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02-21-2017, 07:08 AM
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#15
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Senior Member
Join Date: Jul 2014
Posts: 35,349
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Quote:
Originally Posted by xrated
Hope this helps...
Typical 1500 watt heater being supplied with 120vac draws 12.5 amps, ignoring any additional current from a fan.
Now let's introduce a loose connection at the receptacle that the heater is plugged into.
The loose connection causes a voltage drop at the receptacle, so just for the sake of this discussion, let's say we now have 95 volts instead of 120.
The heater doesn't know anything different, so it's still trying to consume 1500W of power, which it will do, at least for a short time. So, take those numbers...95 volts and 1500 watts and do the math. The heater is now drawing 15.78 amps. That will work until the 15 amp breaker trips from overcurrent
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I'm not buying that theory either.
A dead short back to neutral will increase amps but a second source of heat, the bad connection, is just a series connection.
Take an electric welder. When the welding rod is first touched to the steel, the amps go high ( dead short ). Once the arc is developed the amps drop.
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02-21-2017, 10:14 AM
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#16
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Senior Member
Join Date: Jan 2017
Location: "Murvul", TN
Posts: 1,658
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Quote:
Originally Posted by twinboat
I'm not buying that theory either.
A dead short back to neutral will increase amps but a second source of heat, the bad connection, is just a series connection.
Take an electric welder. When the welding rod is first touched to the steel, the amps go high ( dead short ). Once the arc is developed the amps drop.
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Electric welders are a different animal. They usually fall under one of two types...constant current or constant voltage machines and really aren't relevant to this discussion.
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02-21-2017, 10:43 AM
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#18
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Senior Member
Join Date: May 2001
Location: Spicewood, Tx
Posts: 708
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Quote:
Originally Posted by xrated
Hope this helps...
Typical 1500 watt heater being supplied with 120vac draws 12.5 amps, ignoring any additional current from a fan.
Now let's introduce a loose connection at the receptacle that the heater is plugged into.
The loose connection causes a voltage drop at the receptacle, so just for the sake of this discussion, let's say we now have 95 volts instead of 120.
The heater doesn't know anything different, so it's still trying to consume 1500W of power, which it will do, at least for a short time. So, take those numbers...95 volts and 1500 watts and do the math. The heater is now drawing 15.78 amps. That will work until the 15 amp breaker trips from overcurrent
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Sorry, but you are absolutely wrong. The heating element is a large resistor and it does not have the ability to change resistance value significantly due to a lower voltage being applied to it. It would have to drop by almost 60% for the current to increase from 12.5 amps to 15.78 amps. No resistor draws more current at a lower voltage than it does at a higher voltage. At 95 volts, the element would only draw about 10 amps and certainly not that 15.78 amps.
__________________
Larry Day, Texas Baptist Men volunteer
'13 Silverado LT 3500HD D/A CCSB 2wd, custom RKI bed
'19 Starcraft Telluride 292RLS
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02-21-2017, 11:17 AM
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#19
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Senior Member
Join Date: Nov 2015
Posts: 1,111
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So what we're dealing with is a cct bkr that is thermal activated. (heat triggered) When the load is reduced the heating element in the bkr dose not get as hot. Introducing additional resistance in the cct like a loose termination will cause the breaker to run cooler. Another good example of cct protection for bedrooms in a S&B ,,,, Many municipality's have made it mandatory to install AFCI breakers. AFCI's senses fault's like sparking wires/loose connections that don't draw enough enough power to trip a 15 amp breaker. Connections that "sizzle"cause power spikes that the AFCI detects and trips to prevent fire ,like from a splice, defective outlet, or frayed wires under a rug..
AFCI's also trip on continuous over current.
CLIFFORD
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02-21-2017, 01:25 PM
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#20
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Senior Member
Join Date: Jan 2017
Location: "Murvul", TN
Posts: 1,658
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Well, I've been wrong many times before in my life. It's been late 70s since my formal training and I mostly deal with high voltage stuff not residential. Sorry
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02-21-2017, 01:34 PM
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#21
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Registered User
Join Date: Nov 2016
Posts: 520
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Quote:
Originally Posted by Codtee
Funny story you were wrong, there was a loose connection in the plug at the end of the circuit. So I guess you shouldn't automatically think people are retarded huh. The propane furnace does not work at the moment either due to we are waiting on parts. Just thought you should know. Have a great day.
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Codtee
I guess that means you should always check the connection in the plug at the end of the circuit before you ask questions.
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02-21-2017, 02:13 PM
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#22
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Senior Member
Join Date: Sep 2012
Posts: 8,055
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Quote:
Originally Posted by clifford j
So what we're dealing with is a cct bkr that is thermal activated. (heat triggered) When the load is reduced the heating element in the bkr dose not get as hot. Introducing additional resistance in the cct like a loose termination will cause the breaker to run cooler. Another good example of cct protection for bedrooms in a S&B ,,,, Many municipality's have made it mandatory to install AFCI breakers. AFCI's senses fault's like sparking wires/loose connections that don't draw enough enough power to trip a 15 amp breaker. Connections that "sizzle"cause power spikes that the AFCI detects and trips to prevent fire ,like from a splice, defective outlet, or frayed wires under a rug..
AFCI's also trip on continuous over current.
CLIFFORD
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The problem is I squared R loss. In essence the power dissipated in a connection is the current squared times the resistance. a 10 amp heater current with a half ohm resistance from a bad connection will produce 50 watts of heat. A quarter ohm will be 25 watts. Those numbers can be real for a poor connection. That amount of heat on a small surface = high temperatures with maybe a fire. In the case of a thermal circuit breaker it will cause it to trip at a lower current rating.
A known issue with screw type compression terminals is that the wire relaxes into the shape of the connection thus getting looser. That provides the small extra resistance that creates problems. The answer is to tighten the terminals a few times after installation and spaced in time so the problem has a chance to occur then get corrected. (weeks to months) At some point it will probably not respond to being tightened any more. Tight is snug. Over tightening breaks things well before one reaches the point where it would help
FWIW properly made pressure crimp terminals don't have the problem because the initial crimping force is high enough to deform the wire bundle and terminal into a solid mass. Screw terminals don't have that much clamping force.
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02-21-2017, 03:02 PM
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#23
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Senior Member
Join Date: Nov 2015
Posts: 1,111
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This has been a great discussion, we've all learned something. Now in my late 70's I've learned I've killed too many brain cells :>( Many many years ago I was a tech with The Hubbell Company, mfgr's of industrial power plugs/connectors/switches. Our R&D group did continual testing of conductor terminations. It was just mentioned here that re tightening connections is a good thing. True. Years ago it was discovered that house wiring that used aluminum wire became a hazard. The aluminum small gauge wire had 'cold flow' meaning over time a tight connection would relax, oxidize and the termination would generate high temps causing fires in some cases. This caused new standards and what's now called CO/AL terminal identification , a terminal that is compatible with either material. A termination like a wire nut or a squeeze on connector are excellent connections. A wire nut keeps constant tension for its life. A squeeze type like a 3M crimp type pierce the insulation, grip the conductor and maintain contact pressure. Ordinary crimp connectors using a hand tool for crimping must be done with care to ensure a tight bundle.
This strays from the OP's original problem but some of the information apply's
CLIFFORD
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