Good Read about Circuit Breakers (Debunking Myths)

[ChooChooMan74]

My last camping trip for Halloween weekend, I brought 2 electric heaters with me. I plugged one into a regular wall outlet, and one into the outlet for the electric hot water heater (I would swap out one heater for electric HW during the day, and use residual hot water at night and replug in the heater). I had both heaters on high, 2 heated beds on, refer on electric, and the converter on. I looked at my Progressive Industries monitor, and noticed that I was pulling 38 amps. I was wondering, why am I not tripping anything. I lowered my power use and got under 30 amps, but I needed to know more. So I investigated. This is what I found.

Goodson Engineering wrote:
The first (and most common) misconception is that a breaker trips when its nameplate rating is exceeded. One fire text has stated (incorrectly) that a circuit breaker will trip in several minutes with a small increase in current over its rating[1]. Actually, a 20 amp breaker must trip at a sustained current of 27 amperes (135 percent) at less than one hour, and at 40 amperes (200 percent of wire rating) in less than 120 seconds—far different from what the cited text implies. These two trip points (135 percent and 200 percent) are defined in NEMA Standard AB-1, MCCBs and Molded Case Switches[2]

SOURCE: Goodson Engineering - Forensic Engineering * Fire Investigations

MOD Idea: Put a separate 20 amp into the camper for purposes of running 2 heaters. I know some have done it for a 2nd AC on a 30 amp unit before.

[john_bet]

kellertx5er wrote:
Many myths and electrical misinformation posted in here. My first ever post addressed the myth that "circuit breakers trip at 80% of rating." Depending on any of many factors (age, temp, mfg., etc etc) a breaker could trip at 10% or never trip at all. The misinformation gets even worse when discussing conductor loading and ratings. Any breaker in reasonably good condition should easily carry 100% of its rating continuously at temps <= 104 F.

Pop quiz: according to the NEC, what is the ampere rating for #10 type THHN conductor (<=3 conductors in raceway, ambient temperature 86 deg F)?

a) 25A
b) 30A
c) 35A
d) 40A

A better question is, what is the ampere rating per the NEC of your typical 10/3 cord used as an RV shore cord? Or a 6/4 cord used in 50 amp service for RV's?

[ktmrfs]

ChooChooMan74 wrote:

kellertx5er wrote:
Many myths and electrical misinformation posted in here. My first ever post addressed the myth that "circuit breakers trip at 80% of rating." Depending on any of many factors (age, temp, mfg., etc etc) a breaker could trip at 10% or never trip at all. The misinformation gets even worse when discussing conductor loading and ratings. Any breaker in reasonably good condition should easily carry 100% of its rating continuously at temps <= 104 F.

Pop quiz: according to the NEC, what is the ampere rating for #10 type THHN conductor (<=3 conductors in raceway, ambient temperature 86 deg F)?

a) 25A
b) 30A
c) 35A
d) 40A

Just looked it up. THHN says 40 Amps #10. Is that the new wiring standard? THHN wire? I am trying to find out what the TW UF wires are, that are rated for 30 Amps at #10.

amp rating depends on how the wire is run. std romex has a lower rating than single insulated conductors in a raceway. part/all of the difference is related to the temp rise in the wire at rated current. romex has 3 conductors in an insulated sheath so it will have a higher temp rise than conductors in free air. and romex can end up being run through insulation as well.

[MEXICOWANDERER]

Ummm, I'm into INsignificant voltage drop. No way in hell am I going to force 10AWG to swallow 40 amperes. I wired Quicksilver with 10/3 marine wire to feed a max of three 20-amp receptacles each breaker. Even the 127vac plug is a cheater, because Mexican receptacles are looser than 10 seamen in Po City. Install a good receptacle in a campground, the minute you leave it is exchanged for a garbage receptacle from the owner's house.

I scan all my breakers and all my switches. The shore power plug and receptacle. If I detect heat, then something is wrong.

[wa8yxm]

MOD Idea: Put a separate 20 amp into the camper for purposes of running 2 heaters. I know some have done it for a 2nd AC on a 30 amp unit before.

Same here but more so.. When I'm on 50 amp site I have two additional circuits 15/20 amp duplex outlets (Contrasting color from regular ones to make 'em easy to spot) and 12ga wire straight back to the breaker, One is 15 amp one 20.

Plus a added 15 amp cord and an "Break out" point for my 2nd A/C which also feeds a special 15/20 amp outlet (Single not duplex) cause I do not use Heat and A/C at the same time (and if I did the 20 amp breaker will never hold)

[ChooChooMan74]

pianotuna wrote:
Hi,

It behooves you to replace that "standard" outlet with something sturdier than the "stab" type connectors that may exist in most RV's. I've replaced all of mine except for one.

If there is a 30 amp service for continuous loads it may be best to run at 80% of that capacity (24 amps). Assuming 120 volts that allows 2880 watts for all loads.

It is good practise to check all devices in an RV for wattage using a kill-a-watt meter or similar device. The name plate wattage is approximate on every device I have checked.

I have three shore cords now. The OEM 30, a 20 and a 15 amp.

ChooChooMan74 wrote:
I brought 2 electric heaters with me. I plugged one into a regular wall outlet

PianoTuna, I think I will do that, also. At least the ones I use high amperage devices on like the heater or coffee maker (Thinking about getting rid of that, too, and just perking coffee on the stove)

[ChooChooMan74]

kellertx5er wrote:
Many myths and electrical misinformation posted in here. My first ever post addressed the myth that "circuit breakers trip at 80% of rating." Depending on any of many factors (age, temp, mfg., etc etc) a breaker could trip at 10% or never trip at all. The misinformation gets even worse when discussing conductor loading and ratings. Any breaker in reasonably good condition should easily carry 100% of its rating continuously at temps <= 104 F.

Pop quiz: according to the NEC, what is the ampere rating for #10 type THHN conductor (<=3 conductors in raceway, ambient temperature 86 deg F)?

a) 25A
b) 30A
c) 35A
d) 40A

Just looked it up. THHN says 40 Amps #10. Is that the new wiring standard? THHN wire? I am trying to find out what the TW UF wires are, that are rated for 30 Amps at #10.

[pianotuna]

Hi,

It behooves you to replace that "standard" outlet with something sturdier than the "stab" type connectors that may exist in most RV's. I've replaced all of mine except for one.

If there is a 30 amp service for continuous loads it may be best to run at 80% of that capacity (24 amps). Assuming 120 volts that allows 2880 watts for all loads.

It is good practise to check all devices in an RV for wattage using a kill-a-watt meter or similar device. The name plate wattage is approximate on every device I have checked.

I have three shore cords now. The OEM 30, a 20 and a 15 amp.

ChooChooMan74 wrote:
I brought 2 electric heaters with me. I plugged one into a regular wall outlet

[kellertx5er]

road-runner wrote:
The breaker's thermal overload trip current is specified at 104 degrees F and is greater when the temperature is lower, which would be case when wanting to run a heater. The following document has current vs. temperature graphs on page 6, and the complete trip curves starting on page 19.

http://www.schneider-electric.com.mx/documents/local/qo.pdf

Good reference except ignore the curve on p. 19, it is incomplete. The 'real' curves start on p. 20.

[kellertx5er]

Many myths and electrical misinformation posted in here. My first ever post addressed the myth that "circuit breakers trip at 80% of rating." Depending on any of many factors (age, temp, mfg., etc etc) a breaker could trip at 10% or never trip at all. The misinformation gets even worse when discussing conductor loading and ratings. Any breaker in reasonably good condition should easily carry 100% of its rating continuously at temps <= 104 F.

Pop quiz: according to the NEC, what is the ampere rating for #10 type THHN conductor (<=3 conductors in raceway, ambient temperature 86 deg F)?

a) 25A
b) 30A
c) 35A
d) 40A

[gotsmart]

I centrally located my 1500W ceramic space heater to provide all-around heat distribution. This places it near enough to the kitchen sink - a "wet" zone - to cause concern. Because of this I wanted GFCI protection added to it. I found this at Walmart. It's a single plug 15amp GFCI extension cord.

At the time I did not think about going to a home center. I just checked Home Depot using http://www.homedepot.com/s/gfci%2520cord. They have several similar products.

[road-runner]

The breaker's thermal overload trip current is specified at 104 degrees F and is greater when the temperature is lower, which would be case when wanting to run a heater. The following document has current vs. temperature graphs on page 6, and the complete trip curves starting on page 19.

http://www.schneider-electric.com.mx/documents/local/qo.pdf

[allenm]

I installed a separate 12 gauge line for my heater, It is connected in the 4x4 box where my black cord terminates and is protected with a 15 amp "fast-blow" fuse inside the box.

Except for the microwave and a/c, all my factory outlets have 14 gauge wire. To make matters worse, they use cheap outlets with push-in connectors for the wire instead of screws.

A few years ago on another camper, I had a 1200 watt grill plugged in my outside outlet. I smelled something electrical burning under the kit sink and saw the push-in wires arcing and smoking. Don't ever plug an electrical heater or grill into a factory RV outlet!

[RoyB]

Interesting comments from the Electric Engineering guys... I knew there was longer trip zone involved but didn't know it was that different than the rated trip zone.

I know my main 30AMP breaker will trip - Have that happen on occasion...

I have been using a separate HD Extension Cords connected to the camp ground 20A SERVICE connection for years here on the East side of the US where it is probably alot warmer camping nights than out West. This is primarily for my two oil-filled electric heaters. Wiring in an outside MALE Weathertight Connector would be nice I guess but I just run extension cords into my OFF-ROAD TRAILER through the window flaps. Have heard from other users on here that some campgrounds may ask you to stop using the 20A service in this fashion but I have never run into that...

The one difference from what you was saying is I NEVER set these electric heaters on HIGH. I run then at mid settings. I do not like the idea of heating up the electric cords near the 120VAC receptacle connections. We dress for the cool weather trips and like to get under the blankets at night.

I also use the campground 20AMP SERVICE for almost all of our outside 120VAC requirements...

I set the propane trailer heater on its lowest setting and this is only used as absolute back-up heat. I would use more of the propane heat if we were camping in very cold areas where the underbelly area would need some heat but since we do all of camping in our OFF-ROAD POPUP trailer this is not an issue for us.

Just my thoughts on the way we like to camp in cold weather - Give me a big bon-fire and a long poking stick...

Roy Ken