Conductor size for 100 amp sub panel?

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Bradleyelectric post #39 layout is exactly correct- vliposky

Bradleyelectric post #39 layout is exactly correct- 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...)
 
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
 
Standard calculations

Standard calculations

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.
 
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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

 
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).
 
Thanks

Thanks

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?
 
Is the calculated load on the Panel #2 (that will feed the sub) less than 22A?
(100-78 = 22)
 
vliposky said:
... 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?
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I think your problem is going to be that piece of #4 NM...

infinity said:
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.
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vliposky said:
Calculated load at the sub panel is 78 AMPS
Click to expand...
You're over the 70

vliposky said:
Does 310.15 (B) (6) apply under these circumstances?
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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.
Click to expand...
334.80 says use 310.15.
(NOTE: Portions unrelated not copied)
 
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
Click to expand...


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.
 
Dennis Alwon said:
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).
Click to expand...
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.
Click to expand...

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

Dennis Alwon said:
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.
Click to expand...
I agree.
At the very least put a reference back to 334.80 in there (310).
 
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....
 
Panel 2

Panel 2

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