Air compressor transformer wiring

[Mr. Tanooki]

I’m setting up a 2010 Sullair compressor for my dad’s small machine shop in Florida and the unit is 480V while the shop only has 230V. Transformer is 112KVA and my load calc came out to about 265A on the primary. I was going to run 4/0 THHN in 2” EMT but since it’s right at the edge, do you think it’s smarter to jump to 250kcmil?

 

[VoltagexV]

Good call on double-checking the wire size! You're on the right track, but your load calculation is still likely too close for safety and NEC compliance. Skip both 4/0 and 250 kcmil, and instead go with 300 kcmil THHN. Make sure your 2′′ EMT has enough room for those bigger wires, plus your ground.

 

[h0td0g]

I’m setting up a 2010 Sullair compressor for my dad’s small machine shop in Florida and the unit is 480V while the shop only has 230V. Transformer is 112KVA and my load calc came out to about 265A on the primary. I was going to run 4/0 THHN in 2” EMT but since it’s right at the edge, do you think it’s smarter to jump to 250kcmil?

The step up to 250 kcmil is necessary for ampacity, but you should also factor in voltage drop (VD).
At 265A, even a slight VD can hurt the motor's efficiency and lifespan, especially on a 230V primary run. Given Florida's heat and the potential run length in a shop, the larger conductor will significantly reduce resistance and VD, ensuring the compressor gets the cleanest possible power from that 112 kVA transformer.
It's a small extra cost for a much healthier installation

 

[PGB1]

Hello Mr. Tanooki,
Preface:
All projects should be done under a permit just in case of a fire or injury later. Not having a permit & green tag inspection is an insurance company escape. But that is up to your judgement.

Assuming a permit is on this project: (And is covered by NFPA 70)
One caution to note is whether or not the local authority will call the equipment a Continuous load. If that is the case, you will have to upsize to accommodate 125% of the load.

Assuming 3 current carrying conductors in the raceway or cable at 30-C ambient; now your primaries jump up to 400 kcml with 75-C terminations. (600 kcml if there are any 60-C terminals in the mix)

I'd play it safe and ask the authority having jurisdiction if she will require 125% (and as mentioned by h0tD0g, if a temperature correction will be required.

I have had it judged both ways in the exact type of installation- tool room shops and small machine shops.

Were I the AHJ, I would say it is not a continuous load. Air compressors often have a duty cycle requirement of their own. (Example: Run 20 minutes, off for 10). Plus, you will almost certainly not operate it 3 hours or longer at a time unless it's a production shop and the compressor operates coolant pumps, nitrogen generators, chip or fume evacuators or similar.

The only places where I have automatically wired compressors for continuous are those for pneumatic thermostats, dampers and such in large buildings and in robotic paint operations. Also for pneumatic transformer changeover switching devices.

But in a machine shop? Nope.

Paul

 

[Mr. Tanooki]

Thanks everyone for the input. You've correctly shifted my focus from just the conductor size to the bigger picture of permits and the inspector's final say. @PGB1's point about the AHJ is my main concern now.

Before I move forward, has anyone here actually had an inspector sign off on a similar large compressor install in a small machine shop? I'm trying to figure out if this typically gets approved or if it often gets stopped by main panel or service limitations, regardless of the wire size used for the new circuit.

 

[PGB1]

Thanks everyone for the input. You've correctly shifted my focus from just the conductor size to the bigger picture of permits and the inspector's final say. @PGB1's point about the AHJ is my main concern now.

Before I move forward, has anyone here actually had an inspector sign off on a similar large compressor install in a small machine shop? I'm trying to figure out if this typically gets approved or if it often gets stopped by main panel or service limitations, regardless of the wire size used for the new circuit.

I'm not sure what you are asking above.

If the existing service and equipment are suitable for the existing loads, plus the new compressor load- the rules allow it.
I've installed hundreds of step-up transformers for equipment (including air compressors) over the decades. Same for delta to wye, etc. It is a routine practice. (Be SURE to use a step-up transformer and NOT a step-down installed backwards. People learn this the hard way!)

Size the transformer and the wiring appropriately- remembering the inrush current of an air compressor can be quite large. The machine should have a tag stating the circuit size it requires. If no tag, ask the manufacturer for a copy of the manual & data plate.

The No Surprises Way:
You can always bring to the permit issuing authority details of your existing service and the load. Use the actual load and not a list of circuits breakers. Indicate what each load's job is. (Lighting, tool, convenience outlet, heater... and their load.) Most will be more than happy to review and offer guidance. Also ask if she will require the 125% rule for the new compressor.

And, I really want to emphasize the importance of a permit.
Even if a fire or injury isn't compressor or transformer related, the insurance company can get out of paying should there be a fire or injury.

I have been called as an impartial witness for tort actions and arbitration often.
Example: Someone runs a new circuit for a convenience outlet with no permit or inspection. Later, a roof leak gets a high bay light fixture wet and someone is shocked by that light fixture which was installed with a permit & inspection years ago. Nothing at all to do with each other. Insurance still gets out of paying for the injury and damage because the adjuster found one (unrelated) thing- the new outlet- that was not installed under a permit. Owner pays medical and fire damage out-of-pocket. Sometimes- medical forever.

 

[Mr. Tanooki]

Paul, thank you for the excellent 'No Surprises Way' procedure and the critical point about insurance. This is a much better approach than relying on anecdotes, so I will gather my load calculations and contact the permit office directly, as you've outlined.

 

[PGB1]

But sure to let us know about your load calc's & how things went at the permit office!
Paul