Converting 50amp receptable to 20amp for table saw/jointer

[Rollie Meyers]

If one tries to convert the circuit to 20A will run afoul of NEC article 250.122(B) because the equipment grounding conductor will be to small in a NM type cable. (6 & 8 AWG NM cable has a 10 AWG grounding conductor).

This is a copy & paste from the 2008 NEC. (Unchanged from 2005).

(B) Increased in Size. Where ungrounded conductors are
increased in size, equipment grounding conductors, where
installed, shall be increased in size proportionately according
to the circular mil area of the ungrounded conductors.

Bold by me.

If the cable is 3-conductor W/ ground a small subpanel would be the ticket.

[Tom Godley]

Rollie: Can you explain this further -- I do not understand.

I converted a feed that once was used for an electric dryer - to run a 220v pump. Installed a 20amp breaker and did not use the neutral - Why would a 10awg ground conductor be a problem in this situation.

Thanks for the info!

[Hugh Jardon]

I suspect because under ground fault conditions, the ground conductor becomes the return path for the circuit. If the return path is not rated to carry the current that the live can provide, then it may fail, thus rendering the load live.

[Chip Lindley]

I cannot delete my previous post, so I will insert a disclaimer. What is simple and works, may violate some code, etc. Disregard my unprofessional and unsafe advice.

[Matt Meiser]

Could he put a fused disconnect in place of each 50A outlet with the appropriate fuses and the proper receptacle on the load side?

[Chip Lindley]

This thread has been all over the place, so, would be hard to hijack it.

To further basic understanding, and in the interest of safety, somebody please give me a scenario that could possibly occur to cause damage to motor, starter or wiring of, say...my 1.5hp 8.4a, 230v DJ20 jointer on a 50A circuit! The motor is double protected by a thermal reset and an overload heater in the magnetic starter.

The jointer is plugged into a 25ft. 8-3 extension cord with 6-50 male and female ends. A short 10-3 pigtail adapter has 6-50P to L6-30R. The jointer has an 8ft. 12-3 cord. You have the chance to fry my nice jointer, or burn down my garage. Go for it!

And...what would be different that would not happen with my Lincoln welder using the same 50A circuit which it specifically calls for??

[Rod Sheridan]

Hi Chip, I can't comment on your jointer beacause I don't have any of the "uncommon" information that all electrical components have.

This would be the withstand ratings of the devices, which esentially is how much current can they carry, or interrupt (turn off) without exploding or catching fire.

The circuit that feeds your jointer has a breaker or fuse, and a circuit impedance that limits the maximum short circuit current the circuit can deliver.

The maximum short circuit current is larger on higher ampacity circuits, so the internal components of machines meant to be connected to these circuits will have suitable withstand ratings for the expected maximum short circuit currents. (This means they are larger and more expensive).

Your welder would be just fine on a 50 ampere circuit, your jointer may, or may not be, only the manufacturer, or a complete evaluation by an Engineer can tell you that.

I have two machines that do list the maximum over current protection required, I also have machines that don't.

It's always good practise to reduce short circuit currents through the application of correct overcurrent protection, including in some cases the application of current limiting fuses.

Regards, Rod.

[Tom Godley]

Chip - The concern is not about the basic operation of the tool.

On a modern tool/ machine if the motor develops a short or becomes overloaded the combination of internal overload protection and the application of a thermal overload in the switch should protect against a fire. The same is true for the wires between the motor and the switch -- if the switch has protection built in.

The problem is from the switch to the outlet -- What protects this wire??
The only thing protecting this wire is the breaker in the panel - in your case this wire is almost 35' long. What happens if the switch develops a problem before the overload protection or does not have any overload protection? Or you drop a sharp tool on the wire and damage it in some way? You can have a short that develops and the breaker will allow a 50amp load to be placed on that supply line. Will the 12g section handle the load? How will the damaged area of the 8g extension handle the problem. The overloading of supply lines is a common cause of fires. Have you ever compared the single spark from touching a 15amp vs 20amp line together. Now think about a 50amp line and wood chips in a shop.

Nothing is gained by having 50 amps supply a tool that will run and was designed to run on 20amps.

[Matt Meiser]

So actually you could put a 2 circuit subpanel at each location dirt cheap (meaning about $15/location + breakers) With quadplex breakers you could even put both a 20A and 30A 220 circuit at each location giving you a lot of future flexibility.

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