Wire Size?

[Tom L.]

I am replacing (and upgrading) a subpanel in my house. I am upgrading to a 125 amp panel. The run is approx 90 feet through a garage and crawlspace. The entire run will be in a conduit.

In determining wire size, I came up with needing #4 (just on the edge of needing #3) wire. To be safe I was thinking of going with the #3 for the 2 hot's and neutral, and a #8 ground.

Does this sound correct, or would you recomend a different size depending on my conditions and the length of the run.. Would I be ok with #4 instead?

If you need any more info, let me know
Thanks

[Wgoodrich]

125 amp service entrance conductor serving a dwelling is dictated by NEC Table 310.15.B.6 that calls for 2 awg copper or 1/0 awg aluminum as the minimum conductor size per the NEC.

I am wondering where did you come up with your posted wire sizes?

This 90 feet is this all measured while inside the dwelling?

What is the total distance from your utility transformer and you main service rated panel serving your home.

Did you perform a demand load calculation to find the minimum size main service as required by the NEC to serve your home, or are you guessing the service size?

If you are guesing your service size I suspect you would better serve yourself if you went to the following link on our homewiringandmore.com web site that provides a format so you can perform your own demand load calculation so you will know what minimum service size is required by the NEC to serve your home. Then you can make an informed decision as to how much more than minimum you want to install for a service upgrade.

http://www.homewiringandmore.com/homewiringusa/2002/definitions/demandcalc02/DwlingDmdCalc02.html

After you have performed your demand load calc come back in and tell us the figures you calculated then we will calculate it again for you and let you know how close you came.

At least then you will be making informed decisions concerning you planned project.

Hope this helps

Wg

[Tom L.]

Sorry for any confusion, but I need to clarify a few things from my origional post.

The panel that I am replacing is a non-service rated sub panel that is being fed from my main (200A) service panel. The 90 feet is the distance beteen the 2 panels.

HOpe this helps...
THanks

[Wgoodrich]

A 125 amp nonbservice rated panel [aka sub panel ] still will require a 2 awg copper or 1/0 awg aluminum if you are installing a 125 amp breaker in your main service rated panel to protect that feeder going to that nonservice rated panel.

Don't forget a nonservice rated panel requires a 4 wire feeder. Two hots, one insulated neutral and one insulated or bare equipment grounding conductor.

250.122 calls for a 6 awg copper equipment grounidng conductor to serve that nonservice rated panel that is 125 amp rated.

Don't forget that the neutral bar must be kept isolated from the equipment grounding bar and from the metal case of that nonservice rated panel.

If you are installing THWN or other single conductor wires as your feeder those conductors must be protected by a conduit. If you are using EMT and not a flexible conduit then that EMT may serve as your equipment grounding conductor to serve that nonservice rated panel.

Good Luck

Wg

[Tom L.]

A few things have now changed in my upcoming wiring project.

I moved a few circuits around, and determined that I only need to feed the non-service rated subpanel with 95amps (will use a 100 amp feeder). The wire will still be running through a 90 foot conduit.

Another change is that I will need to also power a electric stove/oven with 50 amps from my main service rated panel. This is located in route to the sub panel. Due to the design of my house, the only practical way to power the stove is running the wire through the same conduit.

I have decieded to run both sets of wires in the same 1 1/2" conduit.
I want to double check my calculations for accuracy and opinion.

The conduit will contain 4 wire service for the sub panel and the stove. (6 current carrying conductors (counting neutrals) and 2 ground wires. I need to de-rate this at 80%. I would need #2 (130A * 80% =104A) runs for the sub panel, and #6 (75A * 60% = 60A) for the stove (both THHN).

For conduit fill.. 3) #2 THHN and a #6 ground, plus 3) #6 THHN and a #10 ground. I plan to use 1 1/2" PVC. Once the Stove wiring branches off in its 1" conduit , I will reduce the feeder to the sub panel to 1 1/4".

Sound good? Did I miss anything?
Thanks for your help!!!

[Wgoodrich]

YOU SAID;
I moved a few circuits around, and determined that I only need to feed the non-service rated subpanel with 95amps (will use a 100 amp feeder). The wire will still be running through a 90 foot conduit.

Another change is that I will need to also power a electric stove/oven with 50 amps from my main service rated panel. This is located in route to the sub panel. Due to the design of my house, the only practical way to power the stove is running the wire through the same conduit.

REPLY;
Is there a reason that you are not using nonmetallic sheathed cables [aka Romex] for either the range or sub panel? Just curious.

YOU SAID;
I have decieded to run both sets of wires in the same 1 1/2" conduit.
I want to double check my calculations for accuracy and opinion.

The conduit will contain 4 wire service for the sub panel and the stove. (6 current carrying conductors (counting neutrals) and 2 ground wires. I need to de-rate this at 80%. I would need #2 (130A * 80% =104A) runs for the sub panel, and #6 (75A * 60% = 60A) for the stove (both THHN).

REPLY;
Table 310.15.B.6 in the NEC says in a residential setting concerning a feeder [your subpane] you may use 4 awg THHN copper conductors if you like to serve a 100 amp breaker that is my understanding is the breaker size in your main service panel protecting this sub panel feeder.

YOu calculations for ampacity deration due to more than three current carrying conductors in a raceway is not even a concern. YOu missed the rules that apply to current carrying conductors concerning ampacity deration in Table 310.15.B.2.A.

Your sub panel it is true has four conductors yet the grounding conductor is not a current carrying conductor. Neither is the neutral conductor. In a feeder to a sub panel you have a true neutral conductor and true neutral conductors also are not counted as as current carrying conducors.

Then again exactly the same applies concerning your range. YOu have a true neutral in that range and an equipment grounding conductor in that range. Neither would be counted as a current carrying conductor. Therefor you have only 4 current carrying conductors total combining the range and subpanel feeder in the same conduit.

Then again TAble 220.19 note 3 tells us that you may install an 8 awg copper 40 amp range cable if you like as a minimum conductor size for the range. If you ran 8 awg and look in Table 310.16 your 8 awg copper wire would be rated in the 90 degree column matching your insulation of THHN wires your ampacity deration calculation would start at 55 amp in the 90 degree column then you multiply that by the 80% deration and your answer would be 44 amps after ampacity deration. Then if you go to NEC 110.14.C.1 says you must use the 60 degree rating in table 310.16 saying that the ampacity is 40 amps then you must compare the two ampacities and use the lower of the calculated and the chart. The Chart is the lower of the two in ampacity and this keeps your ampacity regardless of 4 current carrying conductors in that conduit at 40 amps as it was originally. No deration concern would apply to the range.

Same results in the calculation using the 4 awg feeder wires and for the same reasons using Table 310.15.B.6 allowing the 4 awg copper feeder size to the sub panel with no deration that would apply.


YOU SAID;
For conduit fill.. 3) #2 THHN and a #6 ground, plus 3) #6 THHN and a #10 ground. I plan to use 1 1/2" PVC. Once the Stove wiring branches off in its 1" conduit , I will reduce the feeder to the sub panel to 1 1/4".

REPLY;

Before I calculate conduit fill I would like you to have read this reply and let me know for sure what sizes you plan to run now after reading this reply. Then we can do the conduit fill calcs. Right now not sure what you will plan to do. You may now even opt for nonmetallic sheathed cables instead of conduit. Let me know then we will proceed from there.

Hope this helps a bit

Wg

[SharpT]

Tom,

I am in the process of some similar tasks as you. I am sending THHN conductors in conduit outside under the eves over to a 100 amp subpanel (I'm on slab, no attic or accessible soffits). I have decided to go ahead and use #2 THHN wires because even using #4 wires I have to bump up to the 1 1/4"PVC SCH80, wire is cheap, only 50 foot run. Also my subpanel is for the kitchen/laundry/garage which could see some more serious loads over the years as I acquire more woodworking equipment, potentially could have garage vacuum system going, table saw, dryer, washer, fridge, breadmaker, dishwasher, and oven all at the same time. :shock:

#2 THHN 0.1333 in^2 (two hot conductors) black
#4 THHN 0.0973 in^2 (neutral conductor) black with white tape on each end. :?:
#6 bare ground wire 0.027 in^2

Total wire area is 0.3909 in^2.
Max fill for 1 1/4"PVC SCHD80 (40%) is 0.495in^2.

SharpT

[Wgoodrich]

Sharp T, I didn't see you mention a grounding conductor. If you have a bare grounding conductor then you could use 1 1/4" Sch 80 PVC as you said. If you installed an insulated grounding conductor then you would have to use a 1 1/2" Sch 80 PVC conduit.

2-#2 THWN x .1158 = .2316
1-#4 THWN x .0824 = .0824
3-#6 THWN x .0507 = .1521
1-#8 THWN x .01 = .01 BARE GROUNDING
Total area used == .4761
Area 40% fill 1 1/4" Sch 80 PVC .495 OK.

2-#2 THWN x .1158 = .2316
1-#4 THWN x .0824 = .0824
3-#6 THWN x .0507 = .1521
1-#8 THWN x .0366 = .0366 INSULATED GROUNDING
Total area used == .5027
Area 40% fill Sch 80 PVC .495 TOO SMALL.
Required conduit size Sch 80 PVC if with insulated grounding conductor would call for 1 1/2 Sch 80 PVC .5027 OK.

I noticed a couple of you numbers and my numbers did not match. Are you using 99 or 2002 NEC for your area square inch of insulated conductors?

Curious

Wg

[Tom L.]

I am running in a conduit because I live in the house from hell (electricly speeking.....but its really a nice house). Our main panel is in the far corner of the garage. The sub panel is located in the middle of the house in the laundry room. The attic is not accessable due to a vaulted ceiling. Only choice is crawlspace. I am unable to put a big enough hole in the garage/attic wall to run a feeder cable. Only option is running outside and underground from the garage over to a section of the crawlspace where I can get through the wall via a conduit. Someone previously took this same approach in running a 6/3 cable from our main panel (this is completely seperate from my project though). I decided to make the full run conduit/single strands due to the route, and there are a few areas in the crawlspace that it is very tight to work in. Putting up conduit in these areas will be much easier then dragging 50 pounds of wire through it.

No de-rating and using #4 wire sounds much better for me. I just want to double check and be safe. It will be protected at 100A in the main panel. The sub panel will provide for the entire house's lighting and outlets, kitchen circuits, dryer, and A/C. The run to the panel will be about 90 feet. 8 gague ground. All good?

The range setup is a split system. The oven is wall mounted and the cooktop is next to it. Both appear to meet at a junction box below the cooktop. Any special requirements here? Again, power to this will come from the main panel and will travel about 60-70 feet in the above mentioned conduit.

Thanks again for your help!

[Wgoodrich]

YOu hit the discription fine.

Now as for your cooktop and oven. Most likely your oven is a single of rated at 30 amps though it may be a double oven rated at 40 amps.

If you have a single oven and a cooktop the NEC allows you to install a 40 amp 8 awg branch circuit with a black, white, red, and bare to a junction box then install a tap from that junction box as short as possible using a 10 awg copper conductor set black, red, and bare to your cooktop no neutral is normally used on cook tops. Then install a tap conductor 8 awg copper conductor set black, red, white and bare from that junction box again as short as possible to the oven. Use flex as your conduit style in the tight areas between the cooktop and oven. This is what is called a Branch circuit tap for cooking equipment found in NEC table 220.19 And NEC artilce 210.19.

Hope this helps

Wg

[Tom L.]

THe range is currently breakered (or is that broken? hehe) at 40 amps, with what appears to be red/black 8 gague (goin by memory) with a bare neutral/ground...seems typical for the era (1950's). I cant find a spec plate for the range part, but the oven part is rated at 5.3Kw It is a 1950's Frigidare (Made by General Motors) setup.. It will probably never be replaced as it is built into the kitchen, and was made localy (Dayton, OH). There is a shop that specializes in restoring these units. Keep it at 40? (but with 4 wire of course)

[Wgoodrich]

There is a rule that approaches existing range and oven branch circuits. The tap rule still stands as we discussed earlier. However the white wire is allowed to be missing using the bare wire as both the equipment grounding and hte neutral conductor in that branch circuit if existing. This is true only if you have an SE type cable. You are telling me that your cable has black and red and bare. This depicts an SE type cable. Yes keep it at 40 amps and you may use that existing branch circuit without the white wire.

If you install new branch circuit then you must meet current code again the tap rule is allowed as discussed but with a new branch circuit you must use a black, red, white and bare wire in that branch circuit cable.

Now that cook top will not have a neutral at all so no concern.

If you use the existing three wire branch circuit as discussed then the connections in that oven stay as it is existing.

However if you install a new 4 wire branch circuit then you must enter that oven at the terminal block where the house wiring is connected and remove the jumper between the silver screw of that teminal block [neutral screw] and jumped to the metal frame of hte oven. This jumper needs to be removed. Then the white wire of that new 4 wire branch circuit must be connected to the silver screw on that terminal block in that oven but the green or bare wire of hte branch circuit must be connected to the metal frame of the oven where the old jumper was connected. That jumper must be removed between the neutral screw on the terminal block and the metal frame of the oven when a new 4 wire branch circuit is installed.

HOpe this helps

Wg

[SharpT]

OK, let me try again: :oops:

Feeder from Main Panel to 100 Amp Non-Service Rated Panel

2-#2 THHN x .1158 = .2316
1-#4 THHN x .0824 = .0824
1-#8 THWN x .013 = .013 BARE GROUNDING
Total area used == .327
Area 40% fill 1 1/4" Sch 80 PVC .495 OK.
Area 40% fill 1 " Sch 80 PVC .275 NOT OK.

Also 200.6.B allows me to use black #4 neutral wire as long as I apply a "distincitive white marking at its terminations that encircles the conductor or insulation"

SharpT

[Wgoodrich]

SharpT, looks like we both made a very basic mistake in our print of our calculations. While the area square inch of the 8 awg bare is as you quoted. How to you have a THWN BARE CONDUCTOR. Ha Ha. Don't feel bad look at what I wrote in my reply and I also wrote THWN bare 8 awg. Is that one basic or not and we both did that one?

Your reference to concerning conductors larger than 6 awg.

Now be careful using this marking rule ensuring it is a conductor 4 or larger. 6 awg and smaller must be marked its entire length of those smaller conductors concerning neutral conductors. not just at the points it appears in boxes. The only exception allowing 6 awg or smaller being marked at the boxes or visible points would be if this neutral 6 awg or smaller is contained within a cable sheath. Single conductors 6 awg or smaller otherwise must be marked the entire length of that neutral conductor.

Also note that green is not allowed to be reidentified as a grounded or hot conductor at all.

Hope this helps

Wg

[Tom L.]

Ok, Ive got everything figured out now, and started to run my conduit today. I decided to keep the conduit at 1 1/2 to make pulling wire easier.. got a few bends to pull through (below the max degree's between pull points).

I got through the first half of the crawlspace as planned and decided to start prepping to run the outside/underground leg. I mentioned before that someone else ran a conduit along the foundation in a path similar to mine. Well, I ran across it about 6" down. In my 'homework' I have come across a table stating burial depths for various conduits. Under PVC it mentions general depth of 18". and other depths for under concrete, buildings, driveways, streets, ect. Is there a special rule for running right next to a foundation? My origional plans were to run my conduit next to or below the existing one (and was expecting to find it a bit deeper).

Thanks

[Wgoodrich]

If you installed schedule 80 PVC it would not even have to be buried. Yet the NEC gives no credit for being next to a foundation or any lesser degree of depth due to being next to a foundation. PVC just states if buried 18" minimum. If under 2" of concrete it would be allowed less. In your case you only fit buried not under concrete.

I would check with your AHJ [electrical inspector] He or she may declare it not buried deleting the minimum burial depth in your design being next to the foundation. The NEC does not provide any credit for being next to the foundation. I would suspect your AHJ would say don't worry about it but it is that AHJs call.

Good Luck

Wg