Conductor size for 100 amp sub panel?

[vliposky]

Seems the old forum said I could use 4/3 w/G NM conductor for 100 AMP sub panel (being fed from 100 AMP new service in this residential dwelling application...) Is this still true?

[Dennis Alwon]

It depends -- read art. 310.15 (B)(6). Let us know hwat you think.

[celtic]

Seems the old forum said I could use 4/3 w/G NM conductor for 100 AMP sub panel (being fed from 100 AMP new service in this residential dwelling application...) Is this still true?

310.16 says you need a #1. ( As supported by 334.80)

[celtic]

It depends -- read art. 310.15 (B)(6). Let us know hwat you think.
It isn't a service.

[Dennis Alwon]

Celtic????

Where did you get that..... What are you up to now????

[Dennis Alwon]

read the article I stated services and feeders.

[celtic]

Celtic????

Where did you get that..... What are you up to now????

What do you mean?

[iwire]

If its a sub panel it is likely 310.15 (B)(6) does not apply.

[Dennis Alwon]

The Op is obviously talking about copper. 100 amps.. table 310.16 60C.. Look at #1

Plus you never have to go higher than the service conductors.

[Dennis Alwon]

If its a sub panel it is likely 310.15 (B)(6) does not apply.


We don't know that plus it is a 100 amp service feeding a 100 amp sub. It may apply.

[infinity]

The Op is obviously talking about copper. 100 amps.. table 310.16 60C.. Look at #1

Plus you never have to go higher than the service conductors.


I thought that he mentioned NM cable.

I could use 4/3 w/G NM conductor for 100 AMP sub panel

[Dennis Alwon]

If the service can be #4 then the sub does not have to be larger

[mdshunk]

Do they even make/normally stock 4-gauge NM? I thought you're generally into SER when you get that big?

[Dennis Alwon]

Do they even make/normally stock 4-gauge NM? I thought you're generally into SER when you get that big?


Yes and also #2 NM 4 wire

[Dennis Alwon]

I thought that he mentioned NM cable.


If it is a dwelling with a 100 amp service than #4 is okay--- agreed

A sub panel for 100 amp does not have to be larger than the service conductors. It would make no sense

[infinity]

If it is a dwelling with a 100 amp service than #4 is okay--- agreed

A sub panel for 100 amp does not have to be larger than the service conductors. It would make no sense


You're making a few assumptions. How do you know that the service entrance conductors are #4?

[Dennis Alwon]

You're making a few assumptions. How do you know that the service entrance conductors are #4?

You are correct I am assuming that the service was wired as per Table 310.15(B)(6)

[charlie b]

Plus you never have to go higher than the service conductors.
Care to save me the trouble of looking up that statement? :)

[Dennis Alwon]

Care to save me the trouble of looking up that statement? :)


Art 310.15(B)(6) next to last sentence

[celtic]

If it is a dwelling with a 100 amp service than #4 is okay--- agreed

A sub panel for 100 amp does not have to be larger than the service conductors.

Just caught up to that part....It would make no sense.

Now someone else can learn something too.

[celtic]

Care to save me the trouble of looking up that statement? :)

I just happened to have it open:


6) 120/240-Volt, 3-Wire, Single-Phase Dwelling Services and Feeders. For dwelling units, conductors, as listed in Table 310.15(B)(6), shall be permitted as 120/240-volt, 3-wire, single-phase service-entrance conductors, service lateral conductors, and feeder conductors that serve as the main power feeder to a dwelling unit and are installed in raceway or cable with or without an equipment grounding conductor. For application of this section, the main power feeder shall be the feeder(s) between the main disconnect and the lighting and appliance branch-circuit panelboard(s). The feeder conductors to a dwelling unit shall not be required to be larger than their service-entrance conductors. The grounded conductor shall be permitted to be smaller than the ungrounded conductors, provided the requirements of 215.2, 220.22, and 230.42 are met.

[vliposky]

My plan is to feed a new basement located 100 AMP service (with 100 AMP breaker) with #2, then from ANOTHER 100 AMP breaker in the primary service panel to feed a 100 AMP sub panel (with yet another) 100 AMP breaker (total 3) that will provide power to all circuits (except HVAC & H2O circuits which will be fed from the primary service panel) in this second floor dwelling unit of this residential duplex.... (The first floor unit have it's own 100 AMP service & no sub panel etc...) Does 310.15 (B) (6) apply under these circumstances???? ( I already have #4 CU installed so I want to use what I have if I can....)

[FrancisDoody]

After reading the highlighted sentence from above it only makes sense that a feeder need not be larger than the service entrance conductors to a dwelling unit.

[George Stolz]

I'm a little foggy on what you're up to.

If a feeder comes from a panelboard beyond the service (as in, the feeder does not start at the service) then you may not apply 310.15(B)(6).

Hope that helps,

[charlie b]

Care to save me the trouble of looking up that statement?

Art 310.15(B)(6) next to last sentence

Thought you were going there. Now I need to ask what version of the NEC applies in the jurisdiction under discussion? The reason I ask is that that phrase was revised in 2005. It no longer says that the feeder need not be BIGGER than the service. What it now says is that the feeder need not have a higher ampacity. But ampacity depends on the “conditions of use.” So it is possible that a feeder might need to have a larger size conductor, in order to get the same ampacity as the service conductors.

By the way, I think 310.15(B)(6) does not apply. Neither of the two “sub-panels” under discussion appears to be the “main power feeder” to a separate dwelling unit.

[radiopet]

Agree with Charlie.......the statement of 310.15(B)(6) is for the dwelling itself to which is being fed, the definition of the feeder in the example is from the service disconnect to the lighting and appliance panel...so if the service disconnect was on the lets say outside.....and run to a " remote distribution panel " inside as part of the main power feeder setup...then 310.15(B)(6) could apply.......not for feeders actually feeding other remote distribution panels not part of the main power feeder to a different structure.

6) 120/240-Volt, 3-Wire, Single-Phase Dwelling Services and Feeders. For dwelling units, conductors, as listed in Table 310.15(B)(6), shall be permitted as 120/240-volt, 3-wire, single-phase service-entrance conductors, service lateral conductors, and feeder conductors that serve as the main power feeder to a dwelling unit and are installed in raceway or cable with or without an equipment grounding conductor. For application of this section, the main power feeder shall be the feeder(s) between the main disconnect and the lighting and appliance branch-circuit panelboard(s). The feeder conductors to a dwelling unit shall not be required to be larger than their service-entrance conductors. The grounded conductor shall be permitted to be smaller than the ungrounded conductors, provided the requirements of 215.2, 220.22, and 230.42 are met.

[celtic]

Bob, Charlie and Paul....so what is answer?

Was this correct then??
310.16 says you need a #1. ( As supported by 334.80) Post #3 (http://forums.mikeholt.com/showpost.php?p=671359&postcount=3)

[bradleyelectric]

Why not use 60A breaker for the sub panels? The #4 is oversized for the breakers, but wont hurt. The 60A breakers cost less then the 100A breakers, and you have already pulled the major loads from the main service. Did you calculate the load that will be on the sub?

[infinity]

Does 310.15 (B) (6) apply under these circumstances???? ( I already have #4 CU installed so I want to use what I have if I can....)


No it doesn't. Since the #4 NM is already in place you'll need to use the 60 degree C rating of #4 in table 310.16. You'll need to use a maximum of a 70 amp CB for the #4. Unless your calculated load is greater than 70 amps. In that case you'll need to change something.

[Dennis Alwon]

I am totally confused the op's original question seems to be very different now. Anyone care to explain what we are looking at. Lost-----

[infinity]

I am totally confused the op's original question seems to be very different now. Anyone care to explain what we are looking at. Lost-----

New 100 amp service fed with #2's and with a piece of #4/3 NM cable to a subpanel. He wants to use table 310.15(B)(6) to size the OCPD for the subpanel feeder.

[Dennis Alwon]

Thought you were going there. It no longer says that the feeder need not be BIGGER than the service. What it now says is that the feeder need not have a higher ampacity. [/SIZE][/FONT]

Thank you Charlie--- I had been thinking about this all day today and I realized that the fact that NM cable was being used it would change what I was thinking. I just got the chance to get in and read what has transpired.

[Dennis Alwon]

New 100 amp service fed with #2's and with a piece of #4/3 NM cable to a subpanel. He wants to use table 310.15(B)(6) to size the OCPD for the subpanel feeder.


It's more than that--- he said something about 3 100 amp breakers---Is the actual service 100 amp or is terminology being used incorrectly

[celtic]

It's more than that--- he said something about 3 100 amp breakers---Is the actual service 100 amp or is terminology being used incorrectly
Got room on the lost boat?

I think we went from a "simple" 100A sub being feed from a 100A main in a SFH:
Seems the old forum said I could use 4/3 w/G NM conductor for 100 AMP sub panel (being fed from 100 AMP new service in this residential dwelling application...) Is this still true? (Post #1 (http://forums.mikeholt.com/showpost.php?p=671351&postcount=1) )

...to a 2-family or multi-family dwelling? unit:
My plan is to feed a new basement located 100 AMP service (with 100 AMP breaker) with #2, then from ANOTHER 100 AMP breaker in the primary service panel to feed a 100 AMP sub panel (with yet another) 100 AMP breaker (total 3) that will provide power to all circuits (except HVAC & H2O circuits which will be fed from the primary service panel) in this second floor dwelling unit of this residential duplex.... (The first floor unit have it's own 100 AMP service & no sub panel etc...) Does 310.15 (B) (6) apply under these circumstances???? ( I already have #4 CU installed so I want to use what I have if I can....) (Post #22 (http://forums.mikeholt.com/showpost.php?p=671393&postcount=22) )


I drew this picture...obviously it's not right...but that's what I am getting from the post..
http://img114.imageshack.us/img114/5564/subpanelset7.jpg

[Dennis Alwon]

Quite a change from the OP. Given the new info I would agree that Table 310.15(B)(6) does not apply. A #2 copper will be sufficient if the load is less than 95 amps. The #4 would have to be fused at 70 amps

[celtic]

I agree that 310.15(B)(6) doesn't apply(yes, that's me back peddleing ).....but something STILL doesn't make sense about all this to me....I can't visualize the layout - hence that ridiculous drawing.

[Dennis Alwon]

It ain't ridiculous. I appreciate the effort.

[celtic]

It ain't ridiculous. I appreciate the effort.

Thanks for the thought....but it is ridiculous....100A MCB panels and 100A OCPD in the main - why?

[bradleyelectric]

I'm thinking 100A service switch feeding a 100A panel with a 100A breaker in it feeding a 100A breaker that is being used to backfeed a MLO panel, but I haven't seen it to verify that mess. That's why I was thinking just put a 60A breaker in the Panel being used as the service to feed the 4-3 NM running to the MLO panel that I think is only feeding general lighting. Again nobody verifying this, just what I'm conjuring up in my wild imagination. I see Celtic came up with a MCB instead of the backfed breaker I had the guy buying, still neither is neccesary and you can still feed a 100A MCB panel with a 60A breaker if that is what exists or you have 1 sitting in the truck in good shape from the service upgrade you did yesterday if you want. Not condoning using that old panel, but we do tenant refits all the time where the new tenant uses the existing service without it being tested or reconditioned.

Ok, I did another reread and it seems he possibly has 2 100A MCB panels sitting in a basement. panel 1 feeds the first floor unit. panel 2 feeds the second floor unit HVAC, HWH, and a piece of 4-3 NM run to the second floor. He wants to put the 4-3 on a 100A breaker and have it feed a subpanel that has a 100A breaker of some configuration with unknown load. My suggestion is do a load calc. and make sure a 60A breaker will suffice and call it a day.

[celtic]

Maybe the OP can draw us a picture?

It's really not that hard...basic info is all that is really needed.

[vliposky]

Second floor unit sub panel will have kitchen stove, 220 dryer, 220 4000 watt heater outlet, one bedroom circuit, living room circuit, 2 bath circuits, 2 kitchen counter circuits, fridge circuit, washing machine circuit, lighting circuit, hardwired smoke detector circuit... How many amps do I need to provide for in this sub panel ? (HVAC & HWH will be wired from primary 100 AMP basement service panel...)

[celtic]

How many amps do I need to provide for in this sub panel ?
Do a load calculation as detailed in 220.

[Oakey]

Nothing gets more attention here that the infamous 100 amp sub feed question about 310.15(B)(6) or 310.16. What a can of worms LOL

[vliposky]

Since I am totally unsure of myself, I need help....

[FrancisDoody]

Range 5000VA
Dryer 5000VA
Heat 4000VA
Kitchen 3000VA
Laundry 1500VA
1000SF 3000VA

SF 3000VA
Kitchen 3000VA
Laundry 1500VA
Total 7500VA
-3000VA
4500X0.35%=1575 + 3000=4575VA

Demand 4575VA
Range 5000VA
Dryer 5000VA
Heat 4000VA

Total 18575VA/240=77amps
This is a rough estimate based on the limited information. The 1000 sf area is a guess of the square footage of your project.

[celtic]

Since I am totally unsure of myself, I need help....

Residential Load Calculations (http://mikeholt.com/documents/calculations/formulas/ResidentialLoadCalculations.xls)
Dwelling Unit (780K Spreadsheet)

[Dennis Alwon]

In the figure below is the first panel or the second panel a Lighting & Appliance Branch Panelboard (L&ABP)? Or are they both?

The 6 cir panel control 6 DP circuit for 2-- electric furnaces, 2--Heat Pumps, Well Pump, and the Sub Panel

383
Fig. 1

[LarryFine]

My input on this subject is that, while it may be okay to size the sub-panel's feeder as small as the permissible main feeder (now), if the service is ever upgraded, I'd hate to have to replace the sub-panel's feeder (later).

[vliposky]

Thank you FrancisDoody and all you guys... You have all been very patient and helpful...

Now To restate the question:

I have two 100A MCB panels sitting in a residential duplex basement... Panel 1 feeds the first floor unit... Panel 2 feeds the second floor unit AC, HWH and has a piece of 4-3 w/G NM run to the second floor... Want to put the 4-3 on a 100A breaker and have it feed a 100A MCB panel serving as sub panel ...

Calculated load at the sub panel is 78 AMPS

Does 310.15 (B) (6) apply or not... If so, why and if not, why not?

If not with this layout....(Which seems to me very very basic) where would it apply?

[celtic]

Is the calculated load on the Panel #2 (that will feed the sub) less than 22A?
(100-78 = 22)

[vliposky]

By going with gas HWH, yes, under 22 AMPS

[celtic]

... Panel 2 feeds the second floor unit AC, HWH and has a piece of 4-3 w/G NM run to the second floor... Want to put the 4-3 on a 100A breaker and have it feed a 100A MCB panel serving as sub panel ...



Does 310.15 (B) (6) apply under these circumstances?
I think your problem is going to be that piece of #4 NM...

No it doesn't. Since the #4 NM is already in place you'll need to use the 60 degree C rating of #4 in table 310.16. You'll need to use a maximum of a 70 amp CB for the #4. Unless your calculated load is greater than 70 amps. In that case you'll need to change something. Calculated load at the sub panel is 78 AMPS You're over the 70

Does 310.15 (B) (6) apply under these circumstances? 310.16(B)(6) does not list NM as one of the conductors, as listed in Table 310.15(B)(6).
T310.16(B)(6) lists:
Conductor Types RHH, RHW, RHW-2, THHN, THHW, THW, THW-2, THWN, THWN-2, XHHW, XHHW-2, SE, USE, USE-2

334.80 states:

Ampacity.
The ampacity of Types NM, NMC, and NMS cable shall be determined in accordance with 310.15. The ampacity shall be in accordance with the 60°C (140°F) conductor temperature rating. The 90°C (194°F) rating shall be permitted to be used for ampacity derating purposes, provided the final derated ampacity does not exceed that for a 60°C (140°F) rated conductor.
334.80 says use 310.15.
(NOTE: Portions unrelated not copied)

[Dennis Alwon]

Are we talking about 2 separate apartments????

Let me see if I understand. The two panels in the basement are their own service, ie, 2 services (separately metered). The first MB panel feeds everything on the first floor--heat, kitchen, HW, etc. The second panel feeds the entire second floor load also and nothing else.



310.16(B)(6) does not list NM as one of the conductors, as listed in Table 310.15(B)(6).
T310.16(B)(6) lists:
Conductor Types RHH, RHW, RHW-2, THHN, THHW, THW, THW-2, THWN, THWN-2, XHHW, XHHW-2, SE, USE, USE-2


Art. 310.15(B)(6) clearly states that "....feeders that serve as the main power load to each dwelling unit and installed in raceway or cable...." shall be allowed to use T310.15(B)(6).

I agree with Celtic that T310.15(B)(6) does not mention cable but the art. does.
My call is it is good and T310.15(B)(6) applies--- but what do I know. I am going by the wording not by my common sense.

I think if the NEC wants NM to be rated at 60C then they need to fix this article because it leaves too much room for interpretation.

[celtic]

Art. 310.15(B)(6) clearly states that "....feeders that serve as the main power load to each dwelling unit and installed in raceway or cable...." shall be allowed to use T310.15(B)(6).

2005 NEC 310.16(B)(6) states..in it's entirety:
120/240-Volt, 3-Wire, Single-Phase Dwelling Services
and Feeders.
For individual dwelling units of one family, two-family, and multifamily dwellings, conductors, as listed in Table 310.15(B)(6), shall be permitted as 120/240-volt, 3-wire, single-phase service-entrance conductors, service lateral conductors, and feeder conductors that serve as the main power feeder to each dwelling unit and are installed in raceway or cable with or without an equipment grounding conductor. For application of this section, the main power feeder shall be the feeder(s) between the main disconnect and the lighting and appliance branchcircuit panelboards(s). The feeder conductors to a dwelling unit shall not be required to have an allowable ampacity rating greater than their service-entrance conductors. The grounded conductor shall be permitted to be smaller than the ungrounded conductors, provided the requirements of 215.2, 220.61, and 230.42 are met.

T310.16(B)(6) does not list NM...but it does list SE, USE, USE-2.


I think if the NEC wants NM to be rated at 60C then they need to fix this article because it leaves too much room for interpretation.
I agree.
At the very least put a reference back to 334.80 in there (310).

[Dennis Alwon]

I agree with you NM should be excluded in a more precise way. It almost as if they either forgot to included it or they meant to exclude it. I think it was meant to be excluded--- I missed the last 3 listing of cable in T 310.15(B)(6).

Good catch....

[vliposky]

Panel 2 serves the entire second floor unit (separate meter). Panel 2 in basement serves AC unit and the second floor thru 4-3 wG NM feeding 100A MCB panel....

Re Conductors INSIDE the 4-3... each is marked THHN... the NM (I think means the outer sheath is Non Metallic)

[Dennis Alwon]

The bottom line is that 4/3 NM cannot be used higher than 60C which translates to 70 amps. Table 310.15(B)(6) will not apply.

[vliposky]

If not here, where would the #4 be usable for 100 AMPS?

[celtic]

If not here, where would the #4 be usable for 100 AMPS?
A #4 NM would never be usuable at 100A ~ 334.80 & T310.16

[George Stolz]

Re Conductors INSIDE the 4-3... each is marked THHN... the NM (I think means the outer sheath is Non Metallic)
Not to say I don't believe you, but could you post a picture of the 4-3 NM? I've never seen NM with marked conductors inside the sheath.

If the conductors were in fact marked "THHN" then there would be a viable means to apply T310.15(B)(6) to this feeder, IMO.

[vliposky]

Just talked with my brother who sold for wire manufacturer Southwire for years... (Biggest customer was GM in the old days) He says ALL wire in NM sheath is marked (or should be) by either printing or having been embossed... (Makes sense since at the time the individual conductors are manufactured, no one knows what it's final application will be... ie no one knows it might end up inside a NM sheathed cable...) Will check marks again tomorrow, however... The embossing is hard to read...

[roger]

Vliposky, if you do find that the conductors inside the sheath are identified as of conductor type please let us know who the manufacturer of this NM is.

I have heard others say they have seen conductors marked as THHN/THWN inside NM only to find after closer examination they were not.

Roger

[Dennis Alwon]

If not here, where would the #4 be usable for 100 AMPS?
If the cable you were using was a copper service entrance cable- 4 wire- then you would be able to apply Table 310.15(B)(6).

I have been told by the wire manufacturer that NM cable does in fact use THHN insulation. This, however would not fly without it being labeled on the wire or the jacket of the cable.

I have made a complete turnaround, (no surprise) in my thinking on this. I realize that you may have grounds to stand on, as George has said, if the cable were marked THHN. The fact is the NM cable is not listed in Table 310.15(B)(6) and if it were I still do not believe the NEC's intent is to use NM for Table 310.15(B)(6).

Why? Because if you look at Table 310.15(B)(6) you will see that the NEC lists conductors as well as cables at the top. So pretend that NM cable has THHN wire and is marked. The table still does not list NM.

Maybe it is an unintentional omission you may say. I say this--SE cable is listed and most SE cable has XHHW-2 conductors inside and it is marked on the sheathing. Table 310.15(B)(6) list THHN but not NM cable yet it lists xhhw-2 and it also list SE cable. That would be redundant if they wanted us to use the conductor insulation as a basis for this table.

I believe the NM was intentionally left off Table 310.15(B)(6) because it's temp rating should be used only at 60C.

I would still love to know why NM is only listed as 60C-- I was also told by the wire manufacturer that the jacket on NM is the same insulation as the wire, thus it is rated 90C.

The only way we know that the conductors of NM cable are 90C is because the NEC says it must be art.334.112. The NEC does not specify what temperature the jacket insulation should be.

[bradleyelectric]

never mind

[hillbilly]

If not here, where would the #4 be usable for 100 AMPS?

IMO....If your cable was one of the types listed in 310.15(B)(6), you could use #4 copper in your application.
I've stripped a lot of NM and I've never seen any insulation markings on the conductors.
steve

[vliposky]

Cannot verify conductor marks or absence thereof... Cable is now installed and not stripped enough to read.... CAPEX brand cable manufacturer not located.... Another cable & wire sales guy says NM is probably THHN inside but probably not marked....

[roger]

Cannot verify conductor marks or absence thereof... Cable is now installed and not stripped enough to read.... CAPEX brand cable manufacturer not located.... Another cable & wire sales guy says NM is probably THHN inside but probably not marked....

And this fact takes you back to the conductors only being allowed to be used at 60 deg.

310.11, (the key here) 334.80, and 334.112

Roger