I've done it in the past...... See others do it all the time.
Here you are, putting in new wire on top of substandard 12 wire.
You go through the fuss and time of 'upgrading'.
And your upgrade is - one wire size? To just barely handle the loads you are using today. But just barely.
Your only extra cost in this to do it right is buy a fatter wire. You alreday got all the other set costs of the job.
Why oh why would you carefully count out every watt, and figure your job to just get by?
So you can redo the job again in 2 years, when you realize you need to run some real electric in your barn? When you need to run a calf warmer, or - something.
Why wouldn't person run the wire to feed 50-60 amps to the far building, and have a good job?
To save a couple cents on the wire.
So a person can do it all over again in a couple years, then you got both 12 and 10 gauge wire rotting away, time & money wasted, redoing this a third time.....
I'm a cheap guy, raised by a German penny pincher, tightest person I ever knew. I understand where you are coming from.
But.... Give it some thought? Are you really improving anything? Do you really know what you need down in the far building 3 years from now?
Think that is what others are kinda hinting at here.
An example. In the 1950's dad put an 8 inch drainage tile out to a neighbor's field. The neighbor added on to that tile, and so his water drained down into dad's field, and couldn't go away fast enough, so dad ened up with no gain - field still flooded out.
So he ran a 10 inch tile to his wet spot in the 1960's. That worked until the 1970's.
Then the neighbor and another neighbor hooked up more tile to the old 8 inch line, and it brought so much water down into dad's low field, it again flooded out. Even the 10 inch line couldn't pull water away fast enough.
So last year, a 3rd neighbor over that way talked about tiling and adding on.
I said fine, I'm all for it, but this time the tile will be big enough, period, or we don't do it at all.
So we put in a 15 inch tile through most of my land, and a 12 inch up to his.
This means the 8 inch, 10 iinch, and 15 inch alla re running within 50 feet of each other off in that direction.
Dad could get tiling done for a few cents a foot back then, and a dime a foot the second time.
Neighbor & I spent $19,000 going 2400 feet.
Sure wish dad had put in a bigger tile back when. We'd have had better crops, more yield, all these years as well. And he coulda done it so much cheaper....
This 15/12 inch line is set up big nough to drain all 200 acres that channel in this direction. Period. There is another neighbor wanting to hook on - we planned for that. This time. Only took dad/me 3 tries to get this figured out....
Another example - I got building with 20 amp fuse. Parked both my diesel tractors by it one nite, next morning wife & I wanted to go stalk chopping and plowing, had to plug them both in, 1200 watt heater on each. well, that didn't work, my brain wasn't thinking - came back an hour later and everything was still cold, fuse blown. Now what, new fuse, plug in one for an hour, then plug in the other for an hour, wasted a whole morning on that silliness. 20 amp is a joke in today's world.
What are you planning for? With 10 wire at 125 feet???? What's your future? One doesn't do a remodel for the curent needs, one plans for the future on a bigger projct like this.
Why aren't you running wire to go with a 50 amp box out at the far end?
Don't mean to be hard on you, I understand budget, I understand needing to keep costs down, and this is your deal. Only want to make you think a bit?
--->Paul