Can you use the 90 degree collum when sizeing the conductors for a 25 ft tap. Since the load will never come close to the 90 degree rating . Example 800 amp 120/208 to a 200 amp fused switch. 1/3 would be 266 amp 250 mcm 75 degree is 255. at 90 degree 290. My load would never exceed the 200 amp fused sw so the rating of the terminials would not be a factor?
applying the 25 ft tap rule
- Thread starter marcs11
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- New Jersey
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- Journeyman Electrician
Isn't the 90? conductor really only a 75? conductor when connected to a 75? terminal? I know that the code is somewhat silent on this when it comes to tap rules.
RichB
Senior Member
- Location
- Tacoma, Wa
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- Electrician/Electrical Inspector
never thought about it--90 degree used just for derates--always used the 75 degree column for everything else
- Location
- Chapel Hill, NC
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- Retired Electrical Contractor
I don't believe that would be code compliant unless the taps were connected to 90C terminals. I guess you could bug the wire with 90C terminals and then bug a small piece of 300 kcm with another 90C bug and it would work.
BTW- I believe Polaris connectors are 90C rated
BTW- I believe Polaris connectors are 90C rated
Last edited:
chris kennedy
Senior Member
- Location
- Miami Fla.
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- 60 yr old tool twisting electrician
All except the 750's.
- Location
- Massachusetts
OK, I have to do it. 
How long of a section of larger conductor is required?
How long of a section of larger conductor is required?
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
FWIW
The length used in the UL test for breaker and fusible switch terminations is 4ft. I think this would be a very defensible choice.
The UL test takes into account heat transfer as well as conductor 'pull out' during short circuits.
- Location
- Chapel Hill, NC
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- Retired Electrical Contractor
I am lost on the last 2 posts. What difference does it make if the larger conductor was only 2 inches long if the polaris and the wire is 90C.
- Location
- Massachusetts
The higher heat of the conductor running at 90 c rating will pass through into the next device such as a breaker. .
You need some distance to give the heat a place to radiate off from.
Always remember copper conductors are excellent conductors of heat as well as electricity.
You need some distance to give the heat a place to radiate off from.
Always remember copper conductors are excellent conductors of heat as well as electricity.
chris kennedy
Senior Member
- Location
- Miami Fla.
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- 60 yr old tool twisting electrician
Could you elaborate on this statement.
- Location
- Chapel Hill, NC
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- Retired Electrical Contractor
The higher heat of the conductor running at 90 c rating will pass through into the next device such as a breaker. .
You need some distance to give the heat a place to radiate off from.
Always remember copper conductors are excellent conductors of heat as well as electricity.
Thank you
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
You can defend your choice of 4ft by saying that it is the same as the breaker was tested at.
Another FWIW.
Per UL requirements, the terminals on a UL489 breaker can only be 50?C which is why they need the conductor to be a heat sink. This pretty much busts the 'myth' that a fully loaded conductor runs at the temperature of its insulation.
chris kennedy
Senior Member
- Location
- Miami Fla.
- Occupation
- 60 yr old tool twisting electrician
Thank you sir.
Thanks every one for your informative responces. I'm not sure why the the terminations are such an issue in this case? article 240-21 [2] [1] the ampacity of the tap conductors is not less than one-third of the rating of the overcurrent device protecting the feeder conductors. You should be able to use the conductor 90 degree rating as you would in de-rating. The insulation thhn gives the conductor the ability to handle 290 amps with out break down. Since my load is limited by my 200 amp switch the rating of my terminations should not be an issue. I only need to comply with my 1/3 of 800 amp which is 266 amps. can any one give a code articial that says I can not use the 90 degree collom in this case?
Thanks
marcs11
Thanks
marcs11
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
I believe you know this code article-- it is 240.21(B)(2). Then, Art. 110.14 limits the conductor ampacity to 75C unless the terminals are rated higher. So you cannot use the 90C rating if the terminals are rated 75C otherwise the conductor is not rated 1/3 of the OCPD.
Basically you want to say the load is only rated 200 amps so why can't you use the 90C-- well that is not what the nec requires- it is based on OCPD and ampacity of the conductor.
Basically you want to say the load is only rated 200 amps so why can't you use the 90C-- well that is not what the nec requires- it is based on OCPD and ampacity of the conductor.
iceworm
Curmudgeon still using printed IEEE Color Books
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- North of the 65 parallel
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- EE (Field - as little design as possible)
OK, I have to do it.
How long of a section of larger conductor is required?
I have always applied 310.15.A.2, 10 feet or 10% of the length, which ever is less.
ice
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
I have never read this section that way.
To me, 310.15.A.2 says that you can ignore the smaller ampacity rating if the length is 10ft or 10% of the circuit, which ever is shorter.
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
I agree. Basically if the conductor passes thru an area where the ambient temp is high and derating will be needed then you can forgo that if the conductor only goes thru 10' on a hundred foot run, or 5 feet in a 50 foot run. This section also applies to other factors that may affect derating.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
I didn't think that Article 240 and the taps rules were explicit on determining the tap conductor ampacity from the terminals.
iceworm
Curmudgeon still using printed IEEE Color Books
- Location
- North of the 65 parallel
- Occupation
- EE (Field - as little design as possible)
You're absolutely correct in your interpertation. And your probably right my interpertation is not included. I've probably been clear screwed up the few times this has come up. And I agree that 4 feet will be defensible when (if) it burns up.
In my defense, I can say the thought never occured to me that I would ever have to defend my interpretation.
ice
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