Multifamily Construction R2 feeder calculation

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andy@advancelectric.com

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Hi, I have a project I am working on, which originally was specified from the electrical room ( multiple meters stack) to unit buildings subpanel feeders rated 100 amps with conduit size 1-1/4" with 3x#4 +1#8 THWN CU. All the switchgear is rated 75 degrees terminations. Now, this is where I am confused if I run NMB cable 4-3 as a feeder. Will this meet code? I am concern about 334.80, where says I need to determine the ampacity in accordance with 310.15, but then say the allowance ampacity shall not exceed the 60 degrees which makes this wire only good for 70 amps. If I consider the 83% factor an adjustment factor, so you can use the 75C table per 310.15(B)(7)(2) this wire is good for 85 amps, or NMB cable needs to be always calculated at 60 degrees.

Thanks,
 
andy@advancelectric.com said:
Hi, I have a project I am working on, which originally was specified from the electrical room ( multiple meters stack) to unit buildings subpanel feeders rated 100 amps with conduit size 1-1/4" with 3x#4 +1#8 THWN CU. All the switchgear is rated 75 degrees terminations. Now, this is where I am confused if I run NMB cable 4-3 as a feeder. Will this meet code? I am concern about 334.80, where says I need to determine the ampacity in accordance with 310.15, but then say the allowance ampacity shall not exceed the 60 degrees which makes this wire only good for 70 amps. If I consider the 83% factor an adjustment factor, so you can use the 75C table per 310.15(B)(7)(2) this wire is good for 85 amps, or NMB cable needs to be always calculated at 60 degrees.

Thanks,
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I don't believe you can use the NMB in this fashion. 110.14(C) 1 states circuits 100 A or less. So you can not use the NMB at the 75C rating. Also as you pointed out, that 334.80 says that at the end of the day - NMB is at 60 C
 
andy@advancelectric.com said:
Hi, I have a project I am working on, which originally was specified from the electrical room ( multiple meters stack) to unit buildings subpanel feeders rated 100 amps with conduit size 1-1/4" with 3x#4 +1#8 THWN CU. All the switchgear is rated 75 degrees terminations. Now, this is where I am confused if I run NMB cable 4-3 as a feeder. Will this meet code? I am concern about 334.80, where says I need to determine the ampacity in accordance with 310.15, but then say the allowance ampacity shall not exceed the 60 degrees which makes this wire only good for 70 amps. If I consider the 83% factor an adjustment factor, so you can use the 75C table per 310.15(B)(7)(2) this wire is good for 85 amps, or NMB cable needs to be always calculated at 60 degrees.

Thanks,
Click to expand...
The 83% is not an "adjustment factor," but I'm not sure how calling the 83% an adjustment factor would allow you to use the 75C ampacity.

If you were on the 2020 NEC, you could argue that you could use the #4 NM for the 100A feeder as long as you had not adjustment or correction factors to apply, per 310.12.

Use SE cable instead of NM cable and your problem is solved.
 
David, I still have not purchased the 2020 NEC yet. What would now allow #4 NMB to be utilized for a 100 amp feeder? Its only rated at 70 amps.
 
curt swartz said:
David, I still have not purchased the 2020 NEC yet. What would now allow #4 NMB to be utilized for a 100 amp feeder? Its only rated at 70 amps.
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310.12(B) Feeders: says if no adjustment or correction factors are required, Table 310.12 shall be permitted to be applied.

Table 310.12 says you can use #4 copper for a 100A single phase dwelling unit feeder.
 
david luchini said:
310.12(B) Feeders: says if no adjustment or correction factors are required, Table 310.12 shall be permitted to be applied.

Table 310.12 says you can use #4 copper for a 100A single phase dwelling unit feeder.
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So T310.12 doesn't list specific wire (insulation) types like previous tables did?
 
david luchini said:
Table 310.12 says you can use #4 copper for a 100A single phase dwelling unit feeder.
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NO, bottom of NEC Annex D, Example D7 explains that table is based on 75c.
the requirements in 310.15(B)(7) said:
This table is based on 75°C terminations and without any adjustment or correction factors.
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NM Cable listed for 60°C cannot be used with that table.
 
ramsy said:
NO, bottom of NEC Annex D, Example D7 explains that table is based on 75c.
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The annex example says that the table is based on 75C Terminations. NM cable can be used with 75C Terminations.

But regardless of that, the Annex is not an enforceable part of the Code. Nothing in the text of 310.12 mentions temperature ratings for ampacity or insulation type.
 
ramsy said:
Listings and OEM instructions limit Romex NM cable to 60°C ampacity,
not for use with tables based on 75°C ampacity.

#4cu is rated for 70 A, not 85 A
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Table 310.12 doesn't say its based on 75C ampacity.

And #4 THWN has an ampacity of 85, not 100, but the Code lets you use it for a 100A feeder in this instance.

The table in question doesn't say anything about insulation type , insulation temperature rating, or ampacity. It simply says #4 copper conductors can be used for a 100A single phase dwelling feeder when no adjustment or correction factors are required.
 
david luchini said:
#4 THWN has an ampacity of 85, not 100, but the Code lets you use it for a 100A feeder in this instance.

The table in question doesn't say anything about insulation type , insulation temperature rating, or ampacity. It simply says #4 copper conductors can be used for a 100A single phase dwelling feeder when no adjustment or correction factors are required.
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Yes, if OP avoids Romex or other NM cable brand, which shows #4 @ 70A max ampacity.

See #4 specs on Pg.2 of listed instructions below:
 
The current version of the "83%" rule simply permits a 100A residential service, supplying the entire load of a residence, to be carried on conductors with 83A ampacity. No specific conductors required, just 83A or more ampacity.

Previous versions of this rule specified allowed conductor types and sizes.

As I recall, for 100A, #4 Cu could be used, but NM was not one of the allowed types.

Now you can use NM, but the rules for NM specify that it be installed using the 60C ampacity, even though modern NM-B uses 90C insulation and is likely used with 75C terminations.

If you want to use the 75C ampacity of copper for this application, you will need a different cable type, such as MC.

Jon
 
Thanks so much for the input given by all. Just to clarify even in the future 2020 code you will not be able to use #4 awg NMB or SER cable. Still need to be calculated 60 degrees ampacity correct ?
 
andy@advancelectric.com said:
Thanks so much for the input given by all. Just to clarify even in the future 2020 code you will not be able to use #4 awg NMB or SER cable. Still need to be calculated 60 degrees ampacity correct ?
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Your missing your homework.

Inspectors look for Listed equipment limits described in NFPA-70 110.14(C) that override all NEC idiot tables.

Study this section to the end informational note. Then check the listings. The default limit for existing equipment or cables with no markings is 110.14(C)(1)(a).

Strip back any NM cable sheath to examine if any markings exist on the actual conductors. If no markings are present, or legible on sheath 110.14(C)(1)(a) is the default.

Each combination of wire / termination can have different listed limits, which reasonable persons can understand, unlike the idiot tables.
 
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