Electrical puzzles

MushCreek · May 23, 2012, 05:34:15 AM

Sitting here at camp in the evening, I have time to ponder things (dangerous, I know!) and I have questions about electricity. I am running the camp off of a single 110V, 20A circuit, so I have to be careful not to overload it.

Question 1) The power is coming in through a 10 gauge wire that is 240 feet long. When I check the voltage, it is exactly the same as it is where it plugs in. Shouldn't I be seeing a voltage drop over that distance? I'm using a Fluke digital meter, and it's EXACTLY the same.

2) I have a 1500 watt toaster oven, and a 1500 watt water heater. I accidentally had them both on at the same time, 3000 watts, and it didn't trip the breaker. Even at 120V, that's 25 amps. Why didn't the breaker pop? Out of morbid curiosity, I felt the supply wire to see if it got warm- it was stone cold. Also, there are several other items on the circuit- refrigerator, lights, TV. I suppose if I tried to start a 25A load, such as a motor, it might have popped.

Just curious as to what I'm seeing here. Any electrical gurus want to weigh in?

AdironDoc · May 23, 2012, 06:29:25 AM

Having grown up in a household of electricians and spent a few years as an electrical apprentice, I'll offer my 2 cents.

Your #10 copper wire with an insulation factor of 90 degree C is rated at 30 amps. Code states that conductors can only be continuously loaded to 80%. 30 x .8 = 24 amps. I wouldn't expect any heating.

Now the amount of voltage drop is dependent on, among other things, the load applied at the receiving end. At your full 25 amps, you'd have seen around a 10% drop over your 250ft run. If you had only drawn 1A, you'd have noticed very little if any. On an open circuit without load, you'd not have seen any drop when testing.

I'm not sure why your breaker didn't trip on full load, if as you said, you far exceeded the 20A rating. Were both your appliances at full draw?

Cheers,
Doc

May 23, 2012, 09:26:31 AM

X2 what Doc said.

You won't see a voltage drop unless you put a load on the user side of that supply line. Put your water heater or some heavy electrical device (hairdryer works good too) on the far end, and then measure the voltage.

Circuit breakers are designed to be a little forgiving. If they were not, they would trip every time you flipped on your incandescent kitchen lights. A single 100W light bulb can have in immediate inrush current of 12A or more.

When you are near the capacity of the circuit breaker, it can take a really long time for the breaker to trip. A 15 A breaker is designed to carry 15A, not trip at 15A. Breakers also have a letter designator that indicates at what current they will trip almost instantaneously (sp?). Instant being 100ms or so.

It would not be uncommon for your 15A breaker to allow 25A for awhile. Especially if you were dropping line voltage before you get to the loads (might have been less current than you think). This is not dangerous for most things, but can be hell on some motors. Oddly, if the voltage goes down, they demand more current.

Here is some reference :

http://ecmweb.com/mag/electric_sizing_circuit_breaker/

Here is a trip curve:

http://aircircuitbreakers.blogspot.com/2009/01/time-current-curves.html

Keep in mind that your loads on that line may have been somewhat intermittant as well. Best way to tell is to get a clamp on Ammeter. If you have any question about the integrity of the breaker, replace it. They are cheap enough!

AdironDoc · May 23, 2012, 09:46:41 AM

Agree completely. Breakers may take a while to trip unless a full short exists. I remember asking my father why GFCI breakers were necessary if a breaker is, in essence, the same thing. It's a degree of sensitivity with GFCI's tripping within milliseconds and at fairly low tolerance while breakers will handle lethal overloads for extended periods. Dad, never having the tester handy, was fond of touching the wires together. Pow! "yup, it's hot". I once dropped the end of my extension cord which, as luck would have it, fell into a puddle. The 15A breaker never tripped but the splice in the outlet it was plugged into melted.

MushCreek · July 07, 2012, 04:30:39 PM

An update- I'm using a cheap little fan, and it really slows down when a big electrical load kicks on, so I dug out the old VOM. The voltage is dropping 10-11 volts when my 1500 watt toaster oven kicks on. It's funny that the only appliance that shows a difference is the cheap little fan.

July 08, 2012, 08:29:57 AM

A GFCI isn't really a more sensitive breaker. It looks for an imbalance between the hot and the neutral. If everything is working correctly the current in both the hot and neutral should be equal. If there's a short to ground, it creates an imbalance between the two and the circuit trips. They are required in bathrooms and kitchens so if something gets submerged they trip quick---a breaker may not trip at all if the short isn't complete.
AFCI's are newer and different. Required in bedrooms now---they detect arcing in appliances, sort of an incomplete short that won't trip a breaker but can start a fire. Don't know how those work, some fancy circuitry. There's a neutral wire that comes off the breaker to go to the neutral bar. They aren't cheap.

flyingvan · July 08, 2012, 11:08:10 AM

Update---I was reading how these AFCI's work (I still don't know) but the WIKI article states not all US jurisdictions have adopted the NEC regarding arc fault interruptors for bedrooms. They seem like a good idea to me but I don't want to cost anyone $50 unecessarily

MushCreek · July 09, 2012, 05:47:03 AM

Locally, we are under NEC 2008, which does require AFCI's throughout. It also requires tamper-resistant outlets throughout. Next question- Do my GFCI's need to be on an AFCI breaker? I'm thinking yes.

flyingvan · July 09, 2012, 08:28:18 AM

No---unless there's some local requirement. GFCI's are for potentially wet locations like kitchens, bathrooms, outside. AFCI's are for bedrooms.
This is from the WIki article--"Starting with the 1999 version of the National Electrical Code (NFPA 70) in the United States (US), and the 2002 version of the Canadian Electrical Code in Canada (CSA Standard C22.1), the national codes require AFCIs in all circuits that feed outlets in bedrooms of dwelling units. This requirement is typically accomplished by using a kind of circuit-breaker (defined by UL 1699) in the breaker panel that provides combined arc-fault and overcurrent protection. Not all US jurisdictions have adopted the AFCI requirements of the NEC as written".

I have never heard of backing up GFCI's with an AFCI breaker. I wonder if you'd get a lot of false circuit trips since the AFCI's have circuitry that monitor the neutral, if the GFCI would see that as an imbalance whenever a load was put on it.