker. For a good step-by-step guide for removing and replacing a defective branch circuit CB, go to Replacing a Breaker in Your Panel.
