New MM350P Electrical rant

[Jmb]

I ordered a new 350P last Wednesday (supposed to be here Monday) and I wanted to go ahead and get the electrical done. I went to the Miller site and downloaded the manual and here is what it says.

Input voltage 230V
Input Amperes At Rated output 61
Max Recommended Standard Fuse 90
Min Input Conductor Size in AWG 8
Min Grounding Conductor Size 8

Now if I didn't know better, I might have used a 90 amp breaker with #8 wire. Why would Miller put something like this in the manual?
I know that they are saying the you can use a "MAX" 90 amp breaker and a "MIN" #8 wire, but they don't tell you that if you use that #8 wire you need a 40 amp breaker or if you use the 90 amp breaker you need to use a larger wire (#2 maybe, I'd have to look that one up)

I used #6 from the box to the outlet (about 35') and I'm gonna make up a 50 foot 6ga. So extension cord. I already have a 70 amp breaker and some of the charts that I have seen say the the 6ga will carry 70 amps. I will buy a 50 or 60 amp breaker if you think that is necessary.

[hankj]

Jmb,

If you read NEC Art. 630, you'll find that what the Miller manual says is true. The numbers you quoted are per Art. 310 for 80% duty "standard" circuits. The welder's duty cycle allows the de-rated conductors per Art. 630.

Just talked about that in the "What size breaker" thread.

Hank

[Jmb]

Jmb,

If you read NEC Art. 630, you'll find that what the Miller manual says is true. The numbers you quoted are per Art. 310 for 80% duty "standard" circuits. The welder's duty cycle allows the de-rated conductors per Art. 630.

Just talked about that in the "What size breaker" thread.

Hank

Thanks Hank,
It didn't dawn on me about the duty cycle derating...lol.
But, like you stated in your post on "what size breaker" thread. If I installed the 90 amp breaker with #8 wire and moved, the next owner might be in for a surprise.
Will I be okay with the #6 and the 70 amp breaker that I already have or should I buy a 60 amp breaker just to be safe.

[hankj]

#6 THHN and like inulated wire is good to 75 amps. (Art. 310, Table 310.16) You're good to go.

Hank

[Sberry]

You can use 8 on circuits to 100A for some welding machines, but you have a good point not to leave it that way where someone may assume its a general use 100A circuit. The 6 wire and a 70A breaker (great if you already have one) is fine.

[Jmb]

Thanks a bunch fellas!!!
I should be burning wire by tommorrow afternoon if the machine comes in.