Welder-Sizing Overcurrent Device and Supply Conductors

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naplespete57

naplespete57

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Eastern Kentucky
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Project Manager in Conveyor Industry
Trying to install properly a 480V 3 phase Welder with a nameplate amp rating of 25amps.

To determine the conductor ampacity I look at the duty cycle which is 70 and it is a non motor generator so I believe I should multiply my 25amps by .84. This gives me 21 amps. I would then size my conductors conservatively at 30 amps or #10 wire. 630.11(a)

To size my breaker for one welder. I would use my amp rating of 25amps and multiply it by 200%. This would give me a 50 amp 3 phase breaker.

Am I on the right track here? What am I missing?
 
naplespete57 said:
Am I on the right track here?
Click to expand...

I think you are right on track and could probably go down to 12 AWG.

That said, if the place I was installing this in had semi-knowledgeable people poking around in the panels and I wanted to keep working there I would probably run 8 or 6 AWG to the 50 amp breaker.

Why?

Just so some half knowing maintenance person does not go up the chain of command and say 'Hey that electrician is a hack, he has 10 AWG on a 50 and everyone knows that is good for 30 amps'

Then the customer doubts the installation and you have to explain its' fine etc.
 
Sometimes it is an issue where a contractor comes behind another and calls him a hack, when in fact he knows the code quite well. I tell as many contractors to try and stay away from making comments about other contractors.

And yes, if not installing NM cable, I also think 12 AWG could be installed. If a concern about a customer comes up, show him how you are a cost effective, code complying electrician...maybe heading off a dispute with another electrician.
 
iwire said:
That said, if the place I was installing this in had semi-knowledgeable people poking around in the panels and I wanted to keep working there I would probably run 8 or 6 AWG to the 50 amp breaker.

Why?

Just so some half knowing maintenance person does not go up the chain of command and say 'Hey that electrician is a hack, he has 10 AWG on a 50 and everyone knows that is good for 30 amps'
Click to expand...
Do you use this line of reasoning when sizing fire pump feeders and OCP?:grin:
 
chris kennedy said:
Do you use this line of reasoning when sizing fire pump feeders and OCP?:grin:
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No, but we were not talking about those. :grin:

For that matter I would not always go that route for the welder outlet, just something to consider. I would not be doing it to help out the unqualified I would be doing it to keep the unqualified from costing the company I work for money.
 
Many thanks!

Many thanks!

Appreciate the advice! Sometimes it helps to know that at least you have the right tree to be barking at
 
Just be sure to consider the duty cyle correctly
70% duty cycle means weld 7 minutes, rest 3 minutes, while the welder is on for 10 minutes

most welding I do is at 100% duty cycle, operating at 27v dc, 100A


"Trying to install properly a 480V 3 phase Welder with a nameplate amp rating of 25amps.

To determine the conductor ampacity I look at the duty cycle which is 70 and it is a non motor generator so I believe I should multiply my 25amps by .84. This gives me 21 amps. I would then size my conductors conservatively at 30 amps or #10 wire. 630.11(a)"
 
If the nameplate requirement of the welder is 25A, is there any particular _reason_ to use a 50A breaker? I understand that article 630 gives permission to use a breaker that is 'not more than 200% of I1max' (300% for resistance welders), but is it good design to use this permitted maximum?

I presume that for transformer based welders, there is the issue of transformer inrush current. But wouldn't you want to select OCPD size from manufacturer's information, and wouldn't you want the smallest OCPD that would not be subject to nusiance tripping?

-Jon
 
read the owner's manual for the specified recommended circuit protection

from miler gold star series:
4-13. Electrical Service Guide
60 Hertz Models 300 Amp Model 450 Amp Model 650 Amp Model
Reference: 2005 National Electrical Code (NEC)
1 If a circuit breaker is used in place of a fuse, choose a circuit breaker with time-current curves comparable to the recommended fuse.
2 ?Time-Delay? fuses are UL class ?RK5? .
3 ?Normal Operating? (general purpose - no intentional delay) fuses are UL class ?K5? (up to and including 60 amp), and UL class ?H? ( 65 amp and
above).
4 Conductor data in this section specifies conductor size (excluding flexible cord or cable) between the panelboard and the equipment per NEC Table
310.16. If a flexible cord or cable is used, minimum conductor size may increase. See NEC Table 400.5(A) for flexible cord and cable requirements.
 
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