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Sub panel wire sizing.

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6togo

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I'm ready to run some electric in my new garage but not quite sure on the wire sizing and type. I have a 200amp main panel which jumps over to my first detached garage less than 20away and it's 100amp sub-panel with nothing more than some outlets and lights. I want to pull another 100amp sub-panel off the first garage sub-panel to power your basic garage things like a lift, air comp, lights, outlets and openers. The second garage is a total of around 180ft away which 35' is though the first garage before it hits the ground to the second garage.
I found some charts online which said I needed 2/0-2/0-2/0-#1 direct burial to cover line loss and the 125% rule and use 2/0 SER inside the garage to a junction box. I did see that the under ground cable can't enter the building though and I really don't want to have 2 junctions. Any idea's or suggestions? Perhaps MHF?
 
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mike93lx

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MHF or xhhw (requires conduit).

What's the 125% you are solving for?

I'd check voltage drop at 70-90a as well. Might be able to downsize as it's very hard for a one-man shop to hit anywhere near 100a. For what you listed, 60a would work
 
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6togo

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I could be wrong but I thought code said you needed the cable to be oversized for overcorrect protection. It is a fairly large garage 35'x50' and I may add a few more items like tire machines (110V) and a welder outlet.
 
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mike93lx

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6togo said:
I could be wrong but I thought code said you needed the cable to be oversized for overcorrect protection. It is a fairly large garage 35'x50' and I may add a few more items like tire machines (110V) and a welder outlet.
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The 125% refers to constant loads, not an overall service.

Just use 100a (or whatever you want) for the load and calc voltage drop from that.

It doesn't really matter how much stuff you add. One person can only use so much at a time. About max would be HVAC, lighting, compressor and a plasma cutter.

You arent running a tire machine while striking an arc on your 300a tig while a lift has your 14klb utility truck going up.
 
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mike93lx

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#2mhf would give you about 4% voltage drop at your distance with a 90a load.

I'd run that without worrying unless you were really drawing that much
 
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sparky 1971

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You could get a dual rated 1/0 URD triplex and throw a #4 USE in for a ground, but I would stick with the #2. My shop is running g on 175' of #2 and a 100 amp breaker. I run the plasma cutter and air compressor with no issues. The only time I ever had a problem was when my wife's kid had his camper plugged in with the AC running and the 60 amp breaker tripped while I was cutting and the compressor started. I installed the 100amp breaker about 10 minutes later.

Edit: I must add that a 100 amp breaker is a code violation when on the load side of the main breaker. A 90 amp breaker would be compliant, but I had what I had on hand and didn't care. One of these days I might fix it, but probably ain't gonna happen.

My whole point was that if there was going to be a voltage drop problem, I would have issues. There are times when I have two compressors going, one 3hp 240 volt and another larger 120 volt, along with a bunch of lights and big fans at the same time and can use the chop saw. If the drop was bad, it would show when I pull the trigger on the saw.
 
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wyliesdiesels

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sparky 1971 said:
You could get a dual rated 1/0 URD triplex and throw a #4 USE in for a ground, but I would stick with the #2. My shop is running g on 175' of #2 and a 100 amp breaker. I run the plasma cutter and air compressor with no issues. The only time I ever had a problem was when my wife's kid had his camper plugged in with the AC running and the 60 amp breaker tripped while I was cutting and the compressor started. I installed the 100amp breaker about 10 minutes later.
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#2 al is limited to 90a
 
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sparky 1971

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wyliesdiesels said:
#2 al is limited to 90a
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Yeah, I know that. It was a Saturday afternoon and I had a 100 on my truck. My old main panel in the house is now a sub fed by #2 SER and a 100. That was the summer of the Rona and there were no 90's to be found anywhere. When the new service was inspected I showed the inspector the paperwork showing that the 90 was ordered and he was ok with it. When the 90 finally came in after about three months, I used it on another job.
 
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6togo

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So here's a question say I run 2/0-2/0-2/0-1 from sub panel #1 to sub panel #2. What if the main panel to Sub panel #1 had a smaller feeder would that matter? I'm not sure just what size the wire is I have to pull the cover off and see.
 
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mike93lx

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6togo said:
So here's a question say I run 2/0-2/0-2/0-1 from sub panel #1 to sub panel #2. What if the main panel to Sub panel #1 had a smaller feeder would that matter? I'm not sure just what size the wire is I have to pull the cover off and see.
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No, it's fine. You will just be limited in total draw by the breaker feeding sub #1.
 
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6togo

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So I popped the current sub panel cover and it looks like it's awg 2 for the conductors, 1 awg black with white stripe for neutral and a smaller green which I think might be a 4 for the ground. It's has this written on it for types RHH & RHW-2. So I guess it's MHF?
 
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6togo

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Sorry no it's a bit smaller than the neutral. I must have read the neutral wrong must be a 4 and the ground is a 6. Must be 2-2-4-6 MHF it's in conduit and only about 20' from the main.
 
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mike93lx

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Sounds like MHF. What size breaker is feeding that sub (the breaker in the main, not the breaker in the sub)?
 
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6togo

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This was done a few years back from what I was told by the previous owner and was done by a licensed Electrian.
 
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mike93lx

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6togo said:
This was done a few years back from what I was told by the previous owner and was done by a licensed Electrian.
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Doesn't matter. It is a code violation.

Will it ever cause a problem? Probably not. But it was missed by an electrician and possibly an inspector
 
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mike93lx

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6togo said:
So If I swap that breaker to a 90a that means I'm limited to a 90a in the second sub panel?
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Everything downstream of that breaker is limited to 90a in total. Doesn't matter if that is 0a from the first sub and 90a in the second or any other combo that totals 90a. If you need more than that, you have to run a line all the way back to the main panel
 
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