# Miller syncrowave 250dx Welder installation



## Stubbie (Jan 7, 2007)

Hello

Wow! this post got lost in the shuffle it looks like. So I hope you are still there. You are to be congratulated for providing so much information. We don't always have that luxury. 

220 or 230 or 240 are just nominal voltages and should not be looked at differently. Most motors and welders are factory rated at 230 volts. So don't let it confuse you.

Your welder is rated 230 volts with an input amps of 77 and duty cycle of 60%. So Nec 630.11(A) is used to get the branch circuit conductor size. Because the welders rating is done at 60% duty cycle we use a table multiplier that is simply the square root of the rated duty cycle percentage. So sq.rt. of .60 is .78. We take this times the rated input amps on the nameplate... in your case 77 amps. So 77 x .78 = 60 amps. So our conductors must have at least 60 amp capability. The welder terminations are listed at 75 C by Miller so we go to Table 310.16 in the 75 C column and find that a 6 awg copper conductor is good for 65 amps. So that is our conductor size. Next we have to get the branch circuit short circuit and ground fault protection (Circuit Breaker) for the conductors. This is laid out in NEC 630.12 (A)&(B) and states the maximum circuit breaker can be not more than 200% of the ampacity of the supply conductors. So 2 x 65 amps = 130 amps. So our Circuit breaker to protect the branch circuit conductors calculates at 125 amps... NEC 240.4(G). If the welder doesn't have internal overcurrent protection, which this one doesn't, then the welder must have ocpd and cannot be greater than 200% of the rated input amperage which is 2 x 77 amps or 154 amps. Which ever is smaller the ocpd for the conductors or the calculated ocpd for the welder will be the size of the circuit breaker. In this case the ocpd sized for the conductors is smaller so our choice is 125 amp OCPD in the breaker panel protecting the welder and the branch circuit conductors. The ground wire is sized to the overcurrent protective device of 125 amps using table 250.122 and would be a size #6 awg copper wire.

So Our branch circuit is as Miller stated... Max Circuit breaker 125 amps and branch circuit conductors of 6 awg copper with a ground also 6 awg copper.

You are also required to have a disconnect for the welder which is sized to the overcurrent protection 125 amp ...

Diagram is modified from the owners manual


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## 220/221 (Oct 9, 2007)

> Max Circuit breaker 125 amps and branch circuit conductors of 6 awg copper with a ground also 6 awg copper.


If this is correct and #6 wire can be protected by a 125 amp breaker I am going to just give it up and retire. I didn't understand half of what you said but conventional wisdom says #6 is 60 amps.. 


I'm done. 

Maybe The Home Depot will hire me.:jester:


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## saddletramp (Jan 7, 2008)

Stubbie said:


> Hello
> 
> Wow! this post got lost in the shuffle it looks like. So I hope you are still there. You are to be congratulated for providing so much information. We don't always have that luxury.
> 
> ...






If you drink, I would buy you a beer, (im in Dallas) if you don't pm me your address and the wife will send you some oatmeal chocolate chip cookies!

Thank you for taking the time to explain this to me it makes so much more sense now! Any recommendations on brand names or places online that I could order the parts would be helpful too. 

Thanks again I have never gotten such a helpful and complete answer from any web forum. 

-tramp:thumbup:


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## Stubbie (Jan 7, 2007)

Any of the big box stores or electrical suppliers have your materials. You will need to have some thhn/thwn cut to length. Just get black insulation and if you want mark one of the black wires lets say L2 with red tape to differentiate between legs. Get a green insulted ground wire. The wires will be stranded type. 

As for the disconnect you may use the circuit breaker in the main panel as your required disconnect. I changed the previous drawing to reflect this.

But if you would like a remote disconnect below are some examples of a breaker type, fused type and manual type. You will just have to see if it fits your budget by going to a supply and pricing them. You need 125 amp fuses or a 125 amp breaker and the appropriate enclosure or a non fusible manual safety type switch as shown below. There are many styles and types of disconnects so you will have to ask what they have that is rated at least 125 amps. Ebay is a great place to get disconnects if you want one for the welder. Or just google electrical suppliers and search for safety switches or disconnects.


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## J. V. (Jun 1, 2007)

Stubbie....I gotta give it to ya. Excellent presentation.


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## Stubbie (Jan 7, 2007)

I would also like to add that many individual welder branch circuits can be served with thhn in conduit to a junction box close to the welder then you can splice with wirenuts to 6/3 SOOW cord if you like. Below is an example. This would be in place of the flexible metal conduit shown in the previous diagram from the disconnect to the welder. You would just have a 4 11/16" junction box instead of the disconnect. Miller may even have the cord installed on their units when you get them.

Understanding duty cycle is a big part of understanding how we get the conductor sizes and circuit breaker sizes. This welder was rated a 60% duty cycle which simply means the time out of a 10 minute period the welder can operate at rated output and input amps without overheating. So 60% means you can weld for 6 minutes continuous before you have to let the welder cool down for at least 4 minutes. The internal circuitry of the welder will stop the welding process if the duty cycle is exceeded. It is this consideration of duty cycle that allows you to down size the conductors as the cycling effect of the welder will not overheat 6 awg conductors at 60% duty. If you look at the manual you will notice that if you operate this welder at 40% duty cycle the input amps jump to 96. So the breaker must be sized to handle the inrush currents when striking an arc and the input load amps while undergoing the welding process. So duty cycle changes as you change the output amperages of the welder...ie.... high output settings result in more input amps and lower the duty cycle. 

I should also note that when sizing the circuit breaker to protect the branch circuit conductors and welder you will notice that it says 'not more than' 200%. So If the calculated amperage falls like this one did 130 amps for conductors and 154 amps for the welder you will find that there is no breaker that size in amps. In the case of welders we do not go to the next size up like usual we go to the next listed breaker size down or breaker that is equal to the calculated value because it cannot be more than 200%. So the conductors would have 125 amp breaker as the next listed size down and the welder calculation would have resulted in going down to the listed breaker size of 150 amps. Whichever of these is smallest becomes our breaker size. As an example of why you need this size breaker as we did is that when this welder is set at 40% duty cycle it will have an input amp rating of 96 amps. We must have a breaker big enough to handle the inrush amps when you strike your arc. The 6 awg thhn copper will carry the 96 amps at 40% duty without over heating. Also remember that these values of circuit breakers are maximums and for the conductors are minimums. so you can vary them some. The overcurrent maximum values and conductor minimum values you calculate from NEC article 630 insure that the welder will operate as advertised at rated output and input amps for all duty cycles and output amperages set at the control panel of the welder.


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