new codes for #8 copper wire?

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Old 11-29-07, 09:29 PM
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new codes for #8 copper wire?

I was told by a clerk at the big orange box that I could run a 50 amp breaker at the main box with #8 copper wire. So I post it on this web sight and was told that it was wrong you cannot run a 50 circuit breaker with #8 wire because it is rated at 40 amps. So, I go back to the big orange and tell the clerk what I was told on this web sight and he procedes to show me in black and white the new NEC codes regarding sizing of breakers with wiring sizes. The clerk said, that the newer wire coatings was responsible for the change is in ratings.
 
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Old 11-29-07, 09:36 PM
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Are you talking about THHNs or romex?
 
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Old 11-29-07, 09:56 PM
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He's talking about UF (from the other thread).
 
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Old 11-30-07, 12:08 AM
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let get few facts straght here the UF-B is only rated for 40 amp that is unless you have THHN/THWN indivial conductors they can go higher but IMO i will leave this to baically 40 amp for majorty of resdentail area [ commercal and motor load they have a specal set of rules but i am not going to invoke a boatload of details to make it more confuseing as it now per other thread is this thread related story.


and right now most states is not adopting new 08 code cycle unless each state will make a note when they will take it in effect [ some will start first after of the year and some will be at later date so please check with your state for latest issue of code change.]

oh by the way Godavid ,.. i am sure the inspector in your area will nail ya hard if not carefull with the ratings on wire size. so please stop and drink a good coffee or soda and realx for a min and plan it out in nice way before something else will get worst.

Merci, Marc
 
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Old 11-30-07, 01:26 AM
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Fewer die from over engineering then bare minimum. The people here will help you keep it safe. Big Box is not the ultimate source of knowledge. An hour watching programs on a computer does not make an expert.
 
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Old 11-30-07, 05:53 AM
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You may not use 8 gage UF-B or NM-B for 50 amps. Period. The clerk at the store is wrong.
 
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Old 11-30-07, 08:52 AM
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Originally Posted by godavid View Post
I was told by a clerk at the big orange box that I could run a 50 amp breaker at the main box with #8 copper wire.
The gauge of the copper is only part of the story. You also need to consider the type of insulation on the conductor and the temperature ratings.

Certain #8 copper wires can carry 50A, most notably individual conductors of insulation type THHN installed in a conduit. These circuits are rated to 75C and therefore allow the higher current.

Other types of the #8 copper are only allowed to carry 40A. These include cable types NM-B and UF-B "Romex". These cables are only allowed to be rated for 60C so they must be restricted to lower current.

Article 334: Nonmetallic Cable ["Romex"]
334.80: Ampacity. The ampacity of Types NM, NMC and NMS cable shall be determined in accordance with 310.15. The ampacity shall be in accordance with the 60C (140F) conductor temperature rating.

Article 340: Underground Feeder Cable
340.80 Ampacity. The ampacity of Type UF cable shall be that of 60C (140F) conductors in accordance with 310.15.

Table 310.16: Allowable ampacities of insulated conductors...
Column 60C, Row 8AWG = 40A
 
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Old 11-30-07, 08:59 AM
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What wire gauge for what amps is a pretty simple story up through 30 amps. Above 30 amps, it's not as simple as looking up a number on a chart.
 
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Old 11-30-07, 12:28 PM
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Article 334: Nonmetallic Cable ["Romex"]
334.80: Ampacity. The ampacity of Types NM, NMC and NMS cable shall be determined in accordance with 310.15. The ampacity shall be in accordance with the 60C (140F) conductor temperature rating.

Based on this , How is all the commotion with Light fixtures requiring 90C wiring resolved?
 
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Old 11-30-07, 12:38 PM
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Type NM-B cable is actually rated for 90C (194F) by the manufacturer. However to ensure a margin of safety and compatibility with screw terminals on switches and receptacles, the code requires you to treat it as though it is only rated to 60C when calculating the ampacity. Something like setting the weight limit in an elevator to half of what it can actually lift. This allows plenty of wiggle room for cables installed inside insulation, above light fixtures and various other places where the cable may be exposed to external heat sources.

The "-B" in NM-B and UF-B indicates the 90C rating which can be exposed to high temp light fixtures. Older plain NM cable (>20 years old) is actually only rated to 60C.
 
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