I just got done replacing a bunch of aluminum wiring, outlets, and light fixtures on a circuit up to the first outlet from the breaker. That outlet was replaced with a CO/ALR unit which crosses over the #12 aluminum from the breaker box to #12 copper.

All outlets on that circuit (now) check out as correct and grounded. 3 of them are UL 2003 GFCI outlets.

This is a circuit which shares a common with another circuit on the other phase. I left the other circuit alone, so both returns share the #12 aluminum leg. Both supplies are also #12 aluminum.

After doing work like this, I like to overload the circuit to ensure that the breaker will trip. If there's something wrong, I'd rather have the fire happen in front of me, than when I'm not home.

But this time I couldn't get it to trip. On a single 15-amp circuit I started:
An 11.0 Amp vacuum cleaner,
A 1100 watt iron.
2 300watt halogen lamps

The vacuum cleaner plus 2 halogens normally trips the other circuits, and the iron should definately have brought it over the edge. I left it run for 1 minute, then plugged in a coffee pot and the lights dimmed, but still the breaker did not trip. I unplugged the iron, coffee pot, and vacuum immediately, assuming that something was wrong.

It was 20F in the garage where the breaker box is located, and the breaker was barely warmer than the other idle circuits.

Was it the cold?

Could a wiring problem on the other circuit cause this? I have already foundand reparied several reversed hot-neutrals on other circuits.

 

without a ampmeter on the wire to precisely measure the draw of those appliances you cannot be certain of the load. Just because a vaccume says 11 amps on it does not mean that is what it draws during normal operation more of a guideline(sales gimmic)of what it is capable of for a limited time. It does sound as though you had quite alot on the circuit but it was not scientific enough to warrant immediate replacement of the breaker. however if you do the same load again with a ampmeter and it indicates you are in excess of 20 amps for a conciderable time definately more than 1 min. and the breaker still does not trip then there may be cause for alarm. It is possible for a breaker to go bad and the manufacturer reccomends they be excrecised regularly, but it can take some time for it to trip on a thermal overload(and you say it was barely warm to the touch).

 

I dont remember if you had posted it earlier, but what brand is your panel. Federals have had problems over the years tripping on thermal.

 

Replaced my entire FPE panel and service last month after reading the horror stories about them.

Question for the pros out there: Does the code allows you to feed #12 Cu from #12 Al? Al has to be sized bigger to carry the same load because it's not as conductive, which is why your AWG12 Aluminum circuit has a 15A breaker. AWG12 Al is equivalent to AWG14 copper.

 

To test a breaker you need an ammeter and a stopwatch. Inverse time (thermal-magnetic breakers) have a pretty wide tolerance on the thermal side. Typical values for a 15-30 amp breaker are 200% of rating for 9 seconds to 35 minutes, 300% of rating for 3.7-10 seconds, 400% of rating 2.4-5.5 seconds, 1000% of rating 0.45-1.2 seconds. At very high current the magnetic trip comes into play, for a 15 to 30 amp breaker 500 amps 2.4 milliseconds, 1000 amps, 2.2 milliseconds, and 10,000 amps, 1.8 milliseconds. Note that a single cycle of 60 Hz is 16.7 milliseconds, and so a half is 8.3 milliseconds. Very high currents will exist for only a fraction of a cycle.
These are typical values from one manufacturer. Times from others will vary slightly, but they will all be in this ballpark.

 

It's a GE TM1210.

 

Wow! Thanks for all the info, I didn't realize that the tollerances were that high! For something as mathematically exacting as AC circuitry is, breakers are sure sloppy. I expected something like 16 Amps for 10 seconds would trip a 15 Amp breaker.

Well, I certainly don't want to overload it to 300% myself, considering the finicky aluminum in the circuit.

Given the equipment costs and expertise required to do that, is this something you would personally worry about enough to hire an electrician to check out?

 

The magnetic side is fast and accurate. The thermal side is always the problem. And the thermal numbers are at 40C ambient and a cold start. The tripping mechanism comes into play also. It is purely machanical, powered by a spring. Add up the friction variations, the spring rate variations, the resistance of the heating element, it does wander around. This is one of the reasons that #14, #12, and #10 wire are held well below there actual ampacity.