A family member has built a small (12X20') home shop for welding projects. A subpanel
fed from the 225A home breaker panel will be used to feed a subpanel in the shop.
Installed, the subpanel feed wire will entail a total run of circa 110 feet.

Query: What is the correct size wire for this circuit?
"Professionals" at the wire supply stores have differing opinions; some state
#4 THHN THWN 600V with an 80A or 100A breaker will suffice, others say #2 wire
should be run. The use of #2 wire vs #4 will almost double the cost of the feed due
to present local pricing. My study of various ampacity charts via the internet only
seems to confuse the wire selection process.
There will be no further electrical equipment added.


Projected shop electrical load:
The following is a list of items that are presently included in the shop. As this is
strictly a "one man" setup & operation, not all of the equipment will ever be run at
the same time. EX: it is physically possible to run only 1 welding unit or the plasma
cutter at the same time, etc; although it is possible that the air compressor might
activate during the use of the above items. I mention this as the Hobart welder and
the shop compressor are by far the single highest potential simultaneous current usage items.

Items for the shop include: equipment info is via tags on items
Hobart welder: 230V @ 47.5A (light to extended run time **His max use setting is only)
Wattage: 11,040W 70% of equip tag)**
Miller welder: 115V @18.4A or 230V @ 13.1A
Wattage: 2,116W or 3,013W
Plasma cutter: 208-230V @ 35A (seldom to light run time)
Wattage: 8,050W
Compressor: 5 HP 80 gal tank 208-230V @ 23-25A (seldom to light run time)
Wattage: 5,750W
Drill press: 115/230V @ 9.0A/4.5A (seldom run time)
Wattage: 1,035W
Angle grinder: 115V @ 13.0A (light to extended run time)
Wattage: 1,495W

Following miscellaneous items have no tags showing power ratings or else I do not
have the info available to me at this writing:
1 - hand drill, 120V
1 - bench grinder, 3/4 HP 115V
1 - bandsaw, 3/4 HP 120V
1 - 30" shop fan, 1/3 HP 120V
3 - 48" florescent shop lights 120V
1 - portable shop light of two 500W quartz lights

Comment: Maximum shop "continuous" load usage based on real-world conditions.
His "real-world" load would be the Hobart welder (11,040W**), compressor (5750W),
shop lights (300W ?? ),& portable lights (1000W) as this is a "one-man" operation.
I figure it to be circa 18,090W or 18.1Kw. Throw in a plus-10% fudge factor, round it
off, and call it a max "continuous" draw of 20Kw.
(An even more "real world" draw would preclude the use of both the compressor
& the Hobart in operation at the same time as this is a one man operation)
****Are my above calculations realistic, or am I off course in my thought process?
Please advise!!***
-------------------------------------------------------------

tkrussell

Using #4 copper at 110 feet, and assuming a load of 75 steady current amps, 25 for the AC and 40 for the welder, will produce a 2% voltage drop on the feeder, and #2 will result in 1.3%. Note, this is only for the steady running current.

Feeders should be sized to limit voltage drop to a maximum of 3%. So you can see you are on the edge with the #4. The issue using #4 will be the high starting currents of motors. The welder will have a brief high inrush current, but the motor will have much more that will take more time. I am only talking seconds or less.

But with # 4 there is a chance of the motor trying to start with something else on, and causing a voltage drop that will be visible with the lights flickering, and the motor seeming to have a problem starting. This is what should be avoided, as trying to start a motor under low voltage conditions will affect it's operation and the life of the motor.

The 5740 watts you provide for the 5HP air compressor is only relevant for running load. Look on the nameplate for a LRA (Locked Rotor Amps) value, it may or may not be listed, or a Code Letter. if the Code Letter is either B,C,or D, the LRA will be 92 Amps.

This LRA of a motor must be taken into consideration, esp for long feeder runs, as voltage drop will be a factor if the wire size is not large enough to reduce Vd. Welders do have "starting current", but not like motors, a welder is a transformer coil that needs to be saturated, and the result is called "inrush" current. This can be nearly as high as a motor, but lasts less time.

I understand the cost of copper. But with all the money invested in equipment, I believe this is not an item that should be cut short.

This is why aluminum is often used for large feeders, the ampacity is close to copper, usually to match copper only the next size larger aluminum wire is needed to equal copper in capacity.

If cost is a factor, # 2 aluminum with XHHW insulation, with the same load will result in a 1.9 % Vd, and is rated for 100 amps.

However, to compliment the equipment, I believe you will see the benefit of investing in #2 copper. The owner of the equipment will not have any operational problems once your done with the wiring.