corrections for simulated exam

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marsbar42

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I have Mike Holt's Master/Contractor Simulated Exam. Based on the 2005 NEC. Questions 64 and 65 on page 38 ask for answers dealing with THHN. In your answers on page 51, you list the answer in THHN but your calculations are dealing with 75 degree wire. Not 90 (THHN). Is this a mistake?
 

bob

Senior Member
Location
Alabama
Re: corrections for simulated exam

There are only a few times that the 90C rating is used. The conductor is limited to the 75C rating because the connecting ternimals are rated at 75C. If you had a piece of equipment that had its terminations rated at 90C you could use the 90C rating.
 

roger

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Re: corrections for simulated exam

If the questions deal with derating, the 90 deg column can be used as a starting point, but the 75 deg column can not be exceeded.


Roger

[ December 09, 2005, 02:03 PM: Message edited by: roger ]
 

marsbar42

Member
Re: corrections for simulated exam

the question is: The minimum size branch circuit conductor required for a 50A resistance welder having a duty cycle of 50% is (a) 6THHN (b) 8 THHN (c) 4 THHN (d) 2 THHN The answer given is (b) 8THHN....It states that 8 AWG at 75 degrees is rated at 50A (Table 310.16)...I don't see it. 8 THHN is rated at 90 not 75.
 

marsbar42

Member
Re: corrections for simulated exam

The other question is....The minimum size feeder conductor for five 50A resistance welders with a duty cycle of 50% is...(a) 2 THHN (b) 3 THHN (c) 1 THHN (d) 4 THHN....The answer given is (c) 1 THHN.... They say the conductor size from Table 310.16 is 1AWG at 75 degrees is rated 130A...If the total calculated load is 120.7A...why wouldn't you use 2THHN instead of 1THHN...2 is rated for 130A
 

bob

Senior Member
Location
Alabama
Re: corrections for simulated exam

If you read my first post you should see why you can not use the 90 degree rating. The 75C rating is the one you must use.
 

marsbar42

Member
Re: corrections for simulated exam

I'm not sure you're understanding my question...The answers are all for THHN...THHN is rated 90 degrees.
 

JES2727

Senior Member
Location
NJ
Re: corrections for simulated exam

Mr Marsbar,
Even though THHN is rated for 90 degrees C, we are still limited by the temperature rating of the terminations. Take a look at 110.14(C), (2002 edition). We must use the lower temperature rating. We can use the 90 degree column for derating purposes, but in the end we can not exceed the 75 degree rating, regardless of the insulation.

John
 

roger

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Re: corrections for simulated exam

As everyone is saying, even though the insulation may be rated for 90 deg, it can not be used at this rating due to the fact that the terminations will not be rated 90 deg.

6_15_34_3_2.gif


Roger
 

infinity

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Re: corrections for simulated exam

I would also add that as Roger has shown in his illustration this is for conduit and wire installations and other cables types but not for type NM cable. NM cable would have 90 degree conductors but must be used at their 60 degree ampacity due to 334.80.
 

roger

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Re: corrections for simulated exam

Good point Trevor.

Roger
 

marsbar42

Member
Re: corrections for simulated exam

Okay...I think I'm finally understanding this...the 110.14 really helped, but in the questions they don't specify the terminal ratings...why wouldn't you use the 60 degree column instead of the 75. If you use 60 degree wouldn't the answer for the second question (120.7A) be 1/0 AWG and not 1AWG? Also, why wouldn't the answer for the first question (25.5A) be 10AWG instead of 8. P.S...Thanks to everyone who's been trying to help me with this, I really appreciate this.
 

roger

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Re: corrections for simulated exam

Marsbar, you need to recalculate the welder.

If I'm understanding the question correctly, 25.5A is incorrect.

See 630.31

Roger
 

roger

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Re: corrections for simulated exam

The answer to the second question uses the 75 deg column because of 110.14(C)(1)(b)

(b) Termination provisions of equipment for circuits rated over 100 amperes, or marked for conductors larger than 1 AWG, shall be used only for one of the following:

(1) Conductors rated 75?C (167?F)

(2) Conductors with higher temperature ratings, provided the ampacity of such conductors does not exceed the 75?C (167?F) ampacity of the conductor size used, or up to their ampacity if the equipment is listed and identified for use with such conductors
Roger

[ December 10, 2005, 12:15 PM: Message edited by: roger ]
 

marsbar42

Member
Re: corrections for simulated exam

Now I understand the answer to the last question...I missed the 100A provision...Thanks. I still wonder about the first question, though. The answer they give is 25.5A and 8THHN. Thanks for your help, I thought I must be missing something.
 

jbwhite

Senior Member
Re: corrections for simulated exam

now i am confused about 110.14

roger. i have done this in the past on long runs where wire size can offset cost of equipment.

hmmm an example would be: going from distribution panel to equipment disconnect over 100amps. call the supply house and special order a 90 c breaker. disconnect was already rated 90c. run the wires based on 90c and only adjusted for voltage drop.

From disconnect to machine used 75c columb because of the lugs on the equipment.

would this be legal now?
 

roger

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Re: corrections for simulated exam

JB, if actual listed 90 deg components (i.e. equipment, terminals, lugs, etc...) are used in the complete installation, (from breaker to disconnect in your example) then 90 deg conductors can be used at their 90 deg rating.

Roger

[ December 10, 2005, 01:29 PM: Message edited by: roger ]
 

jbwhite

Senior Member
Re: corrections for simulated exam

thanks roger. we paid for the breakers. but made up for in wire. i have found they are not always off the shelf. and i am not sure if all mfgs make them for every panel.
 

roger

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Re: corrections for simulated exam

You're welcome.

Roger
 
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