Question about AWG and Current Limits

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  #1  
Old 01-29-14, 05:56 PM
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Question Question about AWG and Current Limits

Hi DIYers,

I'm new to the electrical scene and had a few probably very basic questions about AWG and safety. From what I understand, the heat generated from copper wires is entirely resistive, so by P = I^2*R, there should be a maximum current for each wire gauge such that the heat generated per length causes the wire to melt and fail.

However, I am seeing some inconsistencies with how wires are being chosen for certain applications. For example, AWG tables state that maximum amps for 18 gauge chassis wiring is 32A and power transmission is 2.3A. An extension cord I have that is 18 gauge wire is max rated for 10A, while a portable space heater I have uses an 18 gauge power cord, but runs at 120V and 1500W, which by I = P/V is >10A.

The reason I ask is because I would like to find the correct gauge of extension cord to extend my space heater, but I am afraid 18 gauge is not enough, even though the power cord of the space heater is 18 gauge.

Any help would be greatly appreciated.

Thanks!

Rodger
 
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Old 01-29-14, 06:08 PM
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Welcome to the forums!

AWG tables state that maximum amps for 18 gauge chassis wiring is 32A and power transmission is 2.3A.
Besides the obvious question - "What's the source of those tables?" - the ratings for the conductors in extension cords are different. I'm not sure how your 18AWG extension cord got a rating of 10A. I wouldn't connect a 1500W continuous load with anything smaller than a cord with 12AWG conductors.

Here's a handy reference chart: Extension Cord Choices.
 
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Old 01-29-14, 06:17 PM
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1500w /120v=12.5amps

Most 1500w electric heaters come with #16 HPN black rubber/neoprene cable which is heat resistant.
 
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Old 01-29-14, 06:29 PM
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Hi Nashkat,

Thanks for your response. The website where I got the general AWG wire gauge info is here: American Wire Gauge table and AWG Electrical Current Load Limits with skin depth frequencies and wire strength. As for the extension cord, the markings on the cord label it as E135710-F3, but I can't find any info online about a current rating for that cord. However, a small tag on the cord says it is OK for 10A.

Thanks for the extension cord link, it is really helpful. However, just for curiosity sake, why is it ok to use 18 gauge wire for my 1500W space heater's power cord but not okay to use a 18 gauge extension cord (I will be using a short cord ~ 3 feet)? Increased resistance because of the longer wire?

Thanks,

Rodger
 
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Old 01-29-14, 06:35 PM
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Hi PJMax,

You really know your stuff. I just rechecked my heater's cord and it is indeed 16 gauge. As a follow-up question, does this mean that I can use a 16 gauge extension cord to extend my heater?

Thanks,

Rodger
 
  #6  
Old 01-29-14, 07:00 PM
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That cord on the heater is a high heat type cord. It won't melt if draped over heater and the cord itself won't get warm at a very short distance while the unit is in use.

We used to call it the 6' tap rule..... within 6' you can reduce one size. Since your circuit is at least on #14 wire..... the appliance can use one size smaller gauge wire.

The smallest wire I'd recommend at a 3' distance is #14.

I've found that the biggest problem with extension cords is not the wire size but the female cap. That's the end the heater plugs into. The quality of the caps is poor and doesn't properly connect to the plug. After you use a heater on an extension cord..... check where the plug plugs into the extension cord.... it'll be at least warm.
 
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Old 01-29-14, 07:22 PM
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Thanks PJ that makes a lot of sense. The actual wires themselves should be able to sustain the high current, so the weak link is likely the insulation or the connections. I will keep that in mind for my application.

Thanks again!

Rodger
 
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Old 01-29-14, 07:34 PM
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Different insulations will change the ampacity rating for the same size conductor.
 
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Old 01-29-14, 07:34 PM
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From what I understand, the heat generated from copper wires is entirely resistive, so by P = I^2*R, there should be a maximum current for each wire gauge such that the heat generated per length causes the wire to melt and fail.
Late to the show but I just wanted to point out that this is not entirely true. Yes, the wire gets hot due to the resistance of the of the wire, however the amount of current required to make a copper wire to melt is likely well into the hundreds of amps. That said, the insulation is what would start melting before the copper ever would.
 
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Old 01-29-14, 07:46 PM
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Thank you everyone for the great responses. I will be purchasing a 14 gauge extension cord for my application, which should be suitable. Glad I didn't try for the 18 gauge extension and asked here first. Thanks for the knowledge!

Rodger
 
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