Derating THHN

[Knight511]

Ordered a welder that is rated at 25.5amp when running it at 200amp, so I have an idea of what to run to my welder outlets in the shop build.

1. A minimum of 30amp breaker looks like it should be enough. Is there any reason to up size to a 40amp?

2. I have 10awg THHN wire being used for almost everything except grounds. 10awg Cu is rated for 40amps. Shop will not be air conditioned, Texas heat, and the wire will be in 1" EMT. Looking at NEC 310.16, I think I should be derating the THHN based on ambient temperatures. If I use 110F (43C), I would have to deregulate by 0.87 bringing the 10awg down to 34.8amps. This will be the ONLY circuit in the EMT, so it has 3 CCCs.

If I need to use a 40amp breaker, I would need to run bigger wire (8awg), right? So back to question 1, should I use the bigger breaker and get bigger wire now before I run the circuits in the shop?

Thanks.

 

[rlitman]

Actually, someone with NEC 630 handy will chime in and point out that dedicated welder breakers don't get derated. They get up rated due to the welder's duty cycle. Which means that off the cuff, I'm fairly sure that your 40A breaker will be fine on 10AWG (possibly even 50A), and you can plug that welder into a 50A circuit.

 

[pattenp]

Ditto... if used as dedicated welder circuit only the 40A breaker is fine on #10 thhn in pipe. If using as general purpose circuit then things change.

By the way 40A for #10 thhn is used only when derating. Sometimes it's 30A or 35A depending on the circuit use.

 

[Terry D]

Ordered a welder that is rated at 25.5amp when running it at 200amp, so I have an idea of what to run to my welder outlets in the shop build.

1. A minimum of 30amp breaker looks like it should be enough. Is there any reason to up size to a 40amp?

2. I have 10awg THHN wire being used for almost everything except grounds. 10awg Cu is rated for 40amps. Shop will not be air conditioned, Texas heat, and the wire will be in 1" EMT. Looking at NEC 310.16, I think I should be derating the THHN based on ambient temperatures. If I use 110F (43C), I would have to deregulate by 0.87 bringing the 10awg down to 34.8amps. This will be the ONLY circuit in the EMT, so it has 3 CCCs.

If I need to use a 40amp breaker, I would need to run bigger wire (8awg), right? So back to question 1, should I use the bigger breaker and get bigger wire now before I run the circuits in the shop?

Thanks.

Although # 10 copper is allowed to be protected by a breaker larger than a 30 amp for motor and welder circuits and other certain special circuits. A # 10 copper is not rated for a 40 amp load. When a #10 copper is used in a regular branch circuit, The maximum overcurrent protection device is 30 amps. You will find this in 240.4(D). When you look at Table 310.15(B)(16), you will notice for copper conductors from #18 to # 10 there is a double asterisk behind them. Looking at the bottom of the table, it will direct you to article 240. These larger amperages are used in derating calculations only for these size conductors.

 

[nadogail]

My Hobart AC/DC StickMate came with a factory installed 12 AWG power cord. When I asked Hobart's tech support about this they said it was because of the Duty Cycle of the welder.

 

[Knight511]

Although # 10 copper is allowed to be protected by a breaker larger than a 30 amp for motor and welder circuits and other certain special circuits. A # 10 copper is not rated for a 40 amp load. When a #10 copper is used in a regular branch circuit, The maximum overcurrent protection device is 30 amps. You will find this in 240.4(D). When you look at Table 310.15(B)(16), you will notice for copper conductors from #18 to # 10 there is a double asterisk behind them. Looking at the bottom of the table, it will direct you to article 240. These larger amperages are used in derating calculations only for these size conductors.

Thank you for this awesome reply. I am just some regular guy doing the DIY thing, but I have been trying to learn as much about this project as possible, so I can do it right (not just "good enough"). The more I learn, the more I understand in the future when something needs to change or be repaired.

Since I cannot be certain this circuit will ONLY be used for the welder, I will upgrade the wiring for the 2 locations where I may use the welder. I already had planned to run the empty conduit, so this really only makes me buy the wire. Most of the circuits are going to be 120v 20a in the shop, so I will make sure the 10awg wire doesn't get used past a 30amp.