Ephemeral_Glow
New member
- Location
- Chaska, MN, USA
My father-in-law is an electrician (industrial) :thumbsup: and we went over my calculations about the wire I needed for a couple of sub-panels and we disagree on wire sizes. I need someone to tell me whether I'm right or whether I have my head cross-threaded on my neck. :happyno: :? :happysad:
First of all, back-story. I bought a Menards gift card with $7500 when they had an 11% off sale that applied to gift cards.
I used most of the card the same day for another 11% off on cabinets and countertops for the kitchen remodel and have a $1000 or so left on it so I'm inclined to see what wire they have available. Menards sells only two sizes of SER cable, 2-2-2-4 AL and 4/0-4/0-4/0-2/0 AL. SER is basically like Romex as I understand it.
I am used to the Square D QO panels that have two bus bars with "fingers" alternating in the center between the two bus bars. That's the style of my main and I have inherited a box of spare breakers that I intend to use in the new subpanels of the same style.
The difference in our calculations stems from my belief that the load at the sub-panel is divided between the two hot legs, red and black, that feed the subpanel. My wire size determination is half what his is. I explained to him, that if I have a 50 Amp 2 pole breaker on two bus bars, each leg would supply half the load. And, the subpanel is like a giant multi-wire / Edison circuit, in which the 120V breakers and circuits that are on one leg only, their neutrals meet the other neutrals from the other circuits that are on the opposite leg, effectively creating a neutral voltage point at the neutral bus bar in the subpanel, provided that the breakers are arranged in the subpanel and the draw is balanced as close as possible.
My calculation goes like this: The old NEC Table 310.16 shows an ampacity of 100 in the 90 degree column. I use the 90 degree column because the subpanel feeder goes through the attic for most of the run and typical attic temp around here is 125-130 degrees, so I apply a correction factor of 76%. Easy Math, 100 x .76 = 76 Amps . The feeder is also going to run from the main in the basement, up through an outside wall that has thermal insulation, so I'm required not to exceed the 60 degree ampacity after correction factors are applied. The 60 degree ampacity is 75 Amps, so very close to the 76 after corrections using the 90 degree ampacity. To me, that means each leg can carry 75 Amps for a combined 150 Amps of power. I'm happy with that. With all the future additions and a bunch of kids that leave all the lights on like my brother did growing up... Even at a further 80% reduction for continuous load, that's 120 continuous. The only question after that is whether a panel becomes 80% derated for continuous, so I should actually install a 150 Amp rated sub-panel for 120 continuous ?
My main requires a QOC style 4 space breaker for a 150-200A branch. I would want a 4 space for a 125 Amp branch as well, although the main says 2 space is okay for 125A. Yeah right. Anyone know of a big box store that carries 4 space QO breakers ? I've got an electrical supply place picked out otherwise.
In an imperfect world, with a sub-panel imperfectly balanced due to difference in breakers, I'm bound to have a load that is 5, 10, 15, worst case 20 Amps out of balance between the two legs. I could figure on having the legs at max 20 amps out of balance, so I could safely expect one leg to carry 75 and the other 55, for a combined 130 Amps total load. So then only a 125 breaker in the main feeding a 125 Amp subpanel would make sense.
My father-in-law insists that I need each conductor to be rated for 100 amps for a 100 amp subpanel. Is he right or do I have my head screwed on straight ? I certainly thought I did my homework. :dunce:
I know even a 100 Amps is plenty at a sub-panel but I have 200 service and the sub will be upstairs and I will be adding lights to all the rooms in the house, where the builder thought it was appropriate to put one light or two in the center of the ceiling and expect you to work in your shadow everywhere. Really? Everything needs to be redone and all the circuits that go to the upstairs and the 100 foot runs to the garage as well are going to be taken away from the main panel and put on the new subpanels. I predict I will only need 50 Amps from the main to the basement once I get rid of the electric dryer and replace it with gas once it goes out and maybe before if I find a deal on one. And I'm starting a business in my garage that involves welding so if things go well I'll be doubling my garage in a few years and upgrading to 300 or 400A service. I already have a stick welder that is just a beast with a nameplate of 90 amps, even though I'll only use half of that for the rods I use, and have some young kid run the other wire feed welder I have that uses 35-45 Amps. When the upgrade happens, I plan on putting in a new 400 / 320A meter combo in the garage and reverse the direction of feeders to feed the old main and upstairs subpanel from the garage. I suppose I would need two meters too so that I could deduct business electricity on taxes seperate from the house. :angel:
First of all, back-story. I bought a Menards gift card with $7500 when they had an 11% off sale that applied to gift cards.
I used most of the card the same day for another 11% off on cabinets and countertops for the kitchen remodel and have a $1000 or so left on it so I'm inclined to see what wire they have available. Menards sells only two sizes of SER cable, 2-2-2-4 AL and 4/0-4/0-4/0-2/0 AL. SER is basically like Romex as I understand it. I am used to the Square D QO panels that have two bus bars with "fingers" alternating in the center between the two bus bars. That's the style of my main and I have inherited a box of spare breakers that I intend to use in the new subpanels of the same style.
The difference in our calculations stems from my belief that the load at the sub-panel is divided between the two hot legs, red and black, that feed the subpanel. My wire size determination is half what his is. I explained to him, that if I have a 50 Amp 2 pole breaker on two bus bars, each leg would supply half the load. And, the subpanel is like a giant multi-wire / Edison circuit, in which the 120V breakers and circuits that are on one leg only, their neutrals meet the other neutrals from the other circuits that are on the opposite leg, effectively creating a neutral voltage point at the neutral bus bar in the subpanel, provided that the breakers are arranged in the subpanel and the draw is balanced as close as possible.
My calculation goes like this: The old NEC Table 310.16 shows an ampacity of 100 in the 90 degree column. I use the 90 degree column because the subpanel feeder goes through the attic for most of the run and typical attic temp around here is 125-130 degrees, so I apply a correction factor of 76%. Easy Math, 100 x .76 = 76 Amps . The feeder is also going to run from the main in the basement, up through an outside wall that has thermal insulation, so I'm required not to exceed the 60 degree ampacity after correction factors are applied. The 60 degree ampacity is 75 Amps, so very close to the 76 after corrections using the 90 degree ampacity. To me, that means each leg can carry 75 Amps for a combined 150 Amps of power. I'm happy with that. With all the future additions and a bunch of kids that leave all the lights on like my brother did growing up... Even at a further 80% reduction for continuous load, that's 120 continuous. The only question after that is whether a panel becomes 80% derated for continuous, so I should actually install a 150 Amp rated sub-panel for 120 continuous ?
My main requires a QOC style 4 space breaker for a 150-200A branch. I would want a 4 space for a 125 Amp branch as well, although the main says 2 space is okay for 125A. Yeah right. Anyone know of a big box store that carries 4 space QO breakers ? I've got an electrical supply place picked out otherwise.
In an imperfect world, with a sub-panel imperfectly balanced due to difference in breakers, I'm bound to have a load that is 5, 10, 15, worst case 20 Amps out of balance between the two legs. I could figure on having the legs at max 20 amps out of balance, so I could safely expect one leg to carry 75 and the other 55, for a combined 130 Amps total load. So then only a 125 breaker in the main feeding a 125 Amp subpanel would make sense.
My father-in-law insists that I need each conductor to be rated for 100 amps for a 100 amp subpanel. Is he right or do I have my head screwed on straight ? I certainly thought I did my homework. :dunce:
I know even a 100 Amps is plenty at a sub-panel but I have 200 service and the sub will be upstairs and I will be adding lights to all the rooms in the house, where the builder thought it was appropriate to put one light or two in the center of the ceiling and expect you to work in your shadow everywhere. Really? Everything needs to be redone and all the circuits that go to the upstairs and the 100 foot runs to the garage as well are going to be taken away from the main panel and put on the new subpanels. I predict I will only need 50 Amps from the main to the basement once I get rid of the electric dryer and replace it with gas once it goes out and maybe before if I find a deal on one. And I'm starting a business in my garage that involves welding so if things go well I'll be doubling my garage in a few years and upgrading to 300 or 400A service. I already have a stick welder that is just a beast with a nameplate of 90 amps, even though I'll only use half of that for the rods I use, and have some young kid run the other wire feed welder I have that uses 35-45 Amps. When the upgrade happens, I plan on putting in a new 400 / 320A meter combo in the garage and reverse the direction of feeders to feed the old main and upstairs subpanel from the garage. I suppose I would need two meters too so that I could deduct business electricity on taxes seperate from the house. :angel:
