Determining amperage of a sub panel

[CooterBrown 18022]

I want to add a circuit breaker to my workshop’s sub panel. It is fed by a double breaker consisting of two 40 amp breakers.

 

Is the sub panel 40 or 80 amps?

[Bevo 13593]

Ignore the first response - The 20 amp subpanel is fed by a double-pole 240-volt breaker at the main service panel, and this single feed circuit is divided into additional branch circuits at the subpanel. If it is 20 amp, it probably is fed by 12 gauge wire. 30 amp is 10 gauge. Over 30 amps can cause 10 gauge wire to melt and become a fire hazard. If you want 40 amps in the garage, you need to replace the wire feeding the garage with 8 gauge wire.

The load on the subpanel could be anything. Add up the loads and that will give you more detailed info. In other words, say a 100W bulb draws .9A. And then you change out the 100W bulb for an LED which draws 1/12A. Now on that 20A circuit where you could have had 22 incandescent bulbs, you can now have 240 LED bulbs.

Edited March 14, 2021 by Bevo

[Bevo 13593]

This article may be of help. https://www.thespruce.com/calculating-subpanel-loads-1152758

And sorry for not being clear, it is a 40 amp breaker.

Edited March 14, 2021 by Bevo

[CooterBrown 18022]

Currently, I have one 20 amp breaker in the sub panel. There are 4 outlets and one LED shop light on it.

I just want to add a second 20 amp breaker so that I can add 4 more outlets and two additional LED shop lights.

I guess I could could just run it all off the one breaker since I doubt it’d get to 20 amps total anyway. The highest load at once would be running a table saw, dust collector, 18v battery charging station, and lights at the same time.

[Bevo 13593]

2 hours ago, CooterBrown said:

Currently, I have one 20 amp breaker in the sub panel. There are 4 outlets and one LED shop light on it.

I just want to add a second 20 amp breaker so that I can add 4 more outlets and two additional LED shop lights.

I guess I could could just run it all off the one breaker since I doubt it’d get to 20 amps total anyway. The highest load at once would be running a table saw, dust collector, 18v battery charging station, and lights at the same time.

Yeah, that would be the easiest. See how many amps the table saw and dust collector run. Typically, I think a table saw runs 6-16 Amps. Dust collectors run a lot also. https://www.woodmagazine.com/woodworking-tips/techniques/outfitting-woodworking-shop/critical-questions-about-workshop-wiring So, you could potentially have an issue.

If you have an issue with tripping the subpanel breaker, there is a decent chance that the builder put in wire larger than needed for that 20Amp breaker in the subpanel. I don't know electrical code, but I would think that if you have a 40Amp breaker at the main, code would require that the wire from the main to the subpanel would need to be able to handle the 40Amps.

Edited March 14, 2021 by Bevo

[CooterBrown 18022]

Went ahead and added a 20 amp circuit breaker. The table saw and dust collector are now on separate circuits. Each has 4 outlets and two shop lights. Easy Peasy. Thanks for the info.

[Hornbeliever 712]

5 hours ago, CooterBrown said:

Went ahead and added a 20 amp circuit breaker. The table saw and dust collector are now on separate circuits. Each has 4 outlets and two shop lights. Easy Peasy. Thanks for the info.

Did you have a main switch to turn power off at sub panel to install?   Or did you  have to turn off main switch at main panel to make sure no electricity running to sub panel? 

[CooterBrown 18022]

Did you have a main switch to turn power off at sub panel to install?   Or did you  have to turn off main switch at main panel to make sure no electricity running to sub panel? 

Just had turn off the circuit breaker on the main panel that feeds the sub panel.

[Bevo 13593]

10 hours ago, Hornbeliever said:

Did you have a main switch to turn power off at sub panel to install?   Or did you  have to turn off main switch at main panel to make sure no electricity running to sub panel? 

It isn't hard, but you have to know what you are doing. He had a 40Amp breaker at the main. After flipping the breaker, he could see if it turned off power to the subpanel. Once he knew he was good, it is a pretty straight forward project. He could inspect the wires to make sure they could handle the current and then install the breaker and move some wires and install a few more wires for the power tools and new outlets.

[Bevo 13593]

Just now, Bevo said:

It isn't hard, but you have to know what you are doing. He had a 40Amp breaker at the main. After flipping the breaker, he could see if it turned off power to the subpanel. Once he knew he was good, it is a pretty straight forward project. He could inspect the wires to make sure they could handle the current and then install the breaker and move some wires and install a few more wires for the power tools and new outlets. Since it is in the garage, a GFCI circuit is necessary, though.

 

[Gil Bang 25987]

22 hours ago, CooterBrown said:

Went ahead and added a 20 amp circuit breaker. The table saw and dust collector are now on separate circuits. Each has 4 outlets and two shop lights. Easy Peasy. Thanks for the info.

What size wire did you use to feed the new outlets? 

[CooterBrown 18022]

What size wire did you use to feed the new outlets? 

12 gauge.

[Gil Bang 25987]

3 minutes ago, CooterBrown said:

12 gauge.

that's what I hoped for. 

 

Did you also buy 20 amp outlets?   The ones sold in bulk at HD/Lowes are 15 amp. 

[CooterBrown 18022]

that's what I hoped for. 
 
Did you also buy 20 amp outlets?   The ones sold in bulk at HD/Lowes are 15 amp. 

I only needed one. They sell singles at Lowe’s and HomeDepot here.