Many people when they need more outlets grab a power strip. They don’t really consider what the load is they’re running off the strip as a whole. Many will do the worst thing in the world and cascade power strips for more outlets, or plug a very large current item into the power strip. There’s a couple things going on that make ignoring these factors a bad idea. The biggest is that this is the result.
According to fire investigators, a power strip started the fire, which caused an estimated $250,000 in damage. Multiple computers and a space heater had been plugged into the strip, but what role that played remains uncertain.
I’m going to explain now exactly why cascading power strips, and ignoring load concerns is a bad idea and results in the above. First read this article on an introduction on basic electronics. In doing this I’m also going to explain why you never replace a circuit breaker or fuse with anything but a matching value.
Power Losses A Quick Reminder:
Did you see the section involving wire diameter, length, and overall resistance? There’s a minimum wire gauge for carrying different loads of current. The reason being is power losses over the wire. Remember the equation, I2R. This can be used to determine power lost over the wire is dissipated in the form of heat. Lets start off with a simple example.
A Base Load Example:
Residential building codes state that for a 12 gauge wire the maximum current load is 20 amp breaker. We have a computer using a 650W power supply and two LCD displays (~75W). These three are plugged into a single 14 gauge surge protector plugged into an outlet on the 20 amp circuit. Now while that 650W supply may not be operation at full load, we will assume it is for calculation purposes.
Circuit breakers are rated to carry 80% of their rated nominal load continuously. This means that our breaker can supply 16 amps continuously to our load distribution the power strip. This works out to be 1760W = (20*0.8)A*110V that the circuit can supply. Though the surge protector can only safely deliver 1320W = (15*0.8)A*110V due to it only having 14AWG wiring. Most power strips will have a safety breaker that will enforce this limit, however they can fail and on cheap strips they may not even work. Currently our load is only 800W so we are well within the safe range.
How Easy It is To Cross the Threshold:
Now lets say we want to add a space heater since the wife is cold. If we add the 800W oil heater to out power strip with our computer. This moves the load on the power strip up to 1600W. This is beyond the 14 AWG capacity of 1320W. This will cause the temperature of the wires to go beyond their rated values. This can be a serious problem if the power strip is located in an area where heat cannot dissipate. The addition of the power strip also adds extra length increasing your overall I2R losses. This is why you don’t want to put a large load such as a heater on an extension cord. If you do, make sure it is a heavy grade extension cord. The safest method is to plug it directly into a wall outlet. This will still be below the 20A continuous rating, prevents the extra I2R loss from the extra length, and doesn’t overload the the smaller power strip.
So What About Cascading Them:
Now so far we’ve only been really talking about a single power strip, what happens when you cascade them? This is where another lesser known problem rears it’s head. The majority of heating in house hold power distribution circuits are at junctions. Where wires are tied together, plugs and outlets, and screw terminals. This is because the resistance in the joint higher than the surrounding materials. Again, I2R comes in to play with the larger resistance at the joint. That power goes somewhere and it is released as more heat into the system. So adding extra power strips increases the resistance in the line and increases the overall power losses.
So What About Circuit Breakers:
Now how does that apply to circuit breakers though. Say instead of a 12 AWG circuit with a 20A breaker it’s a 14 AWG circuit with a 15A breaker. It keeps tripping and you think it’s faulty. All you have in your chest to replace it is a 20A breaker. You have now allowed a much larger current to flow across the circuit that it is safely rated for. You have upped the trip limit from 12A to 16A, thus your I2R loss is going to increase. That increase of 4 amps increases the power dissipation required of the wire by 1.7x. Again that power is dissipated in the form of heat, in your walls, that retain heat, some packed with insulation. You have turned your house wiring into a toaster element, not a good idea.
So there’s a rough idea of
- Why you don’t cascade power strips.
- Why you plug high power appliances into the wall, or use a large gauge extension cord with a single outlet.
- Why you don’t swap a smaller breaker for a larger one.
- The physics behind what cause the fire above.