I have a circuit breaker which the trip rating is set at 20A. How can I measure the actual current?

Answer 1

The easiest way to measure the current is to buy a "clamp-on ammeter" that allows you to measure the current flowing through a wire without disconnecting anything. See the link below for a picture of the meter.

If a breaker is rated at 20 A, that means the maxiumum continuous load is 20 amps. The higher the load, the faster the breaker will trip. This relationship is know as the Time-Current-Characteristics. This data is available for both the mininum trip and the maximum clear times.

According to the TCC curve for a 20 A Square D QO Molded Case Circuit Breaker, the device has a minimum trip time of 400 seconds at 20 A. (Six minutes and twenty seconds)

At forty amps, the minimum trip is 8 seconds

At ninty amps, the minimum trip is 1 second

At currents above 120 amps the breaker will trip "instantaneously". (Technically 0.005 seconds)

The "Maximum Clear" times can be substantially longer than the published "Minimum Trip" times. There is no good way to accurately estime the amperage based on how long the breaker took to trip.

Two links below:
1 = Picture of a clamp-on multimeter available at Sears
2 = Photo of a meter measuring the currrent in an electrical panel.

Answer 2

The idea of circuit breakers is to stop the current flow before wires in the walls can overheat, melt the insulation and cause a fire. It takes a little time for that to happen and the makers of circuit breakers take that into account. Why? Because there are many situations when the current flow through a household or shop circuit temporarily exceeds the rated amperage. Electric motors typically draw a very high current to get started. That is why most have a heavy capacitor to give them an extra boost to start. If two or more motors on the same circuit happen to start up at the same time, the current draw can easily exceed the rated amperage for a minute. By providing a delay before the breaker trips, manufacturers save users a great deal of inconvenience without compromising safety.

Answer 3

To be more accurate you need to be able to vary and measure the current in small increments until the breaker trips allowing for the thermal trip element to activate.
The use of a variac with an in line ammeter would give the desired accurate current trip value.
The test set can purchased as used by Utility Companies under the trade name Multi-amp.

Answer 4

Use a clamp on ammeter (Hall effect or current transformer)
not very expensive to measure AC current

or just look at the electricity meter
with every other circuit turned off (or fuse pulled etc)

Answer 5

To measure current, use an ammeter. The easiest to use would be a clamp-on AC ammeter. You will need to take the cover off the breaker box and clamp around the wire coming from the circuit breaker. You must be very careful and know what you are doing. It would be safer to turn off the 20A breaker and then use a portable lamp or plug-in outlet tester to find out which outlets are connected to the breaker. Then add up the amp ratings marked on the things plugged into those outlets.

Look at:
http://en.wikipedia.org/wiki/Clamp_meter

http://www.homedepot.com/webapp/wcs/stores/servlet/ProductDisplay?storeId=10051&langId=-1&catalogId=10053&productId=100047661&N=10000003+90401&marketID=401&locStoreNum=8125

http://www.sears.com/shc/s/p_10153_12605_03482372000P?keyword=meter

http://www.radioshack.com/product/index.jsp?productId=2103173&cp=&sr=1&origkw=ammeter&kw=ammeter&parentPage=search

http://www.homedepot.com/webapp/wcs/stores/servlet/ProductDisplay?storeId=10051&langId=-1&catalogId=10053&productId=100062242

Answer 6

To measure the actual current you need a special electricians meter that has a clamp type feature that wraps around the feed line that needs to be measured. This will give you a good estimation of the actual current involved.

Answer 7

Use an ammeter in series of the circuit.....
Or use a clamp meter