'Lectric question

[Jon_E]

OK, I have to run electrical service to my garage. House panel (main) is 200-amp, 40-space panel. It's got a few empty spaces in it, as well as a pair of 240v double breakers that power my woodworking machinery currently in the basement. 100-amp breaker feeds adjacent subpanel. Subpanel, 20 spaces, is connected to manual transfer switch (30-amp breaker) which has critical circuits in it that would typically be run during a generator-fed outage (i.e. well pump, refrigerator, freezer, couple of light curcuits, etc.) This also has a few empty spaces in it. So, it's basically a 100-amp sub except when the generator is being used, then it's a 30-amp sub. I can't explain it, maybe the electrician who installed it can. All I know is that in 8 years, it's never been used.

So I want a 100-amp subpanel in my garage. I have several pieces of 240v machinery, including table saw, bandsaw, dust collector, jointer. More than likely will add to that total and I also have a 10-hp rotary phase converter in case I ever get another 3-phase machine (had a big planer; sold it). Need circuits for 110v tools, 240v tools, lighting, probably a small welder, air compressor. Unlikely to use more than two machines at once, phase converter is probably the big one. I think that used a 60-amp breaker.

Considered a separate service to the garage. Cost prohibitive and awkward. Going to feed garage from main panel in house, have conduit part-way there, need to complete installation.

Should I put the 100-amp breaker for the garage sub in the main panel (I can replace one of the 240v basement "shop" circuit breakers, currently 15 amp) or put it in the 100-amp subpanel? Can you feed a 100-amp subpanel from a 100-amp subpanel?

I do have an electrician friend who is going to assist me, but I am trying to do some research in advance so I don't come across as a complete dope when discussing this with him.

 

[pattenp]

Once you start daisy chaining subpanels you are increasing the potential loads on the first 100A breaker that feed the first subpanel. You are better off putting the second subpanel off the main panel. But you can put the second sub off the first sub if there is a logistical reason.

 

[Jon_E]

No, no particular logistical reason that I can think of. Might be awkward to get a big service cable back out of the main and into the conduit to the garage. The sub is more accessible, that's all. Everything is possible with time and money, I guess.

 

[Slowgsr]

To be honest, just take your electrician friends advice, don't worry about looking like a dummy, nothing worse then a home owner making suggestions off of what they've read online. It can be kind of annoying.

Just saying. It sounds like a very straight forward installation which your friend should have no issues with.

From your 200a main panel, put in a 100a breaker and feed your new panel in the garage, either buy a panel with a main disconnect or backfeed another 100a, you need a local disconnect.

Have fun.

 

[wyliesdiesels]

What is the wire length to the garage?

Remember, the neutral bar in the garage sub needs to be isolated and u need 2 ground rods...

 

[Jon_E]

It's 140 feet to the garage.

 

[CNGsaves]

What country are you in ??

Update GJ Profile with City / State / Country.

Still TOO LITTLE INFO . . . . you'll get best advice with all information.

You may be going at "problem" backwards by "deciding" you want 100A breaker in the main panel to feed your detached garage.
a) What WIRE do you plan on using for that 140 feet ??
b) What SIZE CONDUIT do you plan on using for 140 feet ??

A low cost solution is use MHF (mobile home feeder) aluminum 2-2-2-4 which should give you 90A at the subpanel for just $1.50/ft wire cost . . . so you WOULD put 90A breaker in the main panel in house, but could use a 100A subpanel in garage with it's own main shutoff, along with 2 ground rods and separated ground/neutral in subpanel.

2" sch 40 plastic conduit buried and sch 80 once it's above ground. Suggest you put that 2" conduit in bottom of trench, then backfill 12" dirt then another 1" plastic conduit for low voltage stuff like internet, CATV, phone, security, etc.

 

[Jon_E]

2" Sch80 conduit installed when house was built, but I have to reroute part of it due to moving the location of the garage slightly. Also buried 3/4" conduit which I can't find most of, I think it was wrecked when they dug the foundation hole for the garage. That will be for LV stuff like you said.

HD has 3-3-3-6 Cu for $7.84/ft which is insane if I can run equivalent Al for less than two bucks a foot.

 

[Jon_E]

Where do I find a chart, table or formulae that can tell me the size of the wire and conduit I need, based on the proposed amperage draw and length of the service cable?

 

[pattenp]

When you stated 3-3-3-6 at HD I thinking you are looking at SER cable which is NOT allowed to be placed underground in conduit. Make sure you are looking at wire approved to be underground even if in conduit.

For a full 100A service at 140ft I'd use #2/0-2/0-2/0-1 AL MHF in at least 2"PVC. For the sake of preventing headaches pulling,2.5"PVC will be easier to pull through.

 

[wyliesdiesels]

2" Sch80 conduit installed when house was built, but I have to reroute part of it due to moving the location of the garage slightly. Also buried 3/4" conduit which I can't find most of, I think it was wrecked when they dug the foundation hole for the garage. That will be for LV stuff like you said.

HD has 3-3-3-6 Cu for $7.84/ft which is insane if I can run equivalent Al for less than two bucks a foot.

Is this is the wire youre speaking of- #3 SER? If so, u cant use that underground.

http://m.homedepot.com/p/Southwire-500-ft-3-3-3-5-CU-SER-Service-Entrance-Cable-27757403/202250390/

Where do I find a chart, table or formulae that can tell me the size of the wire and conduit I need, based on the proposed amperage draw and length of the service cable?

As nice as that would be to have a chart or table, unfortunately there isnt one.

There is a formula however.

Use what pattenp suggested. U could use #1 AL but 2/0 MHF will be easier and cheaper.

 

[Jon_E]

So, research since I posted this suggests that for a 140' run, 2/0-2/0-2/0-1 AL URD MHF through 2" conduit will get me 100A at the subpanel in the garage. If I were to "downsize" to a 90A sub, I could go with 2-2-2-4 AL URD MHF and have an easier time of pulling it from the house to the garage. Sound right?

Where do I find a chart that will tell me my expected maximum load based on the equipment I will be using? I can see running a 10hp phase converter with a 7.5hp three-phase motor, and a 5-hp single phase dust collector, all at once.