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Electrical question - wire gauge, amps, and heat?
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dana_leighton
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Posted Mon Dec 27, 2010, 6:20pm
Subject: Electrical question - wire gauge, amps, and heat?
 

I have a question for the electrically-minded. I have a roaster controller I built, which runs my 1500 watt Poppery I. The schematic is very similar to the diagram here. The path in question is Main power --> Fuse holder --> Power switch

The problem I encountered is that today while roasting I saw a few puffs of smoke and a burning smell. I shut down the roaster, and opened the control box. The wire running from the main power plug to the fuse holder has melted its insulation. A photo is below. This would make me think the wire is carrying too much current, but it does not make sense to me. Here's why:

The wire is 14 gauge, and UL rated THHN, THWN, MTW, or AWM that I got at Lowes. The fuse is 15A, and the fuse holder is rated at 15A at 125V. The wire coming from the fuse holder to the main power switch did not have melted insulation. All other wire of the same type was undamaged. Using a Kill-A-Watt, I monitored the current draw and it was running around 13A when the heater was running on its highest heat setting.

From the research I've done, a 14 gauge wire should be able to handle more than 15 amps. Any ideas why this wire melted its insulation? There is evidence the fuse holder was heating up: The vinyl covering of the quick connect on the output wire near the fuse holder was brittle and discolored, but not on the quick connect at the switch end of the wire.

One other anomaly I noticed is that the fuse holder has 2 sized of quick connect lugs: on the input side (connected to the melted wire), the lugs are smaller (.187) than the output side (.250). Perhaps that introduced some resistance?

Thanks for any help you all might provide.

dana_leighton: DSC00004.jpg
(Click for larger image)

 
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randytsuch
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Posted Mon Dec 27, 2010, 6:49pm
Subject: Re: Electrical question - wire gauge, amps, and heat?
 

In my roaster control box, which is similar to yours, I used 12 guage thhn wire, and have had no problems.

But, you're right that 14 guage wire should be fine, not sure why it would melt like that.

One other thing, I didn't use any quick connect type connectors in my box, I either soldered the wire, or used wire nuts to make the connections.

Randy
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Snaxx
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Posted Mon Dec 27, 2010, 8:19pm
Subject: Re: Electrical question - wire gauge, amps, and heat?
 

Dana,

What's the purpose of the fuse?  The outlet you're plugging into is already on at least a 15 amp breaker or fuse, and most likely if in a kitchen, it is on a 20 amp circuit. If you had a problem with something shorting out or the heating element pulling too many amps, the breaker or fuse at the main house panel would trip. (or should!)

As to why the wiring going in and out of the fuseholder started melting, it's probably likely, with the 1500 watt heater load, the connections to the fuse holder loosened up just a bit after running the roaster, because current flow through the quick-connects had just enough resistance to start the heating process, and it just progressed from there to a fire.  The worst part of the insulation char is at the fuseholder and beyond, which does indicate the connections at those points are the likely suspects.  The lack of char at the connection from the power cord just means you had a better connection at that point.  

Ken
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JKalpin
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Posted Mon Dec 27, 2010, 8:57pm
Subject: Re: Electrical question - wire gauge, amps, and heat?
 

...or it might be that something in that wiring was just faulty.  One of the conductors could have been (accidentally) 16 ga instead of 14.  Plug-in connections might have made inadequate contact.

If it were me I would simply build another one.  Use 12 ga wire.  Reduce (or solder) plug-in connections where possible.  When in first operation (assuming it is all insulated) I would feel along the assembly to detect any unusual heating.

 
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dana_leighton
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dana_leighton
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Posted Mon Dec 27, 2010, 9:15pm
Subject: Re: Electrical question - wire gauge, amps, and heat?
 

Snaxx Said:

What's the purpose of the fuse?

Posted December 27, 2010 link

I used the fuse inline because I wanted an extra layer of protection (I'm a belt and suspenders kind of guy) so that in the event the house circuit didn't trip or was slow in doing so the inline fuse would, or would go before the house circuit did.

it's probably likely, with the 1500 watt heater load, the connections to the fuse holder loosened up just a bit after running the roaster, because current flow through the quick-connects had just enough resistance to start the heating process, and it just progressed from there

OK - that makes sense, and argues for Jerry's idea below of soldering to reduce quick connects.

JKalpin Said:

Plug-in connections might have made inadequate contact.

If it were me I would simply build another one.  Use 12 ga wire.  Reduce (or solder) plug-in connections where possible.  When in first operation (assuming it is all insulated) I would feel along the assembly to detect any unusual heating.

Posted December 27, 2010 link

Thanks Randy, Ken, & Jerry - I think I will go ahead and rewire the connection. The quick connects makes it much easier to take it apart if it ever needs maintenance, but if they'll cause an overheated wire, it's not worth it.

Cheers,
Dana.

 
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PJK
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Posted Mon Dec 27, 2010, 11:03pm
Subject: Re: Electrical question - wire gauge, amps, and heat?
 

Hi Dana,

As the others have said the connections must have low resistance.  The blade connectors should be OK but they must be tight.  Keep in mind that 10 milli-ohms will give more than two watts of heat with 15 amperes flowing throug it.  As the connector starts to heat the spring tension gets less which gives rise to more resistance and possible thermal runaway.

I also want to comment on the dimmer feeding the transformer.  It may or not be OK.  Regular dimmers may not be be perfectly symetrical giving rise to DC in the transformer primary. Incandescent lamps "dont care" about a DC component but transformer primarys do.  There are dimmers made for low voltage incandescent lights using standard 60 Hz transformers which address this problem.  There are electronic transformers which  require a different dimmer.  Be sure you get the type for the regular 60 Hz transformers.  The dimmer which you have may be OK but the symmetry or distortion is not controlled if it is just for standard incandescents. You can check for DC in the transformer or see if the transformer is getting hot.  If the transformer is not getting hot you are OK and the grunt dimmer is about one third the price of the dimmer for the low voltage lights.  Some people have been dissing Radio Shack transformers used in this circuit.  I think a good bit of the problem they have is the dimmer.  

Phil

 
Philip J. Keleshian
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dana_leighton
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Posted Mon Dec 27, 2010, 11:50pm
Subject: Re: Electrical question - wire gauge, amps, and heat?
 

PJK Said:

I also want to comment on the dimmer feeding the transformer.  It may or not be OK.  Regular dimmers may not be be perfectly symetrical giving rise to DC in the transformer primary.

Posted December 27, 2010 link

Hi Phil - Thanks for the advice. When I did the construction I used a Lutron variable fan speed controller instead of a regular light dimmer, because I read some of the problems wioth trying to control an AC motor from a regular light dimmer. Do you think that makes a difference? As I understand it, it's designed for delivering AC current suitable for motors, and I assume not introducing harmonics problems, but I am not sure that's what you're referring to. I couldn't find a circuit diagram online to know what these controllers are actually doing to the wave form.

I haven't noticed a big heat problem in the box, but it's mostly a closed box so I might not unless it's a major problem. I could also place the fan speed control after the transformer, before the fan. Thoughts?

 
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wbaguhn
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Posted Tue Dec 28, 2010, 12:37am
Subject: Re: Electrical question - wire gauge, amps, and heat?
 

I've seen similar failures when the conductors are damaged during stripping, crimping or bending.

14 AWG seems borderline in this application - I typically hear numbers like 12 AWG for 13A loads. MTW is good, but building wire generally is less flexible (more prone to damage) than the fine-strand cable I've often seen inside industrial wiring panels.  Bigger conductors should get less warm for a given current.  (<80% seems to be a number the guys on the other side of the wall aim for in designs; 13A is 86% of 15A, but 65% of 20A (12AWG).)

I'm not an engineer, I'm not an electrician, and I accept no responsibility for how you (mis)use this information.  I do know that my heating element burned out before my control box.
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PJK
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Posted Tue Dec 28, 2010, 1:29am
Subject: Re: Electrical question - wire gauge, amps, and heat?
 

Hi Dana,

I think the fan control should be fine.  Celing fans use induction motors which would be no more DC tolerent than a transformer.  

I looked at your schematic again.  I assume the step up transformer connected like the one Mike McGunnes posted some time back and not as shown.

I'll look for his schemo.

Phil



dana_leighton Said:

Hi Phil - Thanks for the advice. When I did the construction I used a Lutron variable fan speed controller instead of a regular light dimmer, because I read some of the problems wioth trying to control an AC motor from a regular light dimmer. Do you think that makes a difference? As I understand it, it's designed for delivering AC current suitable for motors, and I assume not introducing harmonics problems, but I am not sure that's what you're referring to. I couldn't find a circuit diagram online to know what these controllers are actually doing to the wave form.

I haven't noticed a big heat problem in the box, but it's mostly a closed box so I might not unless it's a major problem. I could also place the fan speed control after the transformer, before the fan. Thoughts?

Posted December 27, 2010 link


 
Philip J. Keleshian
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PJK
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PJK
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Posted Tue Dec 28, 2010, 10:34am
Subject: Re: Electrical question - wire gauge, amps, and heat?
 

I found it:

Click Here (mdmint.home.comcast.net)

I notice that Mike put the dimmer after the transformer.  With that arrangement any DC resulting from an unsymetrical dimmer will only pass through the relatively few turns of the secondary, so a cheap lamp dimmer should be OK.   The DC will also pass through the fan motor but that is no big deal since the fan motor is really an AC/ DC motor.

Phil







PJK Said:

Hi Dana,

I think the fan control should be fine.  Celing fans use induction motors which would be no more DC tolerent than a transformer.  

I looked at your schematic again.  I assume the step up transformer connected like the one Mike McGunnes posted some time back and not as shown.

I'll look for his schemo.

Phil

Posted December 28, 2010 link


 
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