Most branch circuits in homes are rated at 15 or 20 Amperes and protected by a 15 or 20 Ampere circuit breaker. In older homes, these same 15 or 20 Ampere branch circuits may be protected by Edison-Base fuses. What constitutes an overload on these circuits? Is it when the current draw on these circuits exceeds 15 or 20 Amperes, respectively? Actually, the answer is that an overload conditions exists when the load on a 15-Ampere circuits exceeds 12-Amperes, and the load on a 20-Ampere circuit exceeds 16-Amperes. Why is that?
Circuit Breaker Design and the NEC.
Circuit breakers are designed and manufactured to carry their full-load current, the Amperes stamped into their toggles, but according to the National Electrical Code, NEC Sec. 384-16(c), they are restricted to use at 80 percent of their rated Amperage. Why is that?
Circuit breakers are deigned and manufactured to meet the Underwriter's Laboratory's Standard 489, Underwriters Laboratories Standard for Safety for Molded-Case Circuit Breakers and Circuit Breaker Enclosures. This standard requires them to be able to not trip open at their rated current. In other words, a 15-Ampere rated CB must be able to carry 15-Amperes continuously without tripping open and a 20-Ampere rated CB must be able to carry a 20-Ampere load current continuously without tripping open. So, why then does the NEC place an 80 percent limitation on them? Why the difference?
The difference exists because of the difference between the way the UL tests and approves them and the way they are used in the real world. The Underwriter's Laboratories test them in the open air, not enclosed in a metal cabinet where heat can build up. Heat is generated by the currentflowing through the individual circuit breakers as well as from the environment surrounding them.
As you can see from the above drawing the current flowing through the circuit breaker flows through a bimetallic strip. The bimetallic strip is made of two different metals, metals that expand at different rates as they heat up. This characteristics of two metals fused together, causes them to bend and release the spring-loaded toggle when the safe current level is exceeded in the open air testing conditions enough free air circulates around them to dissipate heat build up and the bimetallic strip does not bend sufficiently to release the toggle until the rated current is exceeded. Not so when they are installed in a metal circuit breaker enclosure.
How to calculate the safe load current for a branch circuit.
The standard Residential single-pole, 125-Volt circuit breaker Ampere Ratings are:
To calculate the safe load for each, simply multiply their Ampere rating by 125 volts time the .8 load factor.
Calculate the safe loads for residential two-pole 240-Voly 20 and 30-Ampere breaker in the same manner.
The results for each standard rating is shown in the table below.
Determine the current load connected to each circuit.
The wattage of most appliance are marked on them, if not there FLA (Full Load Amperes) all you have to do is multiply their FLA by 125 or 240 Volts to get their wattage rating. To make it easier for you, I have included this chart with some common appliances and their wattage rating.
If the total loads connected to any one branch circuit exceed the safe load capacity, you will need to move some of those loads to another branch circuit.