Please tell me if I did this problem correctly.

[ActionDave]

Thhn is a 90' conductor but if it's terminated to a device that's rated at 75' don't you have to take it at 75' colum?

Correct. What kwired is referring to is thwn is 75 degree wire, thwn-2 is 90 degree.

 

[liquidtite]

Got it thnks

 

[kwired]

Thhn is a 90' conductor but if it's terminated to a device that's rated at 75' don't you have to take it at 75' colum?

You can use 90 deg ampacity as the base for ampacity adjustments, but if you have 75 deg terminations you can not have a conductor smaller then what is in the 75 deg column, even if the adjustments otherwise allows for a smaller conductor @ 90 deg.

So your first step is to determine absolute smallest conductor allowed based on termination rating, say that results in a 12 AWG @ 60C. You then have multiple conductors in a raceway or other "bundling" instance and/or high ambient temperature and need to make adjustments - you can make those adjustments based on the 90C rating of the conductor - but if those adjustments come to 14 AWG you still must use the 12 AWG so you do not overheat the 60C rated terminations. Same would be true if you had 75C terminations, except you use 75 instead of 60 deg ampacity columns.

 

[kwired]

Correct. What kwired is referring to is thwn is 75 degree wire, thwn-2 is 90 degree.

Actually if you had a conductor only marked THHN/THWN, it is 90 deg only when used in dry locations, but can be used in wet locations but only at 75 deg because that is what THWN is.

Today many with most conductors you typically have all three of the ratings mentioned marked on them - meaning take your pick as it is rated for any of those ratings marked on it. Go back a few years and the THWN-2 rating wasn't as common and you had to watch more closely what you had and figure accordingly.

 

[liquidtite]

Please let me know how I did on this problem, or if you would have answered any of these differently. Thanks.


QUESTION:

A person buys a house without a garage, and later decides to have a 2-car detached garage built. The owner?s electrical needs are as follows: Garage is to be powered with a 120/240V subpanel installed in the garage, using a 40-amp underground feeder from the main panel in the basement of the house. The owner also wants to be able to control the outdoor garage lights from inside of the garage as well as inside the house. The lighting circuit is to be rated at 15 amps with no other non-lighting outlets connected to it. Receptacle outlets are to be fed by two 20-amp circuits.

1. What is the minimum allowable size for each of the ungrounded (hot), grounded (neutral), and equipment grounding (green) conductors if THHN copper conductors are to be used?

2. If the minimum size conductors are used, what is the minimum allowable size PVC conduit that can be used?

3. What is the minimum allowable size metal junction boxes to be used in the basement and garage to contain all conductors in Question 1?



ANSWERS:
1. Based on Table 310.16: #8 THHN has a rating of 55 amps, and since there are 6 current-carrying conductors in the raceway, (Table 310.15 B2a), 55 * 0.8 = 44 amps. So use #8 ungrounded (red/black) and #8 white (neutral). Based on Table 250.122, use #10 green grounding wire. For the 3-way common and traveler wires, use #14 black, red, and blue.

2. Since the maximum fill of conduit is 40% (Chapter 9, Table 1), and the sums of the cross-sectional areas of the conductors are as follows: 3@0.0366, 3@0.0097, 1@0.0211, for a total of 0.16 square inches. The area of a 3/4? conduit is 0.442 square inches. 40% is 0.177. So a ?? barely meets the requirement. I would personally use 1? to make the wires easier to pull.

3. Each junction box would contain all 7 conductors in Question 1, as well as a #14-3 NM-B cable and a #8-3 NM-B cable. All grounding conductors count as one. The remaining 6 are paired with a conductor from the cables, giving a total of six #8 and 6 #14, and one #10 grounding. For each #14, 2 cubic inches are needed. For each #8, 3 cubic inches. For a #10, 2.5 cubic inches. Total cubic inches needed is 17.5. Therefore, a 4? square by 1.25? deep barely passes with 18 cubic inches. I would personally use a 4-11/16? square X 1.25? because it would provide more freedom, and a 4? box is not likely to have 1? knockout openings, where a 4-11/16? would.

i thought when feeding a detached garage your only allowed to bring one set of feeders? Not have feeders and branch circuits maybe I'm reading it wrong

 

[GoldDigger]

When the building has a panel with a feeder, the single branch circuit allowance is often interpreted so as to allow a three way outdoor light circuit originating at the house to have its garage-end three way switch inside the garage instead of outside the door.

 

[kwired]

i thought when feeding a detached garage your only allowed to bring one set of feeders? Not have feeders and branch circuits maybe I'm reading it wrong

Last paragraph in 225.30:

"Where a branch circuit or feeder originates in these additional buildings or other structures, only one feeder or branch circuit shall be permitted to supply power back to the original building or structure, unless permitted in 225.30(A) through (E)"

This is from 2014 NEC, but is not indicated as a change so it must have been in 2011 also, but I am certain it hasn't been there much before that. This would allow you to have a feeder to a separate building, and run one feeder or branch circuit back to the first structure, (perfect allowance to let one run a set of 3 way switches between the two buildings) and of course other circuits permitted in (A) through (E) would be in addition to this. Most of A thru E wouldn't apply to a dwelling and detached garage very often - maybe a standby generator feed coming from the garage, or if you had a PV system that is installed at the garage.

 

[liquidtite]

Ok thanks ya the code is always changing it's hard to keep up .
U guys are good.

 

[kwired]

Ok thanks ya the code is always changing it's hard to keep up .
U guys are good.

But this topic is one that has not really changed over many years. There were some minor changes to ampacity tables more recently - particular some 60 deg C conductor ampacities, but the method(s) of determining what ampacity one needs for an application has not changed much at all in at least 25-30 years, maybe more that is just my time of experience. Maybe occasional editorial and composition changes or reorganization of of the NEC itself at times but no real change to the end result of applications.