Help sizing 285 ft 90 amp run

[JamesW84]

Subject to 2011 NEC, but probably not inspected. 200 amp main service in house with one spot left for a breaker.

I would like the largest reasonable amperage I can get, but 90 amps will do. I know I don't need that much now, but I'd like to keep options open in the future for mini-split, etc. I plan to run a 200 amp panel in the shop.

275 ft from the main panel in the house, up through the attic to the exterior, underground to the shop and up to the panel. I have 2 runs of 1 1/2" conduit plumbed into the foundation wall, but now realize that is way too small for the wire I need due to voltage drop. Since XHHW and mobile home feeder are about the same price, but 3" conduit would be over $600, I think I'm ok with direct burial now.

According to this: https://www.elliottelectric.com/Media/CH290-CHD-2-0-SpecificationSheet.pdf, the largest wire that will fit in a 80-125 amp breaker for my panel is 1/0. I assume I'll have to transition to copper to fit into the breakers. Also, the breaker says 75*C wire.

I was looking at URD and mobile home feeder, but since URD cannot be inside a building, I think Mobile Home Feeder might be the better choice. I was looking at this here:4/0-4/0-4/0-2/0 Aluminum Mobile Home Feeder Cable I've used online voltage drop calculators, but (as mentioned in another thread, they don't take into consideration for the insulation and other factors.) Can someone run the voltage drop on that and tell me what amperage that wire would be rated for at 285 ft? Can I have the source, also?

Given the voltage drop, and the size wire the breaker will accept, could I transition from MHF to 1/0 XHHW copper in conduit before entering the shop, and then pull the copper into the 1 1/2" conduit I already have in the foundation? According to this conduit fill chart: https://www.elliottelectric.com/StaticPages/ElectricalReferences/ElectricalTables/Conduit_Fill_Table_PVC.aspx, I can have three 3/0 conductors in 1 1/2, so I should be good, right?

Thanks in advance.

 

[theoldwizard1]

I would like the largest reasonable amperage I can get, but 90 amps will do. I know I don't need that much now, but I'd like to keep options open in the future for mini-split, etc. I plan to run a 200 amp panel in the shop.

The size of the "main" breaker in your out building does not really matter much. It is primarily there to be the disconnect for the building and POSSIBLY prevent a long walk to the other building to reset that breaker.

You have much "deeper pockets" than I do. A 60A feed (i.e. protected by a 60A breaker at the house) is more than enough for a "one man shop", even with a mini-split. If you are going to hold a "welding party" with 3 or 4 welders running at the same time that is another story. Worst case is usually a plasma cutter which draws about the same power as a welder but requires compressed air. I assume you will not have more than a 10 HP air compressor. Again, worst case, turn off the mini-split when using your plasma cutter.


2/0 aluminum wire would give you a 5% voltage drop at 90A and 275'. Still acceptable especially since you will never hit that.
2 AWG aluminum wire would give you a 6% voltage drop at 60A and 275'. Marginal, but if your actual usage was between 40A-50A the voltage drop would be about 5%.

Given the voltage drop, and the size wire the breaker will accept, could I transition from MHF to 1/0 XHHW copper in conduit before entering the shop ...

Common practice to transition at a properly sized junction box on the outside of the building and then feed inside through conduit. If you are transitioning from Al to Cu, make sure you use splices that are rated for both. They make special anti-corrosion "grease" for aluminum wire. Wire fill for conduit is in the charts.

 

[7635tools]

The size of the "main" breaker in your out building does not really matter much. It is primarily there to be the disconnect for the building and POSSIBLY prevent a long walk to the other building to reset that breaker.

You have much "deeper pockets" than I do. A 60A feed (i.e. protected by a 60A breaker at the house) is more than enough for a "one man shop", even with a mini-split. If you are going to hold a "welding party" with 3 or 4 welders running at the same time that is another story. Worst case is usually a plasma cutter which draws about the same power as a welder but requires compressed air. I assume you will not have more than a 10 HP air compressor. Again, worst case, turn off the mini-split when using your plasma cutter.


2/0 aluminum wire would give you a 5% voltage drop at 90A and 275'. Still acceptable especially since you will never hit that.
2 AWG aluminum wire would give you a 6% voltage drop at 60A and 275'. Marginal, but if your actual usage was between 40A-50A the voltage drop would be about 5%.


Common practice to transition at a properly sized junction box on the outside of the building and then feed inside through conduit. If you are transitioning from Al to Cu, make sure you use splices that are rated for both. They make special anti-corrosion "grease" for aluminum wire. Wire fill for conduit is in the charts.

60a feeder? I started out with a 100a panel in the shop and changed it out to a 200a panel 10 years later to accommodate new equipment. Luckily when I ran the underground from the house I did it in 2” so getting the 3/0 copper in there wasn’t a big deal. No way was the 100a panel going to start a 40hp phase convertor. You never know what the future might hold. Shops are always evolving. At least mine is.


Sent from my iPhone using Tapatalk

 

[JamesW84]

2/0 aluminum wire would give you a 5% voltage drop at 90A and 275'. Still acceptable especially since you will never hit that.
2 AWG aluminum wire would give you a 6% voltage drop at 60A and 275'. Marginal, but if your actual usage was between 40A-50A the voltage drop would be about 5%.

As far as I can tell, code says you can't have more than 5% total from the main to the end of the branch circuit. You can have 3% maximum drop on your feeder or 3% maximum on your branch circuit, but they can't combine to more than 5% drop.

I was hoping the voltage drop might be different than what one of the online calculators shows. Someone, I think aceman, said in a different post to not trust the online calculators.

Common practice to transition at a properly sized junction box on the outside of the building and then feed inside through conduit. If you are transitioning from Al to Cu, make sure you use splices that are rated for both. They make special anti-corrosion "grease" for aluminum wire. Wire fill for conduit is in the charts.

Got it. Thanks

 

[Bert_]

As far as I can tell, code says you can't have more than 5% total from the main to the end of the branch circuit. You can have 3% maximum drop on your feeder or 3% maximum on your branch circuit, but they can't combine to more than 5% drop.

I was hoping the voltage drop might be different than what one of the online calculators shows. Someone, I think aceman, said in a different post to not trust the online calculators.

Got it. Thanks

There is no requirement to keep voltage drop on a feeder under 3%. The only mention of it in the code is a fine print note. It is only a suggestion.

That isn't to say we shouldn't care about voltage drop but I don't look at it as a hard rule, because it isn't.

If you are planning on a 100a feeder I would use 60 or 75 amps for the voltage drop calculation. In a home shop you are very unlikely to pull anywhere near 100a. Even if you have enough equipment to draw that much it would be even less likely to do it for more than a few minutes.

 

[wyliesdiesels]

As far as I can tell, code says you can't have more than 5% total from the main to the end of the branch circuit. You can have 3% maximum drop on your feeder or 3% maximum on your branch circuit, but they can't combine to more than 5% drop.

I was hoping the voltage drop might be different than what one of the online calculators shows. Someone, I think aceman, said in a different post to not trust the online calculators.

Got it. Thanks

Code only SUGGESTS that voltage drop be limited to 3% for feeders and 5% for branch circuits.

Ive said that before about online calcs because many of us have found errors in them. Plus i prefer to do the calcs myself

so here is the equation

2xIxR / 1000* FT = VD

I= amps

R= resistance of wire- Found in NEC chapter 9 table 9 (can be searched on google or gotten for free on NFPA with an account). Look under the "effective Z" column for the correct type of wire & conduit. Then look for the correct size of wire in the rows. When you find the correct box of numbers, use the lower number in the boxes, which represents "ohms to neutral per 1000' "

FT= footage (one way length of wire)

When you input the values for I, R and FT, you will get the voltage drop at that amperage. Then, you take this number and divide by 240.

When i did the math for 2/0 AL, I got 8.2v. You then divide by 240, which in this example, equals 3.4%.

So 2/0 AL should suit you fine.

 

[JamesW84]

Code only SUGGESTS that voltage drop be limited to 3% for feeders and 5% for branch circuits.

Ive said that before about online calcs because many of us have found errors in them. Plus i prefer to do the calcs myself

so here is the equation

2xIxR / 1000* FT = VD

I= amps

R= resistance of wire- Found in NEC chapter 9 table 9 (can be searched on google or gotten for free on NFPA with an account). Look under the "effective Z" column for the correct type of wire & conduit. Then look for the correct size of wire in the rows. When you find the correct box of numbers, use the lower number in the boxes, which represents "ohms to neutral per 1000' "

FT= footage (one way length of wire)

When you input the values for I, R and FT, you will get the voltage drop at that amperage. Then, you take this number and divide by 240.

When i did the math for 2/0 AL, I got 8.2v. You then divide by 240, which in this example, equals 3.4%.

So 2/0 AL should suit you fine.

Thanks, that is what I was looking for.

I already pulled 2 gauge AL, thinking it was good for 90 amps. I was going to pull 1/0, but an electrician told me that's way overkill for 90a even with voltage drop.

I found a spreadsheet to do the calcs you mentioned https://www.carelient.com/2019/04/13/voltage-drop-calculations-nec-chapter-9-table-8-vs-table-9-spreadsheet

After looking at the numbers and pounding it in my head that 90% of the time I'll use less than 20 amps (1.52% drop), and 95% of the time I'll use less than 50 amps (3.8% drop), I think I'll leave my 90 amp breaker on at the main panel. Then, I can put a 60 amp breaker (4.56% drop) as the main breaker in the subpanel, and give it a shot. I'll note all this in the subpanel in the shop. Doing it this way would avoid tripping the breaker in the main panel and having to walk 300 ft each way to reset it.

I'm not sure how much help I'll have later if I have to re-pull, so I was trying to get it redone now. Probably smarter to give this a shot first.

 

[mike93lx]

If you already have wire in place, I would do the same and run it til it is a problem.