Existing 100 Amp Subpanel fed with 4-3 Romex (2024)

I was asked to give an estimate for wiring an apartment above an existing residential garage. Currently there is a 100 amp panel located in the garage that is fed with 4-3 romex from a 100 amp breaker located in a 200 amp panel located in the main house.
I'm aware that according to article 334.80, romex is only rated for use in the 60 deg column. Which in this case #4 romex would only be good for 70 amps. As a result, I believe I could do one of the following, correct?

1. I could do a load calculation for the future apartment, and if applicable, I could change out the 100 amp breaker for a 70 amp breaker.

2. I could re-feed the 100 amp subpanel with SER AL. I believe that would have to be sized in the 60 degree column since it will pass through thermal insulation. In that case it would be 1/0 SER AL, Correct?

Correct on both.

Edit: Durn bug eyed rattler beat me by 1/2 second.

jumper said:

Correct on both.

Edit: Durn bug eyed rattler beat me by 1/2 second.

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LOL! Your comment made my day

jt700 said:

I was asked to give an estimate for wiring an apartment above an existing residential garage. Currently there is a 100 amp panel located in the garage that is fed with 4-3 romex from a 100 amp breaker located in a 200 amp panel located in the main house.
I'm aware that according to article 334.80, romex is only rated for use in the 60 deg column. Which in this case #4 romex would only be good for 70 amps. As a result, I believe I could do one of the following, correct?

1. I could do a load calculation for the future apartment, and if applicable, I could change out the 100 amp breaker for a 70 amp breaker.

2. I could re-feed the 100 amp subpanel with SER AL. I believe that would have to be sized in the 60 degree column since it will pass through thermal insulation. In that case it would be 1/0 SER AL, Correct?

Click to expand...

I read in a previous post that the 2017 code removes the derating for thermal insulation for SER cable and certain size romex. Art 338(10)B4. I was told by an inspector that you can apply for a modification for something in an upcoming code (here in Ct anyway). So that would mean you could run #2 SER AL

mbrooke said:

Correct on one and two. If this were Canada, 4-3 NM would be sufficient

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jumper said:

Correct on both.

Edit: Durn bug eyed rattler beat me by 1/2 second.

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Not so fast Y'all....OP is in MA which is on 2017. I dont have a 2017 book yet but I think they got rid of the SER in thermal insulation clause right?

electrofelon said:

Not so fast Y'all....OP is in MA which is on 2017. I dont have a 2017 book yet but I think they got rid of the SER in thermal insulation clause right?

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IDK, prolly best to just rid of MA entirely IMO, the place is full of them darn Yanks.

GerryB said:

I read in a previous post that the 2017 code removes the derating for thermal insulation for SER cable and certain size romex. Art 338(10)B4. I was told by an inspector that you can apply for a modification for something in an upcoming code (here in Ct anyway). So that would mean you could run #2 SER AL

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electrofelon said:

Not so fast Y'all....OP is in MA which is on 2017. I dont have a 2017 book yet but I think they got rid of the SER in thermal insulation clause right?

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You guys are probably right- lemme check that.

mbrooke said:

I'm seeing it for SE but not NM in the 2017.

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I do not see it in the MA amendments.

http://www.mass.gov/eopss/docs/dfs/...s/mass-elec-code-527cmr1200-publichearing.pdf

Looks like MA put in back for both NM and SE.

jumper said:

I do not see it in the MA amendments.

http://www.mass.gov/eopss/docs/dfs/...s/mass-elec-code-527cmr1200-publichearing.pdf

Looks like MA put in back for both NM and SE.

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2017 NEC without any locals.

This is something that should be addressed IMO- at least some panel members want it tackled.

mbrooke said:

2017 NEC without any locals.

This is something that should be addressed IMO- at least some panel members want it tackled.

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You lost me.

We are looking into the 2017 for MA regarding SE/SER and NM ampacity when installed in thermal insulation, correct?

jumper said:

You lost me.

We are looking into the 2017 for MA regarding SE/SER and NM ampacity when installed in thermal insulation, correct?

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Well you were technically... I did not see the OP was in MA originally, so I was going by the 2017 NEC after Gerry pointed it out.

mbrooke said:

Well you were technically... I did not see the OP was in MA originally, so I was going by the 2017 NEC after Gerry pointed it out.

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So what specific issue were saying should addressed and that is one certain CMP members want "tackled"?

I get easily befuddled.

jumper said:

So what specific issue were saying should addressed and that is one certain CMP members want "tackled"?

I get easily befuddled.

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Lifting the 60*C restriction on NM and SE cable. Canada has done this for some years but within the NFPA there is debate on whether this should be tackled. Considering that both modern SE and NM have 90*C conductor and a 90*C jacket, and that values listed in 310.15 are already conservative, there would be no risk in removing the restriction which is a hold over imposed from many decades ago.

The OP here is a perfect example. Technically there is no hazard keeping that NM on a 100amp breaker, but code could force a re-wire. Another common whammy is a car charger. In Canada and elsewhere #3 NM would work, but MC or conduit would be required for the same setup here.

jumper said:

IDK, prolly best to just rid of MA entirely IMO, the place is full of them darn Yanks.

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Hey...let's be nice...it's Thanksgiving. Be thankful of us MA Yanks...we come up with some good Code rules...including that if romex is not in insulation, the 75 degree column can be used.

shortcircuit2 said:

Hey...let's be nice...it's Thanksgiving. Be thankful of us MA Yanks...we come up with some good Code rules...including that if romex is not in insulation, the 75 degree column can be used.

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Well, I 'spose all you fellers ain't all bad.

Happy bird day to all, God bless.

The way I understand is that in the 2017 NEC the 60 degree temperature rating only applies to sizes 10 AWG SE cable or smaller.

4) Installation Methods for Branch Circuits and Feeders.
(a) Interior Installations. In addition to the provisions of
this article, Type SE service-entrance cable used for interior
wiring shall comply with the installation requirements of Part II
of Article 334, excluding 334.80.
For Type SE cable with ungrounded conductor sizes 10 AWG
and smaller, where installed in thermal insulation, the ampacity
shall be in accordance with 60°C (140°F) conductor temperature
rating. The maximum conductor temperature rating shall
be permitted to be used for ampacity adjustment and correction
purposes, if the final derated ampacity does not exceed
that for a 60°C (140°F) rated conductor.

packersparky said:

The way I understand is that in the 2017 NEC the 60 degree temperature rating only applies to sizes 10 AWG SE cable or smaller... (ie) 338.10(b)4(a)

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Point taken, you are correct, if not for 110.14(c)1(a), which governs existing 100A garage-panel terminations at 60c, unless explicitly listed or labeled for 75C.

packersparky said:

The way I understand is that in the 2017 NEC the 60 degree temperature rating only applies to sizes 10 AWG SE cable or smaller.

4) Installation Methods for Branch Circuits and Feeders.
(a) Interior Installations. In addition to the provisions of
this article, Type SE service-entrance cable used for interior
wiring shall comply with the installation requirements of Part II
of Article 334, excluding 334.80.
For Type SE cable with ungrounded conductor sizes 10 AWG
and smaller, where installed in thermal insulation, the ampacity
shall be in accordance with 60°C (140°F) conductor temperature
rating. The maximum conductor temperature rating shall
be permitted to be used for ampacity adjustment and correction
purposes, if the final derated ampacity does not exceed
that for a 60°C (140°F) rated conductor.

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Thank you posting code section, I always like to see the exact wording.