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Discussion Starter #1
I've started thinking about bringing power into my year old barn which has been all DIY. I was planning on a 100 Amp sub panel with a main breaker to disconnect from the line coming over from the house. Now I'm considering possibly going to a 125 amp panel because there really isn't that much cost difference between buying a 100 amp or a 125 amp main breaker panel.

The question is will it be overkill. I will have in the barn, three or four lighting circuits, four or five 120V outlets, a 230v/30 line for the air compressor & possibly another 230v line for a heater & one dedicated 20 amp line for a Lincoln Mig 140 welder. I don't think the air compressor will ever be on at the same time as the heater, but even if they were I think I'm still ok. But in any case, is going to a 125 way over my needs. I highly doubt I will ever have more in this barn then what I have stated. I'm not so sure if more is better in my case. Any opinions? Thanks.
 

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There really isn't any such thing as "overkill" when it comes to this sort of thing. The cost difference is just a few dollars and the extra 25 amp capacity is there if you ever need it. There is really no downside to this other than the possibility that you'd want a larger wire to feed the panel if you want to use it for it's full 125 amp capacity. But you can always install the 125 amp panel and feed it from a 100 amp breaker.
 

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Will the 125A box give you more circuits? That would be the only reason I would go bigger than 100A.

Code here is 6 circuits or less in a sub panel you do not need a main breaker, but I think 6 circuits is too few.
 

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More circuits is good and if your starting fresh and the heavier wire is good as well. I have a 100amp breaker feeding my shed and a 200amp panel in the shed with wire rated for 200. It keeps down the voltage drop and I don’t have to hear my wife complain about the welder or compressor dimming the lights.
 

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As I recall the code when I put my subpanels in a number of years ago; the subpanel could be 80% of the feeding panel's capacity (e.g. a 150A subpanel fed from a 200A main panel). 150A panels are available; but I've never seen one in the big box stores; so I went with 125A subpanels.

Go with as big a subpanel as you can as it will cost more in the future to redo it.

If I recall correctly, a subpanel didn't need a main breaker if it was next to or within a few feet of the feeding panel; but a subpanel that is remote and out of sight from the feeding panel requires a main breaker in the subpanel and an equivalent breaker in the feeding panel.

Panels are also called load-centers.

Hopefully, GTT's electricians will see this thread and tell me I'm right or full of it.
 

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Yes the sub panel itself is similar in price. It's the wire that costs a lot when you have to go a size or two up in gauge to handle the extra current. Also it can increase cost a good bit if you have go up a size in conduit.

Cost for adding the extra amperage is mostly dependant on how long the wire run is.

If your wire is sized properly to limit voltage drop for the amperage of your feeder breaker you won't really notice the difference between 100 and 125.
 

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Code here is 6 circuits or less in a sub panel you do not need a main breaker
Personally, I wouldn't have a subpanel without a main, even if it was just 5" from the primary.

Yes the sub panel itself is similar in price. It's the wire that costs a lot when you have to go a size or two up in gauge to handle the extra current. Also it can increase cost a good bit if you have go up a size in conduit.

Cost for adding the extra amperage is mostly dependant on how long the wire run is.
Wire is cheap w.r.t. labor & time at install vs replacing it later. No need then to worry about:
I don't think the air compressor will ever be on at the same time as the heater, but even if they were I think I'm still ok.
 

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Personally, I wouldn't have a subpanel without a main, even if it was just 5" from the primary.

Wire is cheap w.r.t. labor & time at install vs replacing it later. No need then to worry about:
I agree!

When my Mom's detached garage was wired it only had three circuits, opener, lights, outlets. So the sub panel had no main breaker. A big difference, to her in panel costs, and it met code. It probably could have been run off three breakers from the main panel in her basement? :dunno:
 

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Discussion Starter #9
OK. I want to use a 125 amp main breaker subpanel. From the house to the service entrance at my barn is about 75 feet.

This is all new work. I have a 2 inch underground conduit. I will probably run aluminum wire from the house panel to the barn. Originally I was going to use a 100 amp panel but now I'm strongly considering the 125 amp. setup. The question I now have is what size conductor do I use from the house over to the barn. I'm thinking that a #2 size will work for either 100 or 125 amp & at a distance of 75 feet away, my line voltage drop should be no more than 2%. I tried looking it up on a National electrical code chart, but frankly I'm not knowledgeable enough to under stand all the notes, etc. found on the charts I looked at.

I should ad that maybe I should not concern myself with this part of the job because I will be asking my electrician to bring the power over from the house to the barn. But I like to have an understanding as to what materials are involved. I will be doing all the work in the barn except for the final power hook up. I think for me that is the most dangerous part of the job & I want to leave that to a pro.
 

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OK. I want to use a 125 amp main breaker subpanel. From the house to the service entrance at my barn is about 75 feet.

This is all new work. I have a 2 inch underground conduit. I will probably run aluminum wire from the house panel to the barn. Originally I was going to use a 100 amp panel but now I'm strongly considering the 125 amp. setup. The question I now have is what size conductor do I use from the house over to the barn. I'm thinking that a #2 size will work for either 100 or 125 amp & at a distance of 75 feet away, my line voltage drop should be no more than 2%. I tried looking it up on a National electrical code chart, but frankly I'm not knowledgeable enough to under stand all the notes, etc. found on the charts I looked at.

I should ad that maybe I should not concern myself with this part of the job because I will be asking my electrician to bring the power over from the house to the barn. But I like to have an understanding as to what materials are involved. I will be doing all the work in the barn except for the final power hook up. I think for me that is the most dangerous part of the job & I want to leave that to a pro.

You'd need 2/0 (aka "00") aluminum wire, not #2. 2 Gauge is only rated for 90 amps. 2/0 is rated for 130 amps.

wire-size-chart.jpg
 

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For 125amps
#1 copper THHN(130amp in 75deg column) or 2/0 aluminum(135amps in 75deg column). Typically you use the 75 deg column when wire sizing, that's what all lug/terminals in panels/load centers are rated at.(Maybe I should have said 90 percent of lugs/terminals)

That being said allot of people use #2 copper (115amps in 75deg and 1/0 aluminum(120 amps in 75 deg column) many jurisdictions allow this. You can look in the 90 degree column and these wire sizes will exceed 125amps. ( Again many people do this but never approach the Max amp rating...is it right, no, but I've seen it allot.)

I would use #1 copper THHN if it were up to me.
 

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At a minimum for 100 amps and a 100 foot wire run (75 ft between buildings + 25 ft for wire in and out of panels ) you will need 1 gauge copper or 2/0 aluminum to maintain voltage drop less than 3%.

For 125 amp service you need 1/0 copper and 4/0 aluminum wire to maintain voltage drop over 100 ft.

That's assuming you are pulling the full rated amperage.
 

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Vd=2KIL/Cm or Cm=2KIL/Vd

Vd is voltage drop
2 is to account for the circuit length (there and back). The square root of 3 would be used if it were 3 phase, instead of 2.
K is a constant. 12.9 for copper, 21.2 for aluminium. Based on 75°C
I is the required amperage. For a feeder it can be calculated on the required load or the size of the protecting overcurrent device (the overcurrent device feeding the feeder panel)
L is the one way length of the run
Cm is the circular mils of the conductor

So....what size feeder conductor is required for 125A panel, a distance of 100 feet, 240 volts single phase with a 3% voltage drop? Let's assume a full load of 125 amps, and I threw in 25 extra feet (versus 75) for terminations.

Vd @ 3%=240 times .03=7.2 volts. This is how many volts we can lose.
Cm=2KIL/Vd
Copper: Cm=2(12.9)(125)(100)/7.2= 44792 Cm
Aluminium: Cm=2(21.2)(125)(100)/7.2= 73611 Cm

So we need a 44792 Cm Copper or a 73611 Cm Aluminium conductor to maintain a 3% voltage drop.
A #3 conductor is 52620 Cm, a #1 is 83690 Cm.
Those are the smallest conductors to use in order to maintain a maximum 3% voltage drop. Now, will they handle the 125 amps we're looking to get to the feeder panel?

Let's see what 310.15 (B)(16) says. And mostly likely you're going to be using THWN or XHHW conductors, so let's base it off that.
From the 75° column: Copper #2 @ 115 amps, Aluminium 1/0 @ 120 amps would be appropriate for a 125A panel.

Wait, what?
That can't be. Neither give me 125 amps? Ahh...but there is a 'next size up rule'. In easy terms, The next size higher standard overcurrent device can protect conductors (having an ampacity less than the protecting overcurrent device) IF 3 qualifiers are met.

The protected conductors are not part of a branch circuit supplying receptacles (they're not....it's a feeder circuit).
The ampacity of the conductors do not correspond to a standard amp rating of an overcurrent device
The next higher standard rating does not exceed 800 amps

So I can put a #2 copper or a 1/0 Al on a 125A breaker no problem. Do our conductors then handle the voltage drop we calculated?
Yes. The #2 Cu required for 125A is larger than the #3 we needed for voltage drop. The #1/0 Al we need for 125A is larger than the required #1 for Vd.

So now that we did the voltage drop calculation and comparison, the length makes the Vd irrelevant. (But you may want to consider up sizing the wire one size to help keep the lights in the hose from flickering when the air compressor starts.) I typically don't bother with voltage drop until it's over 150 feet. Even then it's based on the load, not the size of a panel. In other words, when have you seen a typical homeowner install a 125 or 150 amp panel in their barn and actually draw that many amps? Yes it happens but is rare.

To the OP (Maddog)

Are you feeding this barn sub-panel from your existing breaker panel in the house? Meaning you're going to put a breaker in the house panel to feed the barn. Can you get a 125A breaker to fit your house panel? If not and a 100A is the largest you can get to fit the house, then you're limited to 100 amps, no matter what size panel you put in the barn. Your main breaker in the barn will be a disconnect then, which is required for a detached structure anyways. (I'm assuming it's detached as I've never heard of a barn attached to a house, but hey it could happen :laugh:). Being a detached structure it'll require a grounding electrode system also. Driving 2 ground rods at least 6 ft apart, connected with the appropriate size grounding electrode conductor is the easiest.

I just wanted to clarify some things, not pointing fingers. FWIW, I have my Michigan's Master Electrical licence and been studying the NEC for over 15 yrs. BUT, keep in mind, I'm human and make mistakes too (hope my wife didn't hear that :laugh:)
 

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[detailed tech stuff] ... Can you get a 125A breaker to fit your house panel? If not and a 100A is the largest you can get to fit the house, then you're limited to 100 amps, no matter what size panel you put in the barn. ... I have my Michigan's Master Electrical licence and been studying the NEC for over 15 yrs. BUT, keep in mind, I'm human and make mistakes too
An example of why I love GTT. Always a pro somewhere shining the light of facts & reason. :good2:
 

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[detailed tech stuff] ...*
I think I've been reading too many of SulleyBear's post. I got long winded didn't I? :lol:

(No offense Sulley :laugh:)
 

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I think I've been reading too many of SulleyBear's post.
Wow! You must have a LOT of extra time on your hands. :lol:
 

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Vd=2KIL/Cm or Cm=2KIL/Vd

Vd is voltage drop
2 is to account for the circuit length (there and back). The square root of 3 would be used if it were 3 phase, instead of 2.
K is a constant. 12.9 for copper, 21.2 for aluminium. Based on 75°C
I is the required amperage. For a feeder it can be calculated on the required load or the size of the protecting overcurrent device (the overcurrent device feeding the feeder panel)
L is the one way length of the run
Cm is the circular mils of the conductor

So....what size feeder conductor is required for 125A panel, a distance of 100 feet, 240 volts single phase with a 3% voltage drop? Let's assume a full load of 125 amps, and I threw in 25 extra feet (versus 75) for terminations.

Vd @ 3%=240 times .03=7.2 volts. This is how many volts we can lose.
Cm=2KIL/Vd
Copper: Cm=2(12.9)(125)(100)/7.2= 44792 Cm
Aluminium: Cm=2(21.2)(125)(100)/7.2= 73611 Cm

So we need a 44792 Cm Copper or a 73611 Cm Aluminium conductor to maintain a 3% voltage drop.
A #3 conductor is 52620 Cm, a #1 is 83690 Cm.
Those are the smallest conductors to use in order to maintain a maximum 3% voltage drop. Now, will they handle the 125 amps we're looking to get to the feeder panel?

Let's see what 310.15 (B)(16) says. And mostly likely you're going to be using THWN or XHHW conductors, so let's base it off that.
From the 75° column: Copper #2 @ 115 amps, Aluminium 1/0 @ 120 amps would be appropriate for a 125A panel.

Wait, what?
That can't be. Neither give me 125 amps? Ahh...but there is a 'next size up rule'. In easy terms, The next size higher standard overcurrent device can protect conductors (having an ampacity less than the protecting overcurrent device) IF 3 qualifiers are met.

The protected conductors are not part of a branch circuit supplying receptacles (they're not....it's a feeder circuit).
The ampacity of the conductors do not correspond to a standard amp rating of an overcurrent device
The next higher standard rating does not exceed 800 amps

So I can put a #2 copper or a 1/0 Al on a 125A breaker no problem. Do our conductors then handle the voltage drop we calculated?
Yes. The #2 Cu required for 125A is larger than the #3 we needed for voltage drop. The #1/0 Al we need for 125A is larger than the required #1 for Vd.

So now that we did the voltage drop calculation and comparison, the length makes the Vd irrelevant. (But you may want to consider up sizing the wire one size to help keep the lights in the hose from flickering when the air compressor starts.) I typically don't bother with voltage drop until it's over 150 feet. Even then it's based on the load, not the size of a panel. In other words, when have you seen a typical homeowner install a 125 or 150 amp panel in their barn and actually draw that many amps? Yes it happens but is rare.

To the OP (Maddog)

Are you feeding this barn sub-panel from your existing breaker panel in the house? Meaning you're going to put a breaker in the house panel to feed the barn. Can you get a 125A breaker to fit your house panel? If not and a 100A is the largest you can get to fit the house, then you're limited to 100 amps, no matter what size panel you put in the barn. Your main breaker in the barn will be a disconnect then, which is required for a detached structure anyways. (I'm assuming it's detached as I've never heard of a barn attached to a house, but hey it could happen :laugh:). Being a detached structure it'll require a grounding electrode system also. Driving 2 ground rods at least 6 ft apart, connected with the appropriate size grounding electrode conductor is the easiest.

I just wanted to clarify some things, not pointing fingers. FWIW, I have my Michigan's Master Electrical licence and been studying the NEC for over 15 yrs. BUT, keep in mind, I'm human and make mistakes too (hope my wife didn't hear that :laugh:)
I don't think this is correct. This explains it best for me.

Conductor Sizing | Terminal Temperature Ratings | Electrical Construction & Maintenance (EC&M) Magazine

Edit: but like you said, he will have a tough time find a 125 amp snap in anyway. Square D QO is the only one I know of.
 

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