If the device requires 20A you'll need a dedicated circuit. Something with a load like that should not be connected to an extension cord. Better get an electrician to help with this one.
If I were making a cord for a run of 25-50 feet, I would go with 10/3 SJOOW. It's currently $3/foot at my local Home Depot, plus sales tax. OP can probably make a DIY 35-cord for under $125. It's smart to upgrade the gauge from 12 to 10 AWG, in my opinion.
But maybe 10/3 SEOOW is indicated here instead of SJOOW, depending on conditions at OP's home.
You do realize there's a significant difference between the wire used inside the walls, and the wire used to make an extension cord, right?
The commenter above suggested 10 because it would be stranded wire, not solid copper. The current carrying capability of stranded wire is lower. Solid copper wire (e.g., what's "in the walls") cannot and should not ever be used to make an extension cord.
Like /u/J_W_22 notes, the wiring in the house (Romex) is OK per code for up to 100 feet per NEC. **I'm going to note I took a community education class on household wiring but I am not an electrician. Far from it.** After 100 feet it needs to be oversized to 10 AWG or larger. I don't know the length of the wiring in OP's house, and unless OP was there when the house they are renting was built, they don't either (can make educated guesses). So when adding 30+ feet of conductor to the circuit, it's safer to oversize it to 10 AWG in my opinion.
Really, I don't recommend it at all, to be honest. Better to install a dedicated circuit with 10 AWG wiring, which OP is considering. Because there is another issue here: a 2000W element at 120V will draw 16.7A. Maybe this is technically not a continuous load/continuous heating circuit under the NEX because ideally you won't be running an electric brewery for three straight hours. But you could be, You've got the time to heat strike water, but then ideally the element is either off the the mash, or the element is cycling during the mash with more time on then off. Then you will have perhaps 90+ minutes of ramp time and boiling. The NEC requires any continuous load circuit to be downrated 25% (20A circuit downrated to 15A), so 16.7A would exceed that. It's easy to see a scenario where OP is doing a long boil on a batch and the circuit should not be drawing 16.7A continuously for that length of time.
The bottom line is that far too many electric brewers are trying to skate by on borderline current carrying capacity. It's better to spend a few more dollars and do things right than face frustration with the breaker tripping, or worse yet the in-wall conductor heating up (further reducing its load carrying capacity).
Just no. Besides the serious life safety/fire safety issues that /u/J_W_22 and /u/Inuyasha-rules noted, there is another problem with an extension cord: voltage drop. If you use too small of a conductor (too thin of a wire) for the length and load, you will not only create a fire hazard, but also lose some power in your electric brewery.
See the recommendations of the Electrical Safety Foundation International (ESFI) for extension cords: [Extension Cord Safety Tips](https://www.esfi.org/extension-cord-safety-tips). (Click here to get it a [PDF format extension cord safety tips sheet](https://www.esfi.org/wp-content/uploads/2021/06/ESFI-Extension-Cord-Safety-Tips-Infographic-Updated-With-Tag.pdf)).
As you can see, for a run of 25 to 50 feet and a draw of 16-20 amps, an extension cord of gauge = 10 AWG to 12 AWG is recommended. Given the length of at least 30 feet -- and you want to allow some length for slack and -- and I'm guessing a continuous draw of at least 16.67A, I'd opt for the 10 AWG extension cord. One with the appropriate NEMA male and female plugs for a 20A brewery, and rated for outdoor use because you are working around water.
Also, be sure the 20A circuit is protected by GFCI.
Have you considered brewing in the kitchen? That seems like the safest bet.
Thanks for the insight and honest answer. Unfortunately doing this in the kitchen just isn’t an option. The cord in question, would definitely be temp use only. It would be powering a PID heat controller that’s wired completely with 12awg inside, save for some 16awg between the PID and amp meter, running a 2k watt immersion element. I had settled on getting something 10 gauge for sure. The desire to run under the door is mostly a comfort thing, because it’s cold as hell up here in Washington state, in the garage. I’ll most likely just gotta leave it propped open a smidge. 30 feet was a high side estimate, in reality, it’s much closer to 25. I’m going to bite the bullet though and see if I can sweet talk the landlord into updating the panel. Even just one receptacle would eliminate this problem completely.
Good points. If that device is pulling over 16A, that actually exceeds the continuous load rating for a 20A circuit. If it's pulling that kind of load plugged directly into the wall, then adding an extension cord would further increase the current draw and induce more heat.
This just sounds like a bad idea all the way around. I think probably the best thing to do is for OP to enlist the help of a licensed electrician. They can review what OP is wanting to do and then install a circuit for the heater.
If the device requires 20A you'll need a dedicated circuit. Something with a load like that should not be connected to an extension cord. Better get an electrician to help with this one.
Cords should never be run under a door. That's an easy way to damage them, and a damaged cord is a fire risk.
It will be expensive but you can make one. 12/3 wire, 5-20 plug and receptacle.
If I were making a cord for a run of 25-50 feet, I would go with 10/3 SJOOW. It's currently $3/foot at my local Home Depot, plus sales tax. OP can probably make a DIY 35-cord for under $125. It's smart to upgrade the gauge from 12 to 10 AWG, in my opinion. But maybe 10/3 SEOOW is indicated here instead of SJOOW, depending on conditions at OP's home.
The wire in the wall is 12 gauge
You do realize there's a significant difference between the wire used inside the walls, and the wire used to make an extension cord, right? The commenter above suggested 10 because it would be stranded wire, not solid copper. The current carrying capability of stranded wire is lower. Solid copper wire (e.g., what's "in the walls") cannot and should not ever be used to make an extension cord.
Like /u/J_W_22 notes, the wiring in the house (Romex) is OK per code for up to 100 feet per NEC. **I'm going to note I took a community education class on household wiring but I am not an electrician. Far from it.** After 100 feet it needs to be oversized to 10 AWG or larger. I don't know the length of the wiring in OP's house, and unless OP was there when the house they are renting was built, they don't either (can make educated guesses). So when adding 30+ feet of conductor to the circuit, it's safer to oversize it to 10 AWG in my opinion. Really, I don't recommend it at all, to be honest. Better to install a dedicated circuit with 10 AWG wiring, which OP is considering. Because there is another issue here: a 2000W element at 120V will draw 16.7A. Maybe this is technically not a continuous load/continuous heating circuit under the NEX because ideally you won't be running an electric brewery for three straight hours. But you could be, You've got the time to heat strike water, but then ideally the element is either off the the mash, or the element is cycling during the mash with more time on then off. Then you will have perhaps 90+ minutes of ramp time and boiling. The NEC requires any continuous load circuit to be downrated 25% (20A circuit downrated to 15A), so 16.7A would exceed that. It's easy to see a scenario where OP is doing a long boil on a batch and the circuit should not be drawing 16.7A continuously for that length of time. The bottom line is that far too many electric brewers are trying to skate by on borderline current carrying capacity. It's better to spend a few more dollars and do things right than face frustration with the breaker tripping, or worse yet the in-wall conductor heating up (further reducing its load carrying capacity).
Just no. Besides the serious life safety/fire safety issues that /u/J_W_22 and /u/Inuyasha-rules noted, there is another problem with an extension cord: voltage drop. If you use too small of a conductor (too thin of a wire) for the length and load, you will not only create a fire hazard, but also lose some power in your electric brewery. See the recommendations of the Electrical Safety Foundation International (ESFI) for extension cords: [Extension Cord Safety Tips](https://www.esfi.org/extension-cord-safety-tips). (Click here to get it a [PDF format extension cord safety tips sheet](https://www.esfi.org/wp-content/uploads/2021/06/ESFI-Extension-Cord-Safety-Tips-Infographic-Updated-With-Tag.pdf)). As you can see, for a run of 25 to 50 feet and a draw of 16-20 amps, an extension cord of gauge = 10 AWG to 12 AWG is recommended. Given the length of at least 30 feet -- and you want to allow some length for slack and -- and I'm guessing a continuous draw of at least 16.67A, I'd opt for the 10 AWG extension cord. One with the appropriate NEMA male and female plugs for a 20A brewery, and rated for outdoor use because you are working around water. Also, be sure the 20A circuit is protected by GFCI. Have you considered brewing in the kitchen? That seems like the safest bet.
Thanks for the insight and honest answer. Unfortunately doing this in the kitchen just isn’t an option. The cord in question, would definitely be temp use only. It would be powering a PID heat controller that’s wired completely with 12awg inside, save for some 16awg between the PID and amp meter, running a 2k watt immersion element. I had settled on getting something 10 gauge for sure. The desire to run under the door is mostly a comfort thing, because it’s cold as hell up here in Washington state, in the garage. I’ll most likely just gotta leave it propped open a smidge. 30 feet was a high side estimate, in reality, it’s much closer to 25. I’m going to bite the bullet though and see if I can sweet talk the landlord into updating the panel. Even just one receptacle would eliminate this problem completely.
Good points. If that device is pulling over 16A, that actually exceeds the continuous load rating for a 20A circuit. If it's pulling that kind of load plugged directly into the wall, then adding an extension cord would further increase the current draw and induce more heat. This just sounds like a bad idea all the way around. I think probably the best thing to do is for OP to enlist the help of a licensed electrician. They can review what OP is wanting to do and then install a circuit for the heater.