Sizing wire and overcurrent protection of Motors

[wmeek]

I have a 5HP 1phase at 15A nameplate at 230V motor on a Air Compressor

Table 430-248 says 5hp is 28A

So we have :
Conductor sizing 28A X 125%= 35A = #8 Thhn
OCP 28A x 250%(per 430-52 1ph motors max) = 70A ( Can I go down to a 50A Breaker)

 

[ActionDave]

You can go smaller if you want to. The only reason to use the max breaker is to handle the inrush when the motor starts. 50A breaker will likely handle it.

Are you worried about the 70A breaker on 8 gauge wire?

 

[Jraef]

I have a 5HP 1phase at 15A nameplate at 230V motor on a Air Compressor

Table 430-248 says 5hp is 28A

So we have :
Conductor sizing 28A X 125%= 35A = #8 Thhn
OCP 28A x 250%(per 430-52 1ph motors max) = 70A ( Can I go down to a 50A Breaker)

Does the actual MOTOR nameplate say 5HP? Or does a sticker on the side of the compressor say something like "Develops 5HP!"? HP ratings on marketing materials for compressors are not the same as the actual HP rating on the nameplate of a motor as required by UL. So when they use their BS ratings for marketing, they often leave the HP off of the motor nameplate and use kW (or even just Watts). So if it says, as I suspect, "1500W", that's roughly 2HP, not 5HP. The "develops" thing has to do with PEAK torque (breakdown torque) that the motor can put out for a few seconds.

The other possibility is that you have a 3 phase motor, not a single phase motor. If so, and there is an inverter that is providing phase conversion (single phase in - three phase out), then you need to know because that affects the circuit you feed it with. If you have a 3 phase motor and you THOUGHT you had a single phase motor, you have an entirely different problem!

Bottom line though, there is no way that is a NAMEPLATED 5HP motor that is 15A single phase 230V. The laws of physics will disagree with that concept.

 

[wmeek]

Actually the unit has a sticker on it with all the markings. Have not seen it to know if the motor has a tag on it or not . I will just have to go see it myself to determine what is actually there.

 

[wmeek]

So I have a HP 5.SPL 1phase 15A 230V motor on a Air Compressor. Not sure what the SPL stands for but would i still use Table Amps from 430.248 to size both the conductors and OCP.

 

[wmeek]

So after reading Article 430 and seeing that 430.6 A1 says use table 430-248 for amperage values for conductors and OCP I come up with This

28A x 125% = 35A for conductor

28A x 250%( Per 430-52)= 70A

So if I use a #8 THHN Conductor and go with a 2p50A Breaker, am I interpreting this correctly

 

[texie]

So after reading Article 430 and seeing that 430.6 A1 says use table 430-248 for amperage values for conductors and OCP I come up with This

28A x 125% = 35A for conductor

28A x 250%( Per 430-52)= 70A

So if I use a #8 THHN Conductor and go with a 2p50A Breaker, am I interpreting this correctly

28A X 125% = 35A - You could use #10 THHN assuming 75 degree terminations.

However, I don't think for an air compressor such as yours that has a motor marked SPL that this would be the correct application of code as this is not a standard motor and is purpose built just for this "appliance" and you would use the nameplate amps of 15. In my view this would fall under 430.6(A)(1) Exception No. 3 as this is likely a listed product and probably even has a factory cord set.

 

[wmeek]

28A X 125% = 35A - You could use #10 THHN assuming 75 degree terminations.

However, I don't think for an air compressor such as yours that has a motor marked SPL that this would be the correct application of code as this is not a standard motor and is purpose built just for this "appliance" and you would use the nameplate amps of 15. In my view this would fall under 430.6(A)(1) Exception No. 3 as this is likely a listed product and probably even has a factory cord set.

So if 430.6 A1(Ex #3) is used

15A x 125%= 18.75A #12 THHN could be used

15A x 250%= 37.5A which a 2p20A breaker could be used or a 2p30A Breaker

 

[Jraef]

So I have a HP 5.SPL 1phase 15A 230V motor on a Air Compressor. Not sure what the SPL stands for but would i still use Table Amps from 430.248 to size both the conductors and OCP.

SPL means "Special"*, a legal marketing term brought about by class action lawsuits a decade or more ago. That allows them to market the "HP" that it DEVELOPS when the motor puts out Break Down Torque, rather than Full Load Torque. BDT takes place just before the motor stalls and is typically 200-250% of FLT, but the motor can only develop that for a few seconds before in overloads. The motor nameplate itself must state the actual watts on it, which is the CONTINUOUS rating. You can extract the "true" HP by dividing that watt value by 746. Technically the installation instructions are supposed to give you the MCA (Minimum Circuit Ampacity) and MOCP (Max. Over Current Protection) to work with. But all that said, schlocky suppliers continuously violate those rules to sell product, counting on the fact that most of us have better things to do than turn them in, if we even knew who to call...

* My favorite definition of SPL is "Someone Probably Lied", other than the fact that there is no "probably" involved.