Choosing Sub Panel

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  #41  
Old 12-26-18, 09:27 AM
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Originally Posted by Tolyn Ironhand View Post
I forgot that you are using a cable (NM or SE) so you are limited to 60 degree as I mentioned earlier. (post #28)

You need #2 copper or 1/0 aluminum.

@ Pattenp: So you install a 100 amp service in a house and feed it with #4 copper as allowed in 310.15(b)(7) and then install a 100 amp sub panel. You have the feed the sub panel with #3 copper? What sense does that make?
SE cable is only limited to 60deg when run within insulation, otherwise it's 75deg.
 
  #42  
Old 12-26-18, 09:35 AM
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Originally Posted by Tolyn Ironhand View Post
I forgot that you are using a cable (NM or SE) so you are limited to 60 degree as I mentioned earlier. (post #28)

You need #2 copper or 1/0 aluminum.

@ Pattenp: So you install a 100 amp service in a house and feed it with #4 copper as allowed in 310.15(b)(7) and then install a 100 amp sub panel. You have the feed the sub panel with #3 copper? What sense does that make?
It makes no sense but that is how I have been taught the NEC. The code is plain English and says when the entire load of a dwelling is supplied you can use 310.15(B)(7).
 
  #43  
Old 12-26-18, 11:00 AM
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Originally Posted by Tolyn Ironhand View Post
Insulation ratings will be listed on the conductor/cable jacket. However, many non-metallic cables are limited to the 60 degree C rating. THHN and XHHW (the two most used conductors in conduit) are rated 90 degrees.

To clarify even though rated 90 degrees, 90 degrees is only used when derating. 75 degrees is the applied temp from the tables for typical installations.
 
  #44  
Old 12-26-18, 11:19 AM
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What does It mean by only used for derating? Not sure what derating means
 
  #45  
Old 12-26-18, 11:50 AM
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Originally Posted by Jrk5230 View Post
What does It mean by only used for derating? Not sure what derating means
You have to derate the ampacity of wire when you exceed a certain number of current carrying conductors within a conduit. Example 4 to 6 conductors you derate to 80%, so you use 80% of the 90 degree rating. That's just a simplified example. It's a temperature rating adjustment, see NEC 310.15(B)(3)
 
  #46  
Old 12-26-18, 02:44 PM
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You have to derate the ampacity of wire when you exceed a certain number of current carrying conductors within a conduit. Example 4 to 6 conductors you derate to 80%, so you use 80% of the 90 degree rating. That's just a simplified example. It's a temperature rating adjustment, see NEC 310.15(B)(3)
And a 2-2-2-4 cable would be considered 3 or 4 conductors? Assuming 4 counting the ground
 
  #47  
Old 12-26-18, 03:01 PM
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Can I get answer to the two following questions for a 80 amp, 90 amp, and 100 amp sub panel breaker? For some reason the 70 amp breakers available through amazon are about double the price of the 80, 90 or 100 amp breakers. I decided against the heat pump so 100 amp breaker is a bit overkill, even adding a swimming pool possibly later as mentioned below in post #24

So what size wire do I need if I run in conduit strapped to bottom of joists?

And what size wire do I need if I decide to run a cable strapped to bottom of joists?

I'm a little confused after reading down through the posts from Patten and Ironhand. Do I follow 310.15(b)(7) in the NEC or is there another table I am to follow since this service is not the supply for the entire dwelling? This service is for addition only.
 
  #48  
Old 12-26-18, 05:09 PM
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If you use conduit and use as an example individual aluminum conductors in the sizes 0f 2-2-2-6, such as XHHW-2, the ampacity at 75deg is a max of 90A. If you decide to use Al SER cable which comes as 2-2-2-4 will also have a max ampacity of 90A provided it is not run within insulation. A #6 Al ground is what is needed for up to 100A if using individual conductors. The table to use is 310.15(B)(16). The ground is not counted as a current carrying conductor so you have 3 current carrying conductors so derating is not a concern. To have a full 100A you need individual aluminum conductors in sizes of 1-1-1-6 or Al SER in 1-1-1-3. Less common odd size breakers are just more expensive.

Also just use a 100A main breaker panel as the sub. It can be fed from a smaller breaker in the main. If using #2 for the feeder you should find that a 60A breaker and some 50A breakers are the smallest breakers that #2 will fit in the lugs.
 
  #49  
Old 12-26-18, 06:29 PM
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To clarify even though rated 90 degrees, 90 degrees is only used when derating. 75 degrees is the applied temp from the tables for typical installations.
That is correct. Except you are really limited to the lowest rating of any part of the wiring. If there is a lug or circuit breaker that is rated only 60 degree you are limited by that. However, as a general rule, the 75 degree rating is commonly used.

At OP - Regarding your heating question: It really depends on where you live. In my area (Minnesota) heat pumps are not used very much do to our cold weather. In the south they are much more common.
 
  #50  
Old 12-28-18, 07:34 AM
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If you decide to use Al SER cable which comes as 2-2-2-4 will also have a max ampacity of 90A provided it is not run within insulation.
My local lowes store carries 2-2-2-4 AL SER cable. It looks like it has the insulation around the wire as shown on website. When you said provided it is not run within insulation did you mean like batts of insulation? Am I able to feed a 90A breaker with this 2-2-2-4 AL SER cable?
 
  #51  
Old 12-28-18, 07:55 AM
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Originally Posted by Jrk5230 View Post
My local lowes store carries 2-2-2-4 AL SER cable. It looks like it has the insulation around the wire as shown on website. When you said provided it is not run within insulation did you mean like batts of insulation? Am I able to feed a 90A breaker with this 2-2-2-4 AL SER cable?
SER cable if installed where it is covered by batt or loose fill or foam insulation is limited to 60 degree rating because the insulation limits heat radiation from the wire causing the wire to heat up too much. See NEC 338.10(B)(4). See NEC 310.15(A)(2) Exception: where if less than 10% (10ft for max run of 100ft) of the run is in insulation then 75 degree may be used for the entire length.
 
  #52  
Old 12-28-18, 08:46 AM
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Ok thanks for the information. My feeder will be run in open ceiling joists in basement ceiling with no insulation present.
 
  #53  
Old 01-07-19, 11:41 AM
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What size copper cable for 90 amp breaker in main panel feeding 100 amp sub panel?

What size copper if go individual conductors for 90 amp?

I've decided to go with a 90 amp breaker in my main panel to feed my 100 amp sub panel with. Now I just need what size individual wires AND what size cable I need. Both Copper and Aluminum please. Want to price all options prior to choosing the wire. Thanks
 
  #54  
Old 01-08-19, 09:23 AM
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For a 90A feeder minimum sized wires you would need are either:

a) #4,4,4,8 copper individual conductors (THHN/THWN-2) in conduit
b) #3-3-3-5 copper cable assembly (SER, NM)
c) #2,2,2,6 aluminum individual conductors (XHHW, RHW) in conduit
d) #2-2-2-6 aluminum cable assembly (SER) run through uninsulated space
e) #1-1-1-3 aluminum cable assembly (SER) run through insulated space

In some cases the cables might have a neutral wire one size smaller like 2-2-4-6 instead of 2-2-2-6. These are OK for 99% of installations and it mostly depends on what your local supplier has in stock.
 

Last edited by ibpooks; 01-08-19 at 02:11 PM. Reason: item (c) had wrong numbers
  #55  
Old 01-08-19, 09:39 AM
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Originally Posted by ibpooks View Post
For a 90A feeder minimum sized wires you would need are either:

a) #4,4,4,8 copper individual conductors (THHN/THWN-2) in conduit
b) #3-3-3-5 copper cable assembly (SER, NM)
c) #3,3,3,5 aluminum individual conductors (XHHW, RHW) in conduit [not standard sizes, might be special order, probably cheaper to go with #2,2,2,6]
d) #2-2-2-6 aluminum cable assembly (SER) run through uninsulated space
e) #1-1-1-3 aluminum cable assembly (SER) run through insulated space

In some cases the cables might have a neutral wire one size smaller like 2-2-4-6 instead of 2-2-2-6. These are OK for 99% of installations and it mostly depends on what your local supplier has in stock.

#3 Al is only good for 75A, needs to be #2
Also SER comes in 4-4-4-6 Cu and could be used for 90A if not run within insulation.
 
  #56  
Old 01-08-19, 02:11 PM
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Fixed, thanks. I was reading the wrong line.
 
  #57  
Old 01-09-19, 08:33 AM
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I'm going to price wire at a local electrical supplier today. Local Lowes has individual #4 THHN & #8 green THHN. Get the 4 wires would cost me $148 for stranded copper. This seems like it would be my cheaper option if settle with lowes or home depot. Will check local electrical supplier today.

as far as conduit goes, I want to run the wire along bottom face of floor joists in ceiling of basement. They are open with no insulation. Is 1" PVC a good idea or should I go with something like EMT or IMC? PVC is cheaper and should be durable enough to withstand damage. Have 3 kids and would not want anything to happen if conduit was struck by one of them or myself.
 
  #58  
Old 01-09-19, 11:19 AM
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Yes PVC sch 40 gray conduit is good for this application. EMT would also be OK. No need for IMC or RMC. You can use 1" or larger.
 
  #59  
Old 01-16-19, 02:24 PM
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Do I need a separate ground rod for my subpanel? I purchased three #4 THHN stranded conductors and one #8 green stranded conductor for the ground. They dont carry any white neutral wire but instead gave me all three sections of the #4 in black. Will use two hots, 1 neutral and 1 ground. The ground and neutral bus bars will not be connected to each other (I have to remove the connector clip) and the ground rod will be bonded to the panel box but the neutral will not. Does this sound like proper installation procedure?
 
  #60  
Old 01-16-19, 03:53 PM
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If in a separate building you need one or two ground rod(s) (ask your inspector how many). The ground rod(s) are connected with a #6. If two rods wire must be continuous.

Black neutral must be remarked with bands of white (or gray) tape or painted white on both ends. Neutral bar is isolated from panel (bonding screw not used).
 

Last edited by ray2047; 01-16-19 at 04:43 PM.
  #61  
Old 01-16-19, 05:59 PM
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You need to mark the neutral wire with white tape.
 
  #62  
Old 01-17-19, 07:56 AM
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I believe I do not need another ground rod based on what you said Ray. My addition is attached to my existing house. I will get some white tape and mark the black neutral conductor. My sub panel box has a lot of plastic that will need removed. Hope this isn't to challenging to accomplish.
 
  #63  
Old 01-17-19, 09:05 AM
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My addition is attached to my existing house.
So no ground rod needed.

My sub panel box has a lot of plastic that will need removed.
What plastic? There should be no plastic that needs removing. If you mean the plastic insulators on the neutral bar(s) they must remain. You will need to buy and add a ground bar.
 
  #64  
Old 01-17-19, 02:31 PM
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https://imgur.com/a/bXJk3SY

I have to remove the metal clip connector at top of inside of panel and also if you look behind the bus bar on right and left there appears to be black plastic between bus bar and metal panel box. Does it matter which side is the ground bus and which one is the neutral bus? If not, I need to remove the black plastic from the ground side thus making the panel grounded as well, correct?
 
  #65  
Old 01-17-19, 02:46 PM
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Both are neutral bars. As stated before you must buy and add a ground bar. Both neutral bars must be electrically isolated from the metal case. Example: https://www.amazon.com/Square-Schnei...SB4CNRXRVNS2KM
 
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  #66  
Old 01-18-19, 10:43 PM
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Where do I mount this ground bus bar to my panel box at?
 
  #67  
Old 01-18-19, 11:24 PM
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There are usually pre tapped holes. If not use self tapping screws.

See video:

https://www.youtube.com/watch?v=dlVhS7wSZtw
 

Last edited by ray2047; 01-19-19 at 12:24 AM.
  #68  
Old 01-19-19, 07:15 AM
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thanks, I think I found where it attaches. Got the ground bar purchased as well and should be here in a couple days.
 

Last edited by Jrk5230; 01-19-19 at 07:34 AM.
  #69  
Old 02-09-19, 01:33 AM
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I attached the ground bar I purchased seperately. If im on the right path, I will connect my two hots from 90 amp breaker in main panel to the two connections labeled at 'HOT' in photo. My neutral will run from neutral bar in main panel to screw labeled 'NEUTRAL' in sub panel. The ground will run from ground bar in main panel to ground bar in sub panel. My ground bar will remain bonded to metal case and separated from the neutrals.

Does everything look good? I believe I'm ready to make my connections and move forward with the rest of my addition.

One question I do have is, does my ground from sub panel to main panel have to attach to the ground wire in main panel that comes from ground rod outside or can it connect to any screw within ground bar in main panel?

Do I need the green grounding screw for anything shown at bottom of photo?

[IMG][/IMG]
 
  #70  
Old 02-09-19, 05:46 AM
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The ground can connect the ground bar in the main panel.
The neutral bar needs to remain isolated from the grounded chassis. You do not need the green ground screw.

For your own convenience a second ground bar on the other side would be nice. It will make your life easier for all the circuits coming from that side of the panel. However it is not required.
 
  #71  
Old 02-09-19, 06:47 AM
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You will also nee to install a bushing on the end of the PVC connector since your wire is #4 or larger. You also need to install a KO seal on the KO you removed in the middle of the panel.
 
  #72  
Old 02-09-19, 09:38 AM
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I was planning to get the KO seal. Also, I'm not familiar with what bushing are out there for the pvc conduit fitting. Is it something that will just thread onto the end of my male threaded connector that enters panel box? So some kind of female pvc bushing that is
 
  #73  
Old 02-09-19, 11:34 AM
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I've been debating what to choose for heating source and was curious of your opinions on this matter.

I'd go with a ductless mini-split system and not look back. This way you don't have to touch the existing furnace or duct work. These new ductless mini-split heat pumps are very efficient, some have SEER ratings around 20 to 30, and they will continue to heat down to almost 0 degree outside temperature. Do some research and check out Mitsubishi's units.


Note* I have a 3 ton central AC unit as well that is big enough to handle the extra load the 24'x24' addition adds to the system. Therefore another reason a heat pump doesn't seem like the best option.

If that existing 3 Ton AC unit was properly sized, it won't be enough to cool the addition of 24' X 24' (576 Sq Ft). Your low bid of $6270 will be over $10,000 by the time you add upgrading the AC to the new 80,000 BTU furnace.
 
  #74  
Old 02-09-19, 12:11 PM
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If that existing 3 Ton AC unit was properly sized, it won't be enough to cool the addition of 24' X 24' (576 Sq Ft). Your low bid of $6270 will be over $10,000 by the time you add upgrading the AC to the new 80,000 BTU furnace.
When the central air was installed, it was oversized by a ton to account for adding addition SF to my existing system. I was told my existing home only needed a 2 ton unit so this 3 ton should be adequate.
 
  #75  
Old 02-09-19, 02:30 PM
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Is it something that will just thread onto the end of my male threaded connector that enters panel box?
Yes. https://www.homedepot.com/p/Halex-1-...7529/100153897
 
  #76  
Old 02-09-19, 10:51 PM
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awesome, picked up the KO seal and bushing today at lowes. Thanks for the help.
 
  #77  
Old 02-12-19, 04:32 PM
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I have 4 conductors running from main panel to sub panel. I marked one of the black 4 gauge conductors on both ends with tape so I know which one is my neutral. Being that the neutral wire has black insulation, how do I make this code compliant with white tape or whatever means is accepted? Can I just tape the first couple inches of insulation at each of the two terminations to denote that it's a neutral white wire?
 
  #78  
Old 02-12-19, 05:36 PM
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Tape the ends white is okay. Youn can do just a couple inches.
 
 

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