Installing a subpanel in attached garage - Firewall Concerns

[Greg S]

So I am installing a 100A subpanel in the garage to handle a plethora of power equipment including a couple of welders and a plasma cutter. This will be #4 copper 4-conductor cable run from the main panel in the basement (approx 40 foot run).

I have laid out my plan for outlet locations and wire routes, as well as lighting fixtures, but I am running into a question that is difficult to get an answer for. If anyone can help, I would appreciate it.

I wish to install a surface mount subpanel on the wall that separates my attached garage from my house. I know that it is important to maintain the integrity of this 'modified 1-hr firewall'. That being said, my house seems to be a little unique in a couple of respects so I want to throw this out there to anyone who might have some insight.

The wall separating my garage from my house is, as far as I know, up to code (with the exception of a rather large hole the previous homeowners drilled in it for a dryer vent-- which I am planning on repairing). My basement wall (standard concrete block foundation) makes up the lower half of this wall... To better explain, the wall that separates my house from my garage is concrete block up to waist level, and up-to-code 1-hr modified firewall from waist level on up to the roof.

I have pored over the NEC handbook and can not find any reference to whether or not what I want to do is 'legal' and I want to have some guidance before I submit my plans to the electrical inspector. The only reference to any of this I found was in building code, and it was vague at best (subpanels shall not penetrate 1hr wall). So, now that you know the background info, here are my string of questions:

1. Is it 'up-to-code' to install a surface mount subpanel on the wall separating my attached garage from my house?

1. Knowing that it is not allowed to penetrate the firewall itself, is it up to code to run the feeder cable for the panel through the waist-level concrete block wall, or is that also considered part of the modified 1hr wall?

2. If 1 and 2 are OK, is it ok to run the feeder cable up along the side and then through the top of the surface mounted box to attach to the main lugs?

1. If I am going down the wrong track completely, can someone please tell me a best-practices method for running feeder cable through or around a firewall from a basement to a garage?

Any help or insight anyone can provide would be greatly appreciated.

[Eric Tonks]

I think you will find that your answers are not really NEC electrically based, but are based in the building code regarding firewalls.

A true firewall should not be penetrated with anything that is flammable. You would not want to recess the panel into the firewall, I don't think mounting it on the surface would be a problem, but that will be decided by your local code, speak to your building inspector's department. Your feeder line probably should be encased in rigid conduit through the firewall and into the panel. Any space around the conduit should be mortared to seal it if it penetrates the masonry, and filled with drywall compound if it goes through a drywall firewall. Again your local building code will rule.

Yes the masonry foundation wall is a firewall and should be treated the same. There are special fire seals that I have used to protect plastic vacuum lines that penetrated a public gathering building firewall. You install one on each side, it is filled with a soft pliable material that expands with heat to seal the opening. 3m Company makes them. Again your local building and electrical code takes precedent.

[Dan]

Straight from the International Association of Electrical Inspectors. They permit the installation of panel boxes in this wall with addition of a second layer of rock behind the panel. It is also permitted to surface mount a panel, if you prefer. This really isn't a fire-wall, the code only requires a 1/2" layer of gypsum board on the garage side of the wall, if it's on the same floor. occupied space above the garage is a little different. That said, your local inspector may have dreamed up something different, but he's flying solo if he did.

Dan

[KJS]

On Wed, 6 Oct 2004 18:19:38 -0700, Greg S wrote (in article ):

Greg -

The NEC has very limited guidance on this. Section 300-21 (1999 NEC) basically says that penetrations in fire-rated walls shall be firestopped to maintain the rating. IIRC, there are more specifics in other building codes, but I can't give you chapter and verse.

From my practical experience, you may run cable or conduit through a 1-hour rated wall (either the stud portion or the block - they're both part of the firewall), but the hole must be sealed with a product that will expand when heated, effectively closing the penetration. The stuff is called "putty pads" in the trade, and I think it's properly called "intumescent putty" (whatever that means).

Take a look at the products from

Good luck,

- Kenneth

[operator jay]

If you can access alt.engineering.electrical you could try crossposting there too.

j

[John Grabowski]

See my responses mixed in below:

According to table 310.16 under the 75 degree column #4 wire is only rated for 85 amps. It is only permissable to use #4 for a 100 amp service, not for a subpanel feed. You need to use #3 conductors. That is assuming that your conductors and terminals are rated for 75 degrees Celsius. If not, then you need to use #2 sized conductors.

I've done it a few times according to what the fire inspector will permit. Sometimes I've had to add an additional piece of drywall behind the panel. Sometimes they will accept a piece of plywood as an addition to the firewall which also facilitates the mounting of the panel and circuit wiring coming out from it.

You should protect the cable with a conduit sleeve. You can use PVC, but it may not be rated for the firewall. You can use metal conduit, but it will need to be grounded.

Yes, but it is neater to enter the panel from the bottom and run the individual conductors inside to the top. You can also turn the panel upside down and have the lugs on the bottom on some panels approved for bottom feed.

Since you will be submitting plans to the electrical inspector and getting a permit, you should consult with him and the fire inspector on this firewall issue. They will appreciate it more if you ask them before you start the job instead of just doing it and hoping for the best.

It sound as though you are off to a good start and are on the right track.

John Grabowski

[Greg S]

Thanks everyone for your advice, I really appreciate it. The advice given has made my planning a lot easier. I do have one question however... Everything I have read suggests that as long as the feeder cables for the subpanel come off of a main panel breaker that it is acceptable to size the feeders according to service entry guidelines for 100A and above... I will definitely check with the inspector before I buy/run the cable, but any input anyone has on this would be appreciated... I was under the impression that SER 4-4-4-6 would be the wire of choice for a 100A subpanel.

[John Grabowski]

That is certainly true for the MAIN power feeder, but you are installing a branch circuit to feed a subpanel. Instead of fooling around in newsgroups and how-to books you should just ask the person who will approve or disapprove of your choice; The electrical inspector. You should also check the panel where the subpanel feed will be coming from to see if a 100 amp branch circuit is allowed. It should be stated on the label either on the cover or inside the panel (Something like: max. branch circuit...). The inspector is likely to look for this.

Do you really need 80 amps @220 volts continuous load for your garage? If not, install the #4 cable and put a 70 amp circuit breaker on it.

John Grabowski

[Greg S]

I did check with the electrical inspector and you were right indeed. The subpanel feeder cables are size according to 310-16. As for max branch circuit capability of my panel-- that is 140A.

It certainly doesn't seem so, but I have a welding unit that draws 50A at max rated output. Most of the time when I am using it it will be drawing around 30A or so... My plasma cutter takes about 40A at max, and I have a 120 Gal 2 stage compressor with an inductive motor. So with the cutter and the welder going at the same time (which can be a common occurance in my house) there exists the possibility of trippage if the compressor kicks on. When I had all of this equipment set up at my previous shop, each device had it's own breaker off the main panel.

50A for the welder, 50A for the cutter, 30A for the compressor, etc. I just wouldn't want to size it too small and then be kicking myself in the head for not going one size bigger and dealing with constant lights-out. The $$ difference in the breaker is $30 and the difference in the cable is $120. So for the $150 extra for SER 2-2-2-4 and a 100A breaker, it's cheap insurance. I am also thinking about adding another welder which would definitely put me over the edge if they were both running at their higher amperage settings. I am not an electrician, but it would seem to me that if I had two welders, operating even intermittantly at the same time (i.e. both drawing 50A for even 30 seconds) and then my inductive motor on my compressor kicked on, that I would have a branch trip, right?

Again, thanks for all the help. I really appreciate it.

[John Grabowski]

I wasn't sure if you were a hobbyist or a working professional. Either way it sounds as though you will need the extra power. Keep in mind when considering your load that circuit breakers are normally rated for 80% continuous load. They can go to 100% or more for short durations. Perhaps you may want to consider a 125 amp feed.

John Grabowski

[phil-news-nospam]

| I wasn't sure if you were a hobbyist or a working professional. Either way | it sounds as though you will need the extra power. Keep in mind when | considering your load that circuit breakers are normally rated for 80% | continuous load. They can go to 100% or more for short durations. Perhaps | you may want to consider a 125 amp feed.

So what happens if you run a breaker at say 99% for a loooong time. Will it trip before it dies? Or will it die (sparks, flames, and a big kaboom) before it trips?

[Greg]

The U/L white book (listing DIVQ) confirms John's assertion that breakers are designed to run continuosly at 80% or less "unless otherwise marked". They are tested at 6 times capacity though, so the failure on a 99% continous load should simply be a "trip" not fire and smoke.