Adding subpanel. Tell me if this is OK.

[Gerald O]

I'm planning what I'll need to do for feeding a 150A subpanel in my garage that is soon to be built. This will be an additional attached garage. The house has a 200A main distribution panel that is full. I want to feed the new sub with 2/0 AL wire from a 150A subfeed lug kit that can plug into the existing panel.

All the ampacity charts that I've seen indicate that the 2/0 AL wire should suffice for 150A, but they don't list distance. My one-way distance will be about 60' give or take depending on routing. Will the 2/0 wire do it?

In the main panel, is it code-legal to move some of the existing 20A circuits that are on full-size breakers over to the thin profile breakers that double-up in a slot in order to free enough space for the subfeed adapter, or will I have to upgrade to a new panel with more slots?

Question on grounding the sub-panel: I can plan to put a UFER in the new garage footings. Should I ground the sub to that? If so, do I still need a 4th wire GEC from the main connected to the sub ground?

Thanks

 

[Zeke]

It's not very polite to tell a newbie to search the forum, but... there are numerous threads on sub panels, bonding, grounding, feeder size, etc. In fact, it may be the most common theme on the Lighting and Electrical forum.

 

[Gerald O]

It's not very polite to tell a newbie to search the forum, but... there are numerous threads on sub panels, bonding, grounding, feeder size, etc. In fact, it may be the most common theme on the Lighting and Electrical forum.

I know how to do a search. I've already searched. I've read dozens of thread on similar issues but did not find answers to the questions I asked.
Perhaps my questions would be better addressed to a code/inspection forum rather than a hobbyist forum.

 

[Alchymist]

I'm planning what I'll need to do for feeding a 150A subpanel in my garage that is soon to be built. This will be an additional attached garage. The house has a 200A main distribution panel that is full. I want to feed the new sub with 2/0 AL wire from a 150A subfeed lug kit that can plug into the existing panel.

All the ampacity charts that I've seen indicate that the 2/0 AL wire should suffice for 150A, but they don't list distance. My one-way distance will be about 60' give or take depending on routing. Will the 2/0 wire do it?

In the main panel, is it code-legal to move some of the existing 20A circuits that are on full-size breakers over to the thin profile breakers that double-up in a slot in order to free enough space for the subfeed adapter, or will I have to upgrade to a new panel with more slots?

Depends on the panel - some are listed for dual breakers, some are not. Check the panel.

Question on grounding the sub-panel: I can plan to put a UFER in the new garage footings. Should I ground the sub to that? If so, do I still need a 4th wire GEC from the main connected to the sub ground?

Yes, use the UFER - and yes, you still need the 4th wire ground.
Thanks

doihjiosdgjh

 

[Gerald O]

...

Thanks. Most of the discussion I've seen has been about detached garages. Didn't know if the fact that it is attached makes a difference with respect to needing a separate earth ground. The panel already has some of the thin breakers. Most are 15A, one pair is 20A. So I know it will hold them. Just wondering if they are legal still. Also whether there is a limit on how many I can use. I'm worried that the panel may be considered too full to add the subfeed.

I've identified another panel with more slots than this one by the same maker, Siemens, that will swap right in, and it's not that expensive. But I'd rather avoid the added cost and work if its not required.

 

[aandpdan]

In the main panel, is it code-legal to move some of the existing 20A circuits that are on full-size breakers over to the thin profile breakers that double-up in a slot in order to free enough space for the subfeed adapter, or will I have to upgrade to a new panel with more slots?

It depends upon your existing panel. How many circuits can it handle? You can't always use tandem breakers.

Question on grounding the sub-panel: I can plan to put a UFER in the new garage footings. Should I ground the sub to that? If so, do I still need a 4th wire GEC from the main connected to the sub ground?

You will need a 4th wire to your subpanel. You do NOT need UFER or any extra ground rods if it is an "attached" garage.

 

[Alchymist]

It depends upon your existing panel. How many circuits can it handle? You can't always use tandem breakers.



You will need a 4th wire to your subpanel. You do NOT need UFER or any extra ground rods if it is an "attached" garage.

While he doesn't "need" the ufer, as long as he's pouring footers, it's but a moment's work to run the ground to the re-rod. What's not to like?

And as I said about the tandem breakers - check the panel - it will specify where and how many can be used.

 

[theoldwizard1]

In the main panel, is it code-legal to move some of the existing 20A circuits that are on full-size breakers over to the thin profile breakers that double-up in a slot in order to free enough space for the subfeed adapter, or will I have to upgrade to a new panel with more slots?

Yes, tandem breakers are legal.

 

[Gerald O]

It depends upon your existing panel. How many circuits can it handle? You can't always use tandem breakers.

Good question. The panel is a 30/40 type--30 slots 40 circuits. "Slots" are obvious, but what defines a "circuit" for this purpose? For example, is a double pole 240 taking up two slots counted as a single circuit?

You will need a 4th wire to your subpanel. You do NOT need UFER or any extra ground rods if it is an "attached" garage.

Makes it easier for me I guess. Seems to be a somewhat arbitrary distinction when it comes to electricity. Perhaps distance from the main would be a more rational criteria.

 

[aandpdan]

While he doesn't "need" the ufer, as long as he's pouring footers, it's but a moment's work to run the ground to the re-rod. What's not to like?

Code does not require it, or any additional ground rods be installed.

He can certainly put it in but there is no need to.

 

[Gerald O]

Guess my only issue with the ufer is that I'll need to call for an electrical inspection (on top of the building inspection) before placing the concrete. Otherwise I'd consider it a no brainer.

 

[Gerald O]

Wire size seems to be more of a mystery. Are calculations on actual loads (continuous/non-continous), distance, temperature, blah, blah, going to be asked for by inspections or do they just consider the size of the breaker and the wire and be done with it? If I can get away with 2/0 AL wire it will make things much easier. Not even sure that I need 150A service.

Let's see: HVAC, air compressor, water heater, abundant lighting, and vehicle lift all running at the same time seems to be a realistic scenario.

 

[6speed]

...
All the ampacity charts that I've seen indicate that the 2/0 AL wire should suffice for 150A...

This is true only under the 90*C column. However most if not all conductor terminations and connectors are rated at a lower temperature.

 

[aandpdan]

It would be a good idea to discuss this with your AHJ (wiring inspector) before you begin. Most will help you out some.

Take the time to do a load calculation. It's not hard, and is much easier than "guesstimating." Leave yourself some overhead.

Remember too, the lift and air compressor are not on all the time. Is the water heater to be on the subpanel or is it already existing?

What type of wire do you want to run?

 

[pattenp]

Just an FYI.. 2/0 Al is good for 135 amps @ 75C degrees. For 150A you need 3/0 Al. 1/0 Cu is good for 150A.

 

[Gerald O]

Take the time to do a load calculation. It's not hard, and is much easier than "guesstimating." Leave yourself some overhead.

The hard part is the uncertainty of not actually having any of the actual equipment picked out yet. Guestimating is my only option, which tends toward overbuilding for it.

...Remember too, the lift and air compressor are not on all the time. Is the water heater to be on the subpanel or is it already existing?

My concern there is for the sizing of the OCP. Don't want to be tripping the breaker under reasonable use scenarios. I could go with a 125A breaker for the subfeed, but I'm thinking that might be on the margin. It will be a second smallish water heater just for the garage to supply a single bathroom with shower.

What type of wire do you want to run?

Whatever is the least costly, easiest to work with, legal, and safe.
It will probably be routed through the crawlspace of the house.

 

[pattenp]

Aluminum SER will be the cheapest. Easy to work with doesn't compute when getting into wire sizes of 2/0 and larger.

........
Whatever is the least costly, easiest to work with, legal, and safe.
It will probably be routed through the crawlspace of the house.

*

 

[aandpdan]

If you really think you'll be anywhere near maxing out the 150 amp subpanel, I'd be looking at your service.

Besides HVAC, do you have electric hot water, stove, or dryer?

SER type cable would be great.

 

[Gerald O]

I'm pretty sure that the only fairly continuous loads will be the HVAC (all electric), hot water, and lighting. That shouldn't amount to a lot. Just want to size for those occasional situations when all that is going plus lift and compressor. My back of napkin figures have that all coming to around 90 amps based on a quick sampling of manufacturers specs. I figured 150 amps would leave sufficient head room. 125 might even be enough. I'll have other power hungry tools and equipment, but it's not likely that they'd be in use at the same time since not more than two guys are likely to be working in the shop at once.

No stove, but maybe a microwave.

 

[Gerald O]

Aluminum SER will be the cheapest. Easy to work with doesn't compute when getting into wire sizes of 2/0 and larger.







*

The SER 3/0 cabling seems to be less readily available than 2/0 or 4/0.

 

[Higgins]

We installed a 125 Amp service when we built our 2nd garage. I don't remember what the gauge size was,000, or 0000 as I got a deal on it and that was over 12 yrs ago and I flat *** don't remember. However, we did up-size the wire to handle a 200 amp panel if we needed it!

We also ran the AL cable in 2 1/2" (maybe 3 ") conduit as our soil is very rocky and prone to nicking the burial cable and causing issues over time. Not only that, had to cross the septic field feed!. ComEd loved the installation and the local inspectors was happy with everything being in conduit! Wish we had installed condiut when ComEd replaced the feed to our house, but that just water under the bridge! That's another story!!

AL

 

[Gerald O]

Ok, I got some useful replies in this thread. I realize that the opinions and info here might or might not be relied on 100% but several useful things were pointed out:

1) Nobody said "no, your plan totally won't work because..."
2) The temperature derating of the conductor ampacity for 75 deg connections was something I was not aware of.
3) Confirmation that 2/0 AL is too small for 150A service.
4) Grounding to both ufer at the garage together with the required GEC from the main is OK and recommended.

This gives me something to go on when discussing the specs with the contractors and getting quotes.

Thanks to all who took the time to help.
-Gerald