Unusual sub panel question...

15 years ago

Sorry in advance if this seems simple (or dumb)but I am not a pro. I have some wiring knowledge, but on this project would prefer to hear from the experts. This is not an inspection job, just want to make sure it is safe.

My goal is to wire a sub panel (main lug type, no breaker in sub) to my attached garage. I have an unused 60 amp breaker in the main panel & would run 6/3 to the sub panel from it. I want to accomodate a 50 amp breaker for a 220v welder, a 20 amp breaker for a 110v welder & a 20 amp breaker for a 110v (1500w) heater. All three of these would be dedicated usage with nothing else on them. The two welder circuits would never be used at the same time (& could be labled as such), however the dedicated heater circuit would be on at the same time as either of the welders, in the winter time anyhow. I know that the breakers I put in the sub are sufficient for the usage of each individual appliance, but was worried about the 60 amp breaker in the main panel. If I was using the 220v welder & the 110v heater at the same time is the 60 amp breaker going to protect it.

I know I will be putting a total of 90 amps of breakers (1 - 50 amp, & 2 - 20 amp) in the sub, off of a 60 amp breaker in the main, but the max amps it could draw is 70 amps based on how it will be used, & won't actually hit 70 amps as the heater doesn't produce a full or even the magic 80% allowable for a 20 amp circuit.

Would this be ok.

I have open slots in my main panel so I could wire a 12/3 seperate (not to the sub panel) but it would save me time & money if the above solution would be acceptable.

Thanks everyone in advance for you input!

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Comments (6)

joefixit2
15 years ago
We can't answer that until you give us all the information on the welder nameplate, including the duty cycle %,and all the info from the heater nameplate as well. In case you have a code book there is a section dealing with welder circuits, article 630.
billhart
15 years ago
>>is the 60 amp breaker going to protect it.
As near as I can tell, everything is protected, assuming it is all installed correctly. The 60A breaker will protect the 6/3 wire. (I assume 6/3 w/ground.)

The issue is whether there will be nuisance tripping if you use the welder heavily while the heater is on. Perhaps the additional info that has been requested will help assess whether it will be a problem.

You might want to review the FAQ on Subpanels that can be found from a link on the top level forum page.
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Given the loads you describe, you should be well within the capacity of a 50-amp sub-panel and still have room (and ampacity) for another circuit or two. This assumes that the electrician wired the panel with a 120/240V feed, which would be the usual practice. (It's a two-pole or "double" 50-amp breaker in your main panel, right? Also, the use of the 12/3 doesn't make any sense unless the panel has a two-pole feed.) That being the case, keep in mind that each of the two legs provides 50 amps of current at 120V. If you have no 240V loads, you can think of that as like having two 50-amp feeds, or 100 amps of available power for 120V circuits. IMHO, you'd be better off running a new circuit for additional receptacles than tapping the warmer drawer circuit because, technically speaking, there's a potential code violation lurking when you don't follow the manufacturer's specifications for a dedicated circuit.
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brian0123
Original Author
15 years ago
Sorry guys I tried to be thourough.

Yes I did mean 6/3 w/ground.

The welder is a Lincoln AC-225 (Stick Welder)- Input info: Single phase (220V), 50A, 60hz...Output(max) info 225A/25V/20%duty cycle.

The Heater is a Marvin 7060 radiant heater Single phase 110V, 60hz - 2 settings Low 750W / High 1500W (if my math is correct this would be in the neighborhood of 13A at the high setting).

I looked through the FAQ yesterday before posting but didn't really find an answer.

I hope this provides you engouh info.

I believe I will be protected ok, but wanted input before I wire it all up.

Thanks again,
Brian
terribletom
15 years ago
Just a note about the ampacity of #6 wire...

While it's quite commonly used for nominal "60-amp subpanels", 6/3 NM is actually rated for only 55 amps. The so-called "roundup rule" permits a 60-amp breaker.

If you start with 55 amps and figure the load demand for the heater at nearly 16 amps (1500W / 120 X 1.25 = 15.625),
that leaves roughly 39 amps for the welder, and with two resistive loads on the panel, there's not much room to discount for diversity when both heater and welder are in use.

As a practical matter, your might be OK with your original proposal since a welder requiring a 50-amp circuit is unlikely to draw a full 50-amps for sustained periods. (That's almost got to be true of a welder with a duty cycle as low as 20%!)

Nevertheless, this sounds to me like a situation where the reasoned, conservative approach would be to bump up to #4AWG, in which case you could comfortably accommodate sustained loads at the full 60 amps or, better still, use a 70-amp breaker.

The difference in wire size would surely add to the cost, but I wouldn't allow the fact that you already have an unused 60-amp breaker to drive the solution. The cost of a new two-pole breaker is pretty modest in the scheme of things. (The price of the wire is a different matter, depending on the length of the run.)
terribletom
15 years ago
Adding another footnote:

My load load calc above refers to only one of the two poles in the subpanel, since the heater is 120V and pulls all of its power from one pole.

If you haven't already purchased the heater, have you considered a 240V unit with the equivalent heating output?

It's not an altogether trivial difference, since a 240V heater draws only half the amperage on each of the two poles. If you used a 240V unit, it'd only require about 7.5 amps on each of the poles (1500W / 240 X 1.25) and you'd use a double-pole 15-amp breaker.

It dawned on me that you may not be fully understanding the capacity of a two-pole circuit when I reread the following in the OP...

I know I will be putting a total of 90 amps of breakers (1 - 50 amp, & 2 - 20 amp) in the sub, off of a 60 amp breaker in the main...

Assuming that the two 20-amp 120V circuits are on opposite poles (the usual case), that's not true! On the contrary, the maximum would only be 70A (50+20 on one pole; 50+20 on the other pole).
terribletom
15 years ago
Sigh. This is one of those times when I really wish I could edit comments. It's also one of those times when I wish I'd reread the code section (230) carefully.

Sorry for trashing your thread with so much verbiage, Brian, but I think you're fine with the 6/3. With that low duty rating (20%), the wire does not have to be sized to pull a full 50A for the welder.

Fuggedabout upping to #4 wire. It's bad advice on my part.

Unusual sub panel question...

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joefixit2

billhart

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Unusual sub panel question...

Comments (6)

joefixit2

billhart

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brian0123Original Author

terribletom

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