100A subpanel installation questions

DerStig · Feb 19, 2020

Hello forum,

I am in the process of installing a 100A subpanel to my 200A main panel. Here is my plan followed by my questions:

- The subpanel is to serve the following purposes: 20A breaker for grizzly table saw. 50A breaker for quincy 5HP compressor. 20A breaker for master blaster air dryer. 20A breaker for washer/dryer in laundry room. 20A breaker for garage doors and lights and some outlets. 20A breaker for bendpak HD-9 lift.

- The run from main panel to the subpanel will be 90% in unfinished basement ceiling. Rest in finished garage.

- The total distance is less than 50 feet.

- Speaking with the inspector of my town, there are no special requirements. They follow NEC 2017. Whatever the book says is what is needed.

Questions:

- I would like to use copper wire because I dont want to deal with AL. With that in mind, what type of wire/size do you recommend?

- What size of PVC conduit is needed? Yes I know I can bore holes in joists and run SER. But i want to have a clean/professional (and safe) installation.

- I have plans to build a detached garage later this year. The detached garage will have 2 lifts, another compressor. It will be used mainly for car stuff whereas this current attached garage as a workshop/woodworking. Common sense might say dont do the subpanel now, do it to the detached garage and move all your stuff there but I simply cannot have woodwork and expensive sports cars under the same roof. While some of the things in the current attached garage can move to the new garage (master blaster for one), some needs to stay (compressor, table saw, etc). So the question then is could I then run another 100A subpanel from this subpanel to the new garage OR do I need to run it from main panel?

This plan calls for 2 compressors in my house. There is good reason for this which is my sprinkler lines and their drain locations to blow out the water during winterizing being in the basement of the house. And the distance from where the detached garage will be to the sprinkler lines is in excess of 100ft making it very difficult to have only 1 compressor. Point being, I need 2 compressors. One in workshop/house/current garage and another in the detached garage, thus having 2x50A breakers in the system (one in each subpanel).

Your help is appreciated.

theoldwizard1 · Feb 19, 2020

First the size on the "main" breaker in a sub-panel real does not matter. That breaker is just a convenient switch to kill all power to the rest of the breakers in the panel (electricians call it a "disconnect"). What matters is the size of the breaker in the "main panel" and the size of the wire. Install a 200A sub if you need a lot of breakers !

Second, Your 5 HP compressor does not need 50A. Likely 30A, but I can not find a definitive document at the moment. Oversizing the breaker and wire to the compressor is NOT a good idea.

Yes, you can run a sub from a sub. Assuming they each have the same "main" breaker, the maximum amount power you can draw is limited by the breaker in the "main panel".

DerStig · Feb 19, 2020

theoldwizard1 said:
First the size on the "main" breaker in a sub-panel real does not matter. That breaker is just a convenient switch to kill all power to the rest of the breakers in the panel (electricians call it a "disconnect"). What matters is the size of the breaker in the "main panel" and the size of the wire. Install a 200A sub if you need a lot of breakers !

Second, Your 5 HP compressor does not need 50A. Likely 30A, but I can not find a definitive document at the moment. Oversizing the breaker and wire to the compressor is NOT a good idea.

Yes, you can run a sub from a sub. Assuming they each have the same "main" breaker, the maximum amount power you can draw is limited by the breaker in the "main panel".

Thank you.

The quincy compressor which sells for $1299 that is 5hp states in their product page “breaker recommended 50A”.

But even if its 30A, you can see I have quite a lot going on in the current garage. It is quite likely washer/dryer running while table saw or compressor or both running easily getting close to 60A constant load.

sberry · Feb 19, 2020

This is a how to thread vs what should I do.

DerStig · Feb 19, 2020

sberry said:
This is a how to thread vs what should I do.

I’m sorry I don’t understand what you mean.

pattenp · Feb 19, 2020

What do you mean "Deal" with AL wire? AL is not a problem. Also boring holes to run SER is not an unprofessional way to install. Personally it seems AL or CU SER would be the best way to go and not mess with conduit.

DerStig · Feb 19, 2020

pattenp said:
What do you mean "Deal" with AL wire? AL is not a problem. Also boring holes to run SER is not an unprofessional way to install. Personally it seems AL or CU SER would be the best way to go and not mess with conduit.

Deal with in a sense I dont want to worry about OX paste or making sure panel/breakers on each end accept AL wire.

I would have to bore 1 1/2” diameter holes in 19 joists total, some of them show cracks in center where the whole would be bored. There are also water pipes in the way running parallel to the joists. I just think conduit looks cleaner.

I have rewired my entire basement (7 circuits) using EMT. I replaced over 250 ft of romex with THHN in 3/4” EMT. Now I have literally 2 EMT lines running across the basement vs lots and lots of romex that appear amateurish. Yes, it is code compliant but it doesnt look as good. I’d like to keep the same type of design with this sub panel as well.

mike93lx · Feb 19, 2020

pattenp said:
What do you mean "Deal" with AL wire? AL is not a problem. Also boring holes to run SER is not an unprofessional way to install. Personally it seems AL or CU SER would be the best way to go and not mess with conduit.

Kevin Essiambre · Feb 19, 2020

DerStig said:
Deal with in a sense I dont want to worry about OX paste or making sure panel/breakers on each end accept AL wire.

The cost savings by using aluminum is worth having to deal with aluminium.

Every single panel and circuit breaker I have come across is dual rated for CU/AL. Of course, it IS possible to buy something that isn't rated for aluminium, but at that point I would question why you're ordering weird things...

I'm not even going to say anything about the EMT for your general circuits.

Sent from my new phone. Autocorrect may have changed stuff.

BB Sig · Feb 19, 2020

2 AWG Copper for 50' run.
1 AWG AL for 50' run.

I am going through something similar right now. For cost, I'm going AL. The fear of Aluminum is from when the aluminum wiring was made of a different alloy. The new AL alloy is okay to use and saves a lot of money.

My first experience with AL wiring was in a house built in the late 1890's. That wire got replaced. Fabric insulation and bad alloy...

Norcal · Feb 19, 2020

BB Sig said:
2 AWG Copper for 50' run.
1 AWG AL for 50' run.

I am going through something similar right now. For cost, I'm going AL. The fear of Aluminum is from when the aluminum wiring was made of a different alloy. The new AL alloy is okay to use and saves a lot of money.

My first experience with AL wiring was in a house built in the late 1890's. That wire got replaced. Fabric insulation and bad alloy...

Only need 3 AWG for copper.

dcg9381 · Feb 19, 2020

Norcal said:
Only need 3 AWG for copper.

Here's a chart:
https://www.cerrowire.com/products/resources/tables-calculators/ampacity-charts/

Note, best to use a volt drop calculator, but 50 feet isn't very far.

I too, use copper for places where I just don't want to deal with the bulk of aluminum.

I think you'll get by with 100A just fine - your drier appears to be gas (it's 20A, not 30A @ 240v)

pattenp · Feb 19, 2020

As Nocal said, #3 Cu for 100A. Use 75C on the chart.

tlmartin84 · Feb 19, 2020

Is a "breaker" required in the main 200 amp panel?

Or can you come off of a set lugs and feed to the disconnect breaker in the 100 amp sub panel?

And lastly, when he does his detached shop, will he be required to use 2 ground rods? Or is one rod allowable on a sub panel?

He will be required to run a 4 wire system, the ground can be smaller gauge as well, correct?

mike93lx · Feb 19, 2020

tlmartin84 said:
Is a "breaker" required in the main 200 amp panel?

Or can you come off of a set lugs and feed to the disconnect breaker in the 100 amp sub panel?

And lastly, when he does his detached shop, will he be required to use 2 ground rods? Or is one rod allowable on a sub panel?

He will be required to run a 4 wire system, the ground can be smaller gauge as well, correct?

You need to protect the wire feeding the sub from overload. Unless you put in 200a rated wire, you need a breaker for it.

2 rods unless you have the testing equipment to prove low enough resistance with one. A second rod is far cheaper than that.

theoldwizard1 · Feb 19, 2020

tlmartin84 said:
Is a "breaker" required in the main 200 amp panel?

Or can you come off of a set lugs and feed to the disconnect breaker in the 100 amp sub panel?

A breaker is required, at least 99% of the time. Some places MIGHT allow you to double tap the meter, BUT that is not common and certainly not for for a wire run of anything more than a few feet.

The typical thing to do is replace the meter pan with a combination meter pan load center (breaker panel) that has 4-8 circuits.

BB Sig · Feb 19, 2020

Norcal said:
Only need 3 AWG for copper.

I've never seen 3 AWG available in Lowe's or HD. You can special order it but I'd rather just pick it up.

Norcal · Feb 19, 2020

BB Sig said:
I've never seen 3 AWG available in Lowe's or HD. You can special order it but I'd rather just pick it up.

Not going to find much of anything at those places.

checkthisout · Feb 19, 2020

theoldwizard1 said:
A breaker is required, at least 99% of the time. Some places MIGHT allow you to double tap the meter, BUT that is not common and certainly not for for a wire run of anything more than a few feet.

The typical thing to do is replace the meter pan with a combination meter pan load center (breaker panel) that has 4-8 circuits.

With a double tapped meter, you would still need a disconnect for the feeder so that all the disconnects for the property are "grouped" correct?

tlmartin84 · Feb 19, 2020

Noone has answered the OP's question about conduit size?

Also, what about running the SER wire in conduit? No reason that can't be done is there? (Other than it being a PITA)

alfredeneuman · Feb 19, 2020

BB Sig said:
My first experience with AL wiring was in a house built in the late 1890's.

If it was built in the 1890's in all likelihood is would have been tin plated copper wire because the bare copper would cause the natural rubber insulation to deteriorate.
It's commonly mistaken for aluminum.

DerStig · Feb 19, 2020

tlmartin84 said:
Noone has answered the OP's question about conduit size?

Also, what about running the SER wire in conduit? No reason that can't be done is there? (Other than it being a PITA)

Yes and on top of that, I am seeing different wire sizes. Some say #3 CU some say #3 AL. Which is it?

Also, I believe SER isnt allowed in conduit anymore. Whether its underground or above is irrelevant. You need either individual THHN wire or other jacketed cable.

Lastly, depending on the size you are mentioning, what size of conduit? 2” PVC is huge, I really would love if I can use 1 1/2”.

Let me put this here without coming across in the wrong way, cost is not a concern to some extent. I am an engineer by trade (and by passion). I really do not like plastic jacketed yellow stuff all over my ceiling hence I replaced it with EMT and it looks much cleaner. Same reason, as much as code says its OK, I feel uneasy routing 100A cable through 19 joists and some more run all in open ceiling. As I also said, there are water pipes in some parts of the ceiling that will create awkward turns of this very big SER cable (one of the lines is a 1 1/4” copper sprinkler line).

And another important point is where the panel will be and how the 100A cable will enter will be from the bottom of the panel which will mean an inspector can deem that cable “subject to damage” hence requiring it to be in conduit. So some part of it will have to be in conduit anyways.

alfredeneuman · Feb 19, 2020

DerStig said:
Some say #3 CU some say #3 AL. Which is it?

Also, I believe SER isnt allowed in conduit anymore. Whether its underground or above is irrelevant.

#3 Cu

SER is allowed in conduit .
It just can't be used undergound

pattenp · Feb 19, 2020

If you want a conduit system forget the SER and use individual wires of THHN/THWN. For 100A use #3 Cu for the conductors and a #8 Cu for the EGC. You can use 1.25" EMT or PVC.

Edit: If you can't locate those sizes go with #2Cu and a #6Cu EGC. The 1.25" conduit will still be okay.

DerStig · Feb 20, 2020

pattenp said:
If you want a conduit system forget the SER and use individual wires of THHN/THWN. For 100A use #3 Cu for the conductors and a #8 Cu for the EGC. You can use 1.25" EMT or PVC.

Edit: If you can't locate those sizes go with #2Cu and a #6Cu EGC. The 1.25" conduit will still be okay.

Thank you.

Shouldnt EGC be #5 for a #3 cu current carrying conductor? Or #4 for #2 CCC?

Also, can you point me to the part of NEC that discusses:

- Size of wires
- Size of conduit for wires

Thanks again

mike93lx · Feb 20, 2020

Plenty of tables are available online for wire and conduit sizing

DerStig · Feb 20, 2020

mike93lx said:
Plenty of tables are available online for wire and conduit sizing

Thank you but I dont know what temperatures to use for example for this chart:

http://wiresizecalculator.net/wiresizechart.htm

How do I choose?

DerStig · Feb 20, 2020

I am trying to use this calculator: http://wiresizecalculator.net/#calc_goto

I am probably doing something wrong, can you check my inputs?

- Voltage 220
- Amps 100
- Single phase
- 60 C insulation (I thought you use the lowest value)
- copper
- raceway
- 1% voltage drop
- 50 feet distance

It tells me 2/0 wire which is much bigger than 2-2-2-4 or 3-3-3-5.

BB Sig · Feb 20, 2020

3% voltage drop

Barry
North Florida

mike93lx · Feb 20, 2020

DerStig said:
Thank you but I dont know what temperatures to use for example for this chart:

http://wiresizecalculator.net/wiresizechart.htm

How do I choose?

Depends on the wire, as is indicated right in the table you linked

mike93lx · Feb 20, 2020

DerStig said:
I am trying to use this calculator: http://wiresizecalculator.net/#calc_goto

I am probably doing something wrong, can you check my inputs?

- Voltage 220
- Amps 100
- Single phase
- 60 C insulation (I thought you use the lowest value)
- copper
- raceway
- 1% voltage drop
- 50 feet distance

It tells me 2/0 wire which is much bigger than 2-2-2-4 or 3-3-3-5.

Your voltage and drop are both wrong.

This may not be DIY for you

DerStig · Feb 20, 2020

mike93lx said:
Your voltage and drop are both wrong.

This may not be DIY for you

What are the right values?

There is a first for everything, but thank you for your insight.

DerStig · Feb 20, 2020

BB Sig said:
3% voltage drop

Barry
North Florida

If I use 3%, its saying even bigger wire.

alfredeneuman · Feb 20, 2020

A 100 amp load with a length of 49' (since you said less than 50') @ 240 Volts shows a VD of 1.97% with #3 copper.

Using the same figures with #1 aluminum, the figure is 1.91%

DerStig · Feb 20, 2020

alfredeneuman said:
A 100 amp load with a length of 49' (since you said less than 50') @ 240 Volts shows a VD of 1.97% with #3 copper.

But thats 90C rated wire. My research tells me one should go for lowest temp rating to be safe, 75C gives #2 and 60C gives #1.

pattenp · Feb 20, 2020

NEC table 250.122, EGC for 61A-100A is #8Cu or #6AL, For 101A-200A is #6Cu or #4Al.
Conduit tables C.1 EMT or C.9 PVC in back of book.
Use 75C for sizing individual conductors.
At a full load of 100A, VD is of no issue using #3 CU until you get up to 150ft or more.

DerStig · Feb 20, 2020

pattenp said:
NEC table 250.122, EGC for 61A-100A is #8Cu or #6AL, For 101A-200A is #6Cu or #4Al.
Conduit tables C.1 EMT or C.9 PVC in back of book.
Use 75C for sizing individual conductors.

Thank you.

Its interesting everything I read indicates EGC being 2 gauges lower than CCCs, which means #2 would go with #4 etc, but your reference states otherwise.

pattenp · Feb 20, 2020

DerStig said:
Thank you.

Its interesting everything I read indicates EGC being 2 gauges lower than CCCs, which means #2 would go with #4 etc, but your reference states otherwise.

The NEC is minimum. Nothing says the EGC can't be bigger. You can run amuck of the NEC when over sizing conductors and not increasing the EGC proportionately above what table 250.122 shows. So technically going from a #3Cu to a #2Cu for 100A may require a #6EGC instead of a #8EGC. NEC 250.122(B)

alfredeneuman · Feb 20, 2020

DerStig said:
But thats 90C rated wire. My research tells me one should go for lowest temp rating to be safe, 75C gives #2 and 60C gives #1.

You're on your own then. Good luck!

wyliesdiesels · Feb 20, 2020

Wow this is being made way harder than it has to be.

#3 cu is good for 100a at 49’ with 75* c temp terminations

Use a #8 cu for the EGC.

Voltage drop is not a concern at this length.

The correct values are 3% for feeder and 5% for beanch circuit

At 49’ i wouldnt even bother doing voltage drop calcs

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