Reply by CaptainKlutz

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Posted on Is it possible to run a 2hp Dust Collector and Saw on a single 240v/30amp dryer circuit?

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5032 posts in 2782 days

#1 posted 03-30-2021 06:21 PM

Interesting debate.

- Building codes like NEC, only codify permanent structures. Once the power comes out a receptacle, there are other federal regulations, like UL, or CSA that cover the appliance plugged into wall. Which means users can make all kinds of unapproved (possibly unsafe) cables/splitters.

- Motor circuits are different than standard 15/20A circuit with multiple outlet’s used for lights, radio, or other household appliances. Other NEC articles govern the rules for household power. While one can debate that circuit sharing is common, and safe; these do not apply to large motor surge loads covered by article 430.

- Codes requires all motors 1HP+ to have a disconnect, magnetic relay, and overload protection. Agency certification (UL/CSA/etc) issues are reason some mfg do not provide a power plug for their larger machines. They avoid needing certification for disconnect device, and place risk on buyer to add proper device.

- NEC treats large motors with special rules. Complicated and convoluted article 430 is enough to make your head spin. Here is summary.

- NEC Article 430 has specific rules for motors sharing current a circuit. If you want to split a motor circuit permanently per NEC, each branch must have:
- Proper rated wire to a separate disconnect switch for each branch
- Fuses/breaker protecting downsize circuit
- Magnetic relay that keeps power off in case of power loss.
- Thermal overload

The sneaky part to NEC 430 becomes when you have a 30A receptacle you want to repurpose.
NEC code doesn’t care if your temporary extension cord catches on fire due overload.
So you can use a 30A dryer plug to dual 15/20A receptacle splitter. The reason that most dryer plug to 6-20 adapters are not UL listed, is they don’t provide circuit protection for the smaller 20A circuit.

Is using 20A appliance on 30A circuit safe? Depends:

Your receptacle/plug is considered a disconnect device. As Whyme mentioned, if both devices have magnetic starter relay and overload protection, the appliance connection be reasonably safe (as it meets intent of UL requirements).

The probability of your extension cord overheating is very small, thanks to overload protection. If an overload fails, hopefully you are in shop using the tool; can see the smoke due motor failure, and disconnect power before wiring overheats. Also highly likely the motor windings will short and 30A breaker will trip before extension cord catches fire, though the 20A cord and motor will be damaged due lack of circuit protection.
Part of reason I would take this risk; is 2HP motor surge currents are relatively low. If you asked about sharing a 50A receptacle between 3-5HP motors, would recommend a sub-panel or fused disconnect solution.

Bottom line: Splitting a 30A into (2) 15/20A receptacles without breaker/fuse for each circuit can be done, but it puts some risk on user. It does not mean you can ignore the physics when setting up temporary solutions. Still need to proper rate the wiring and devices in the circuit.

+1 ibewjon – Just because anyone can buy electrical parts, does not mean you should not consult/hire a professional.

When it doubt with household wiring, hire a professional.
Be safe, not sorry.

PS – Not a licensed electrician. Just an engineer who designs, builds, and sells electrical appliances.

-- If it wasn't for bad luck, I wouldn't have no luck at all, Doom, despair, agony on me… - Albert King - Born Under a Bad Sign released 1967

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