Help with Husky 60 Gallon Compressor

[Macgyver_ga]

I bought some 10/2 nm-b wire and a 30 amp breaker this weekend. I'll install it sometime in the next week or so.

Luckily GE also makes a 30-amp two pole slim breaker so I don't have to take up anymore space in the panel.

 

[Crazy68Dart]

Technically because its 3HP and because #12 NM-b is limited to 20a, it needs to be wired with #10 NM-b or #12 THHN in conduit

Hi, I am laying out my electrical as well. Question on this. Can you explain this further? Charts that I have researched, at this wire size (larger sizes there is some separation in capacity), the current capacity is the same. Is THHN rated differently due to its insulation properties or is it because it is encapsulated in conduit? Or is it strictly a NEC requirement?

FWIW, I am planning on using NM-b, stick build, in the walls before I insulate and drywall.

OP, not trying to hijack this at all. I can start another thread if need be. Sorry.

 

[Norcal]

Hi, I am laying out my electrical as well. Question on this. Can you explain this further? Charts that I have researched, at this wire size (larger sizes there is some separation in capacity), the current capacity is the same. Is THHN rated differently due to its insulation properties or is it because it is encapsulated in conduit? Or is it strictly a NEC requirement?

FWIW, I am planning on using NM-b, stick build, in the walls before I insulate and drywall.

OP, not trying to hijack this at all. I can start another thread if need be. Sorry.

Although NM cable made since around 1985 has 90° rated conductors, the NEC limits it to 60° ampacity, so 8 AWG THHN is 50A, 8/2 NM cable is 40A, the -B suffix on NM-B cable means it has 90° conductors.

 

[Crazy68Dart]

Thanks.

FWIW, this is the matrix I was looking at:

http://www.cerrowire.com/ampacity-charts

 

[pattenp]

Thanks.

FWIW, this is the matrix I was looking at:

http://www.cerrowire.com/ampacity-charts

That table is correct for when the small conductor limit is used for #14 #12 and #10, for such as general outlet branch circuits. Otherwise THHN #14 = 20A, #12 = 25A and #10 =35A when used for motor circuits, such as hardwired compressors.

 

[Crazy68Dart]

That table is correct for when the small conductor limit is used for #14 #12 and #10, for such as general outlet branch circuits. Otherwise THHN #14 = 20A, #12 = 25A and #10 =35A when used for motor circuits, such as hardwired compressors.

Thanks. So, is there also a rating for NM-B for motor circuits?

 

[md21722]

The amperage rating for THHN / NM-B does not vary based on usage. The difference is that NM-B must use the 60C amperage ratings, whereas if one is using THHN (in conduit) then the 90C rating applies The 90C amperage ratings are higher, as indicated by the above post. For example, #10 NM-B is 30A but #10 THHN is 35A. By code a 5HP motor uses 28A FLC at 230V and the wire size used must be 125% of the FLC. So that means for a 5HP motor, one would need to use #8 NM-B or #10 THHN. When following these rules, the code then allows one to install a maximum breaker 250% of motor FLC. So that means a maximum of a 70A breaker could be used as long as the motor has thermal protection or a magnetic starter with heaters (even though one is using #8 NM-B or #10 THHN). Bottom line, NEC has specific rules for motor circuits that differ to the rules commonly associated with residential circuits or resistive loads such as those used on electric baseboard heaters.

 

[wyliesdiesels]

Thanks. So, is there also a rating for NM-B for motor circuits?

Nope.

The amperage rating for THHN / NM-B does not vary based on usage. The difference is that NM-B must use the 60C amperage ratings, whereas if one is using THHN (in conduit) then the 90C rating applies The 90C amperage ratings are higher, as indicated by the above post. For example, #10 NM-B is 30A but #10 THHN is 35A. By code a 5HP motor uses 28A FLC at 230V and the wire size used must be 125% of the FLC. So that means for a 5HP motor, one would need to use #8 NM-B or #10 THHN. When following these rules, the code then allows one to install a maximum breaker 250% of motor FLC. So that means a maximum of a 70A breaker could be used as long as the motor has thermal protection or a magnetic starter with heaters (even though one is using #8 NM-B or #10 THHN). Bottom line, NEC has specific rules for motor circuits that differ to the rules commonly associated with residential circuits or resistive loads such as those used on electric baseboard heaters.

The 90* c column is for derating purposes only. U cannot use the 90* c rating for sizing conductors that do no need to be derated.

 

[kingchevy]

The amperage rating for THHN / NM-B does not vary based on usage. The difference is that NM-B must use the 60C amperage ratings, whereas if one is using THHN (in conduit) then the 90C rating applies The 90C amperage ratings are higher, as indicated by the above post. For example, #10 NM-B is 30A but #10 THHN is 35A. By code a 5HP motor uses 28A FLC at 230V and the wire size used must be 125% of the FLC. So that means for a 5HP motor, one would need to use #8 NM-B or #10 THHN. When following these rules, the code then allows one to install a maximum breaker 250% of motor FLC. So that means a maximum of a 70A breaker could be used as long as the motor has thermal protection or a magnetic starter with heaters (even though one is using #8 NM-B or #10 THHN). Bottom line, NEC has specific rules for motor circuits that differ to the rules commonly associated with residential circuits or resistive loads such as those used on electric baseboard heaters.

Just curious, if you can use a breaker 70A breaker on #8 Romex, where is the overcurrent protection for the wire say if the starter were to fail?

 

[wyliesdiesels]

Just curious, if you can use a breaker 70A breaker on #8 Romex, where is the overcurrent protection for the wire say if the starter were to fail?

Depends on the application.

However, contactor which starts the motor doesnt provide oveecurrent protection. Thats the job of the overload relay- which is either heaters or solid state.