Derating and Conduit Fill Welding Machines

[killer76]

I have been asked to assist with installation for Welding Machines and Plasma Cutter. A co-worker said he’s not too sure about the calculation given from third party engineering company. I have some information for reviews, to see if this sounds correct.
(2) conduit runs from panel board
Indoor installation, ambient 104deg. +/- THHN 8CCC in 1 “ EMT
Not sure if this is enough information on the machines

Mfg. requirements / Recommend Installation
Welder-1 25A fused #12AWG 480V, 1 Ph.
Derating to 3/C #10 from 25A, 2P, CB to 25A fused Disc
Welder-2 15A fused #14AWG 480V, 3 Ph.
Derating to 4/C #12 from 15A, 3P, CB to 15A fused Disc
Plasma Cutter 20A fused #12AWG 480V, 3Ph.
Derating to 3/C #10 & 1/C #12 from 20A, 3P, CB to 20A fused Disc

Mfg. requirements / Recommend Installation
Welder-1 25A fused #12AWG 480V, 1 Ph.
Derating to 3/C #10 from 25A, 2P, CB to 25A fused Disc
Welder-2 15A fused #14AWG 480V, 3 Ph.
Derating to 4/C #12 from 15A, 3P, CB to 15A fused Disc
Welder-3 35A fused #10AWG 480V, 3 Ph.
Derating to 3/C #8 & 1/C #10 from 35A, 3P, CB to 35A fused Disc

 

[Julius Right]

In my opinion for 8 loaded conductors 70% it has to be the reduction factor.
As I understand you don't intend to run all the cables in 1" EMT but only 8 loaded conductor-2 alternatives.
On the first alternative 2 welders and Plasma Cutter you chose the correct size for welders but for Plasma Cutter for 25/0.7=28.6 A you'll need- I think -10 awg.
For the second alternative for the third welder [35/0.7=50 A] you need #8 I think.
The total area of cables will be 0.1454 inch^2 for 3 welders and 0.1431 for 2 welders and cutter.
Permissible 1" EMT 40% area[Article 358 — Electrical Metallic Tubing (EMT)] it is 0.346 inch^2

 

[don_resqcapt19]

You need to take a look at Article 630 for conductor and OCPD sizing for welders.