Need help with wiring

[Jacks Garage]

I have a 200amp service going from the power pole to my house. From the house to the 24' x 24' garage i have 100amp service. I just had a 24' x 30' garage built 75' away from the first garage and i want 100amp service in it. I want to connect the new garage to the first garage. All wires are underground. What kind of wire will i need to do this cheaply and safely. Thank you very much in advance.

 

[G_P]

Dont think thats going to work. I dont think you can run a subpanel off of another subpanel.

 

[justsam]

How far away is the house that is feeding the first garage?

Since you are feeding the second garage from the first, and the first from the house, all the distances must be considered to calculate voltage drop, and therefore wire size.

Is work being done in both garages concurrently, or is it more a case of one "OR" the other. Is there a case where there could be something exceeding 100 Amps total with both garages operational?

You are looking at some pretty serious wire gauges here. Could you scale back to 60 or 90 Amps at the second garage?

 

[Rock knocker]

Dont think thats going to work. I dont think you can run a subpanel off of another subpanel.

It's done time and again commercially.

The issue may be finding a 100A breaker to feed sub-2 that will fit into sub-1. You may be able to do parallel and feed-thru work arounds because the wires are ultimately protected by a 100A OCD at the house.

Zeke'll know.

 

[G_P]

It's done time and again commercially.

The issue may be finding a 100A breaker to feed sub-2 that will fit into sub-1. You may be able to do parallel and feed-thru work arounds because the wires are ultimately protected by a 100A OCD at the house.

Zeke'll know.

Learn something new every day around here!

 

[JohnX14]

You can run 100 amps to the 2nd garage from the first garage. You should be able to just put a 100 amp plug in breaker in the first subpanel. Most likely the bussing is rated for 100 amp per bus, meaning you can't have a breaker on the opposite row of the panel next to the 100. You can run 1.25" pvc underground from one to the other and feed with either #2 aluminum or #4 copper. You will need 4 conductors, and will need to establish a grounding electrode system at the 2nd garage. (drive 2 ground rods, for example)

 

[theoldwizard1]

... and feed with either #2 aluminum or #4 copper. ...

Not according to the Southwire Voltage Drop Calculator !

First do really NEED 100A in you new detached garage ? 60A would run a good sized electric heater and 1 or 2 major power tools (welder, compressor, large table saw, dust collector) with plenty left over for lights and hand power tools.

At 60A, you can use 2-2-2-4 MHF and skip the conduit except above ground (use a Schedule 80 sweep to transition from below to above ground) and inside buildings.

There is NO issue installing a 100A panel in the new building and feeding it from a smaller (60A) breaker in an existing sub-panel.

This will save you a BUNCH OF $$$ !!

 

[pattenp]

You can run 100 amps to the 2nd garage from the first garage. You should be able to just put a 100 amp plug in breaker in the first subpanel. Most likely the bussing is rated for 100 amp per bus, meaning you can't have a breaker on the opposite row of the panel next to the 100. You can run 1.25" pvc underground from one to the other and feed with either #2 aluminum or #4 copper. You will need 4 conductors, and will need to establish a grounding electrode system at the 2nd garage. (drive 2 ground rods, for example)

Nope, #4 Cu is good up to 85A as a branch feeder and #2 Al is good to 90A.

Not according to the Southwire Voltage Drop Calculator !

First do really NEED 100A in you new detached garage ? 60A would run a good sized electric heater and 1 or 2 major power tools (welder, compressor, large table saw, dust collector) with plenty left over for lights and hand power tools.

At 60A, you can use 2-2-2-4 MHF and skip the conduit except above ground (use a Schedule 80 sweep to transition from below to above ground) and inside buildings.

There is NO issue installing a 100A panel in the new building and feeding it from a smaller (60A) breaker in an existing sub-panel.

This will save you a BUNCH OF $$$ !!

Why do you continue to site just 60A for 2-2-2-4 MHF. #2 aluminum such as MHF can be over current protected up to 90 amps.

 

[ez-duzit]

Your first garage is wired for 100 amps. Do not add another 100 amp load to this circuit.

 

[pattenp]

Your first garage is wired for 100 amps. Do not add another 100 amp load to this circuit.

There is nothing to say he can't run a second 100A subpanel off the first 100A subpanel. He would need to keep mindful of actual concurrent loads between the two garages to not trip the 100A breaker back in the main service panel.

 

[ez-duzit]

There is nothing to say he can't run a second 100A subpanel off the first 100A subpanel. He would need to keep mindful of actual concurrent loads between the two garages to not trip the 100A breaker back in the main service panel.

Sorry--not true.

The first garage's 100-amp service is fine for its existing distance. Adding an EXTRA 100-amp load 75 feet beyond is not going to work.

 

[Beemer533]

Your first garage is wired for 100 amps. Do not add another 100 amp load to this circuit.

He isn't adding a 100A "load", he is adding a 100A sub panel...

 

[ez-duzit]

**Bull.**

 

[Alchymist]

Sorry--not true.

The first garage's 100-amp service is fine for its existing distance. Adding an EXTRA 100-amp load 75 feet beyond is not going to work.

Is true. All depends on his overall loads. He could go several hundred feet if all he wanted was lights and a few small power tools. Nothing preventing him from running a sub panel from the sub panel in the first garage. Just a matter of how much power is used vs wire sizes and distances.

Seen many installations where several sub panels were daisy chained.

 

[Bib Overalls]

He needs to upgrade his service to 320/(400) amps with 200 amp disconnects to the house and first garage. The first garage gets a 200 amp pane and feeds a 100 amp sub panel in the second.

 

[theoldwizard1]

Why do you continue to site just 60A for 2-2-2-4 MHF. #2 aluminum such as MHF can be over current protected up to 90 amps.

According to the Southwire calculator you need 1/0 for 90A at 75' for direct buried aluminum.

 

[pattenp]

According to the Southwire calculator you need 1/0 for 90A at 75' for direct buried aluminum.

The Southwire VD calculator is way on the conservative side and doesn't allow for much input.

Here's a better VD calculator.
http://www.nooutage.com/vdrop.htm

Also the NEC says for #2 Al @ 75C that 90A is the max over current protection. Your 1/0 that you site is a wire size that limits VD to less than 1.8% @ 90A by the Southwire calculator.

Edit: Another thing is the Southwire calculator limits the wire use to 60C, and #2Al @ 60C is 85A so the calculator uses 1/0 @ 60C which is 100A. In our case we can use #2 Al @ 75C which is 90A. Bottom line is stop using that crappy Southwire VD calculator.

 

[Alchymist]

He needs to upgrade his service to 320/(400) amps with 200 amp disconnects to the house and first garage. The first garage gets a 200 amp pane and feeds a 100 amp sub panel in the second.

How can you justify this without even knowing his loads in the two garages? This would mean whole distribution system would have to be upgraded, including the feed to the first garage. This would be highly expensive.

 

[theoldwizard1]

Bottom line is stop using that crappy Southwire VD calculator.

Roger !

 

[Jacks Garage]

You guys are great and very knowledgeable. To answer some of the questions. Only one garage will be operational at a time. The first garage is 15' away from the house. This site is great, wish I would have found it sooner.

 

[ez-duzit]

...Only one garage will be operational at a time. The first garage is 15' away from the house...

If the conduit is big enough, you can replace the 15' of cable to the first garage with cable sized properly for the 180' round trip from the main panel to the new garage (15' + 75' times 2). But I would still prefer to see the 2 sub-panels each run directly to the main panel.

Since you are hiding this remodel, you probably won't bother to try to meet code, either. But at least you can try to make it safe.

 

[wyliesdiesels]

Wow! Gotta love the novice and overkill answers like 'gotta upgrade to 320a service"...

Without knowing wat tools and equipment the OP has,
specifically a load calc, the answers people(not including the sparkies pattenp or alchmyst) are giving on this thread are totally wrong!

Bottom line is stop using that crappy Southwire VD calculator.

Roger !

Or just do the math on a calculator!

If the conduit is big enough, you can replace the 15' of cable to the first garage with cable sized properly for the 180' round trip from the main panel to the new garage (15' + 75' times 2). But I would still prefer to see the 2 sub-panels each run directly to the main panel.

Since you are hiding this remodel, you probably won't bother to try to meet code, either. But at least you can try to make it safe.

Did u do a load calc to come up with this?

Cite the code that says he has to do this!!

 

[ez-duzit]

Wow! Gotta love the novice and overkill answers like 'gotta upgrade to 320a service"...
Did u do a load calc to come up with this?

You've never even installed an electrical panel, have you?

 

[wyliesdiesels]

You've never even installed an electrical panel, have you?

Really? Youre gonna pull out thr ad-hominems?

For your information, not that it matters, Ive been an electrician for 10yrs... Ive worked for several contractors including a pump company, electrical service company and currently a motorola service center as the in house electrician. Ive eh nevermind...none of this matters since it seems u want to start a ******* contest!

 

[ez-duzit]

You're actually serious. OK. Then you should know better, even though the OP claims only 1 garage will be used a time. What are you trying to save? 15' of wire?

 

[Steroblan]

My first shop has a 100amp feeder to it from the 200amp house panel. First shop feeds a second shop 85ft away fed by a 90 amp breaker with 1/0 ga. Installed by contractor and permitted.

 

[wyliesdiesels]

You're actually serious. OK. Then you should know better, even though the OP claims only 1 garage will be used a time. What are you trying to save? 15' of wire?

Now better than what? To recommend something thats unnecessary AND without even knowing the calculated load or the loads all together for that matter. Yes for that i do know better.

Looking back through the comments the OP never even stated what equipment he has or will be using. This shouldve been one of the first questions...for all we know the OP may just have a corded drill and a soldering iron...

So OP what equipmnet do u have? AC, electric water heater, space heater, compressor welder, etc??

 

[ez-duzit]

100-amp service in both garages=200 amps.

 

[wyliesdiesels]

Nope doesnt work that way. U dont add up breakers to come up with service size. If that were the case then u would have VERY large main services. U use calculated load.....

 

[ez-duzit]

It makes no difference if he plans on having only one 75-watt bulb on. Each 100-amp panel supply must safely carry its rated amps.

 

[Alchymist]

100-amp service in both garages=200 amps.

It makes no difference if he plans on having only one 75-watt bulb on. Each 100-amp panel supply must safely carry its rated amps.

Not true again.

While the OP has a 60 amp feed (stated above), let's assume for a minute it is really a 100 amp feed, with adequate wire size feeding the first garage, on a 100 amp breaker in the main panel. If he then runs adequately sized wire to a second garage and installs a second 100 amp sub panel (assuming he can find a 100 amp breaker to fit the 1st sub panel) he still has a 100 amp service connecting both garages. He can draw power in any combination between the two garages UP TO 100 AMPS. If he exceeds 100 amps between the two garages, the 100 amp breaker in the main panel will trip. Actually it's a race between the main breaker and the 100 amp main breaker in the 1st sub panel.

 

[CoopVA]

Not true again.

While the OP has a 60 amp feed (stated above), let's assume for a minute it is really a 100 amp feed, with adequate wire size feeding the first garage, on a 100 amp breaker in the main panel. If he then runs adequately sized wire to a second garage and installs a second 100 amp sub panel (assuming he can find a 100 amp breaker to fit the 1st sub panel) he still has a 100 amp service connecting both garages. He can draw power in any combination between the two garages UP TO 100 AMPS. If he exceeds 100 amps between the two garages, the 100 amp breaker in the main panel will trip. Actually it's a race between the main breaker and the 100 amp main breaker in the 1st sub panel.

^^^This^^^


Sent from my iPad using Tapatalk

 

[CoopVA]

Nope doesnt work that way. U dont add up breakers to come up with service size. If that were the case then u would have VERY large main services. U use calculated load.....

And ^^^This^^^


Sent from my iPad using Tapatalk

 

[Rock knocker]

It makes no difference if he plans on having only one 75-watt bulb on. Each 100-amp panel supply must safely carry its rated amps.

It's time for you to be quite now. Baring a load calculation and a situation where a panel(s) are purposely and often put in a over-loaded condition, there is no limit to the number of circuits and gross amperage of these circuits on a panel and it's subs.

 

[pattenp]

100-amp service in both garages=200 amps.

Easy does it ez-duzit. Your lack of electrical understanding is really starting to show through.

 

[ez-duzit]

...it's a race between the main breaker and the 100 amp main breaker in the 1st sub panel.

This is why both sub-panels should be run from their own 100-amp main panel breaker.

 

[Rock knocker]

"Should" is different from "needs", "has to" or "must". There is no danger or code violation in running things as the OP has stated.

 

[wyliesdiesels]

O brother. Some people dont know when to quit!

 

[ez-duzit]

****

 

[Alchymist]

This is why both sub-panels should be run from their own 100-amp main panel breaker.

No, it only means that whichever breaker is the more sensitive to the trip point will trip first. Two identically rated breakers will have a small difference in their trip points. Doesn't mean that the OP needs separate feeds.

If the 60 amp service he has in place is enough to handle the equipment needs for both garages, it's foolish to way over kill with a new 400 amp service. If he needs more than the 60 amps, his next step would be to see if an upgrade from the main panel to 100 amps to the 1st garage would handle his needs, and would be the least expensive upgrade. And very few garages need more than 100 amp service. Multi employee shops, yes, single man - not usually.