Metal Garage Electrical Service

[Joshua5438]

I am building a 30x50 metal garage. I am having a new 200amp meter installed for service. I am searching and looking for ideas on how to mount both the meter socket and the interior breaker panel. The two panels will be back to back. The metal building siding will be running horizontal if that makes any difference. I am thinking of using Unistrut for the meter socket and possibly wood framing something on the inside for the breaker panel. Any suggestions or pictures would be great. Also, the panel will be connected to a ground rod but does the metal building need to be bonded in some way? If so, can anyone point me in the right direction on how this is done?

For reference I am using the Milbank 200 meter socket and a GE PowerMark 200amp 40 space load center.

 

[wwhitney]

As to the mounting, what style of metal garage is this, what is the framing material and pattern? If this is a kit or made to order, can you have it designed with some additional framing where the electrical service will be to facilitate mounting?

On the grounding electrode system, what is the foundation type for the building? If, for example, there's a new concrete slab with thickened edges, and the bottom of the thickened edge is directly in contact with the earth, then before /as the concrete is placed you need to install a concrete encased electrode (Ufer). Which would be sufficient for grounding electrodes, and there would be no need for a ground rod. If there is no footing directly in contact with the earth, then you could use 2 ground rodss, at least 6' apart; 1 ground rod is only sufficient if you do a ground resistance test and get lucky with a low reading, so it's easier to just install 2 rods.

As to bonding the metal building frame, yes, that is required. The size of the bonding jumper is based on the size of the service entrance conductors; for example, for SECs up to 3/0 Cu or 250 kcmil Al (the minimum size in either case for a 200A supply), a #4 AWG Cu bonding jumper is sufficient. As to how to connect it to the metal framing, I'm not so familiar with that; if it's a beam skeleton, you can look at the options for beam grounding clamps.

Cheers, Wayne

 

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[Joshua5438]

As to the mounting, what style of metal garage is this, what is the framing material and pattern? If this is a kit or made to order, can you have it designed with some additional framing where the electrical service will be to facilitate mounting?

On the grounding electrode system, what is the foundation type for the building? If, for example, there's a new concrete slab with thickened edges, and the bottom of the thickened edge is directly in contact with the earth, then before /as the concrete is placed you need to install a concrete encased electrode (Ufer). Which would be sufficient for grounding electrodes, and there would be no need for a ground rod. If there is no footing directly in contact with the earth, then you could use 2 ground rodss, at least 6' apart; 1 ground rod is only sufficient if you do a ground resistance test and get lucky with a low reading, so it's easier to just install 2 rods.

As to bonding the metal building frame, yes, that is required. The size of the bonding jumper is based on the size of the service entrance conductors; for example, for SECs up to 3/0 Cu or 250 kcmil Al (the minimum size in either case for a 200A supply), a #4 AWG Cu bonding jumper is sufficient. As to how to connect it to the metal framing, I'm not so familiar with that; if it's a beam skeleton, you can look at the options for beam grounding clamps.

Cheers, Wayne

It is a total metal structure. The framing will be 2.5" square tube. They do not offer any type of additional framing for meter location. I am thinking of using 2x2 wood or metal and frame an H between the interior studs. Then I can connect like you typically would in a wood framed structure.

They foundation will be a slab on grade, 5" thick. No thickened edges or additional footers. It will have a vapor barrier under 100% of the slab so not sure if that is considered "direct contact". I am planning for two ground rods at this point. I need to look the rod length requirements up as it's been a while. Good point on the 6' apart.

I have 4/0 wire available to connect the meter to the load center. Will that still require a #4 AWG copper wire?

I will keep researching on the building bonding. I am sure it's a simple connection but not finding much info.

 

[wwhitney]

No, with 5" thick everywhere, and 100% undercoverage with a vapor barrier, your slab won't be a Ufer. So you will need 2 ground rods. 8' in contact with the earth, meaning for an 8' ground rod, fully buried, including the grounding lug (listed for direct burial) and GEC.

(Edit) If you use 4/0 Cu, you will need #2 Cu for the building bond. 4/0 Al would be OK with #4 Cu bond. Not sure of the best way to bond to the tube, you could drill and tap and use a grounding lug, I think, not 100% sure.

Cheers, Wayne

 

[Joshua5438]

No, with 5" thick everywhere, and 100% undercoverage with a vapor barrier, your slab won't be a Ufer. So you will need 2 ground rods. 8' in contact with the earth, meaning for an 8' ground rod, fully buried, including the grounding lug (listed for direct burial) and GEC.

(Edit) If you use 4/0 Cu, you will need #2 Cu for the building bond. 4/0 Al would be OK with #4 Cu bond. Not sure of the best way to bond to the tube, you could drill and tap and use a grounding lug, I think, not 100% sure.

Cheers, Wayne

ok. So two 8' rods, 6' apart and fully buried. One continuous #4 Cu (I meant to say I am using 4/0 Al) from both rods to the load center.

I am thinking a bonding lug mounted on one of the wall posts would suffice but not totally sure either. This needs to connected to the neutral, ground in panel or ground rod? I am reading mixed opinions.

 

[wwhitney]

One thing to note about a 4/0 Al conductor: the 75C ampacity is 180A. That's not a standard breaker size, so you can protect it at 200A, the next largest size. But you still only have a 180A service. Which is fine if your load calc for the building is 180A or less. But if it is 180.5A to 200A, you need to use 250 kcmil Al instead (or 3/0 Cu).

As to the bonding, the service panel (enclosure containing the upstream-most OCPD on the service) is where EGCs originate. Upstream of there, there is no EGC, everything is bonded to the neutral (e.g. the meter can). Downstream of there neutral and grounds are always kept separate, and any bonding is done to the EGC.

In the service panel, neutrals and EGCs are commonly just interspersed. Best practice would be to have separate neutral and grounding bars, with the grounding bar mounted on the case, the neutral bar connected to the service neutral, and a main bonding jumper connecting the neutral bar to the case or grounding bar (often just a long screw that goes through the normal insulators the neutral bar is mounted on to connect to the case). The GEC (the unspliced #4 coming from the ground bars) is to be connected to the service neutral per NEC 250.24(A)(1), so it goes to the neutral bar. 250.140(C) says the #4 copper for your bond to the metal building frame can connect basically anywhere: to the service neutral bar, to the service grounding bar or enclosure, to the GEC itself, or to one of your ground rods.

Cheers, Wayne

 

[Joshua5438]

One thing to note about a 4/0 Al conductor: the 75C ampacity is 180A. That's not a standard breaker size, so you can protect it at 200A, the next largest size. But you still only have a 180A service. Which is fine if your load calc for the building is 180A or less. But if it is 180.5A to 200A, you need to use 250 kcmil Al instead (or 3/0 Cu).

As to the bonding, the service panel (enclosure containing the upstream-most OCPD on the service) is where EGCs originate. Upstream of there, there is no EGC, everything is bonded to the neutral (e.g. the meter can). Downstream of there neutral and grounds are always kept separate, and any bonding is done to the EGC.

In the service panel, neutrals and EGCs are commonly just interspersed. Best practice would be to have separate neutral and grounding bars, with the grounding bar mounted on the case, the neutral bar connected to the service neutral, and a main bonding jumper connecting the neutral bar to the case or grounding bar (often just a long screw that goes through the normal insulators the neutral bar is mounted on to connect to the case). The GEC (the unspliced #4 coming from the ground bars) is to be connected to the service neutral per NEC 250.24(A)(1), so it goes to the neutral bar. 250.140(C) says the #4 copper for your bond to the metal building frame can connect basically anywhere: to the service neutral bar, to the service grounding bar or enclosure, to the GEC itself, or to one of your ground rods.

Cheers, Wayne

Didnt realize that. I will look for the 250kcmil but I do not recall ever seeing it at the box stores. I may end up getting the 3/0 copper as I will only need about 10 total feet. I would rather wire it for 200amp and not need it than wish I had.

That all makes since for the ground and bonding. I keep thinking sub panel where ground and neutral are separate. My panel has two full length neutral bars but you recommend adding the ground bar and only connecting ground to that? Then add the screw to bond the two neutral bars to the case with the screw?

 

[wwhitney]

That all makes since for the ground and bonding. I keep thinking sub panel where ground and neutral are separate. My panel has two full length neutral bars but you recommend adding the ground bar and only connecting ground to that? Then add the screw to bond the two neutral bars to the case with the screw?

I'm 100% confident that would be compliant, and I'd probably do that because it seems neater. I'm 95% confident that if you don't have many circuits, you could just use one of the neutral bars for EGCs and one for neutrals. [But you'd still need the bonding screw to bond the case.] I'm not sure if mixing the neutrals and EGCs on the same bar is proper, but I know it's often done. [Note that just one neutral conductor is allowed per busbar hole, but if the panel listing allows, you could put two EGCs in one busbar hole. The label should mention that. But one EGC per hole seems neater.]

Cheers, Wayne

 

[wwhitney]

Didnt realize that. I will look for the 250kcmil but I do not recall ever seeing it at the box stores. I may end up getting the 3/0 copper as I will only need about 10 total feet. I would rather wire it for 200amp and not need it than wish I had.

If the building were a dwelling unit, then there's an 83% allowance, meaning that you'd only need 162A of conductors for a 200A service, so the 4/0 Al would be sufficient for a calculated load of 200A. So you will see alot of 200A residential services with 4/0 Al. But that 83% allowance doesn't apply to a garage, so the 4/0 Al is only good for 180A in your case.

Cheers, Wayne

 

[Joshua5438]

I'm 100% confident that would be compliant, and I'd probably do that because it seems neater. I'm 95% confident that if you don't have many circuits, you could just use one of the neutral bars for EGCs and one for neutrals. [But you'd still need the bonding screw to bond the case.] I'm not sure if mixing the neutrals and EGCs on the same bar is proper, but I know it's often done. [Note that just one neutral conductor is allowed per busbar hole, but if the panel listing allows, you could put two EGCs in one busbar hole. The label should mention that. But one EGC per hole seems neater.]

Cheers, Wayne

Thanks! Would I need all three feed wires to be 3/0 Cu or can the neutral be a smaller wire and if so, what size?

 

[wwhitney]

Thanks! Would I need all three feed wires to be 3/0 Cu or can the neutral be a smaller wire and if so, what size?

Yes, the neutral can be smaller, but as to how small, you need to a do a load calculation. The smallest size it can be is the same as the GEC, so #4 Cu or #2 Al if you your ungrounded conductors are up to 3/0 Cu or 250 kcmil Al.

If your load calculation ended up at 190A of load, say, of which 90A is attributable to 240V only loads, then you know that your neutral conductor won't carry more than 100A. So based on NEC Table 310.15(B)(16) using the 75C column, you could use #3 Cu or #1 Al.

BTW, if you only need 10' of conductors, there are plenty of places that will cut that wire to order and ship it to you, not sure if that's cost effective. Or find an electrical supply house instead of a big box store.

Cheers, Wayne

 

[Joshua5438]

Yes, the neutral can be smaller, but as to how small, you need to a do a load calculation. The smallest size it can be is the same as the GEC, so #4 Cu or #2 Al if you your ungrounded conductors are up to 3/0 Cu or 250 kcmil Al.

If your load calculation ended up at 190A of load, say, of which 90A is attributable to 240V only loads, then you know that your neutral conductor won't carry more than 100A. So based on NEC Table 310.15(B)(16) using the 75C column, you could use #3 Cu or #1 Al.

BTW, if you only need 10' of conductors, there are plenty of places that will cut that wire to order and ship it to you, not sure if that's cost effective. Or find an electrical supply house instead of a big box store.

Cheers, Wayne

Great to know! I know my house is (like you stated before) 200amp service feed with 4/0 Al and 2/0 Al for neutral. I was thinking that I would be safe to go with 3/0 Cu and 1/0 Cu for my neutral. I also do not want to be way over kill either. Just trying to make sure I am safe up to my max limit.

 

[wwhitney]

Just want to confirm that you are talking about a separate utility service drop for your metal garage, not a feeder from your house. Because a feeder from your house to the garage would have some different rules that would affect some of my comments. You mentioned a meter, so that should be a separate service, but it's good to be clear.

Cheers, Wayne

 

[Joshua5438]

Just want to confirm that you are talking about a separate utility service drop for your metal garage, not a feeder from your house. Because a feeder from your house to the garage would have some different rules that would affect some of my comments. You mentioned a meter, so that should be a separate service, but it's good to be clear.

Cheers, Wayne

You are correct. I was just giving the reference to the house as I notice it uses the two sizes down for the neutral. Sorry to confuse things. This is a totally separate meter and feed.

 

[Joshua5438]

I was just reading that a code change in 2017 made a rule about the neutral needing to carry 83% of the load. So 83% of 200A is 166A, which I believe would need a 2/0 Cu? This could be hear say as I am not totally up to code on service feeds.

 

[wwhitney]

I was just reading that a code change in 2017 made a rule about the neutral needing to carry 83% of the load.

Negative, the only 83% is the rule I mentioned about dwelling unit services and feeders carrying the entire dwelling unit load. The neutral load is determined by a load calculation as described.

Cheers, Wayne

 

[Joshua5438]

Negative, the only 83% is the rule I mentioned about dwelling unit services and feeders carrying the entire dwelling unit load. The neutral load is determined by a load calculation as described.

Cheers, Wayne

But the internet said... haha. Thanks for confirming that. I may run all three wires 3/0Cu or only drop the neutral one or two sizes. I am still adding things regularly to the load calc. I want to have the option to reach 190A if I really needed to and not limit myself on what I will originally have installed. Thanks again for all of your help!

 

[Joshua5438]

I have decided to use unistrut to install the meter socket and the main breaker panel. Do you know of any code violations or issues with drilling new mounting holes in the back side of the main breaker panel? I need to span the unistrut 3 feet between two vertical wall posts and the predrilled mounting holes on the rear are in alignment with the back feel knockout that I need to use to connect the two panels together.

 

[wwhitney]

I have decided to use unistrut to install the meter socket and the main breaker panel. Do you know of any code violations or issues with drilling new mounting holes in the back side of the main breaker panel? I need to span the unistrut 3 feet between two vertical wall posts and the predrilled mounting holes on the rear are in alignment with the back feel knockout that I need to use to connect the two panels together.

The metal enclosure that a panelboard comes in is just that, an enclosure, and you can customize it as you wish, or in theory make your own of sufficient size and steel thickness. You can get panelboards in enclosures without any knockouts or mounting holes, and then just make your own holes where required. You can punch new KOs in your existing enclosure if the factory KOs are in the wrong place (although that may be trouble if the desired location partially covers a factory KO, you may need to reinforce the remaining partial factory KO). You can drill your own mounting holes. It may be wise to plug any unused mounting holes, certainly any of significant size. If you knockout a factory KO and don't use it, a KO plug to fill it is required.

Cheers, Wayne

 

[Joshua5438]

The metal enclosure that a panelboard comes in is just that, an enclosure, and you can customize it as you wish, or in theory make your own of sufficient size and steel thickness. You can get panelboards in enclosures without any knockouts or mounting holes, and then just make your own holes where required. You can punch new KOs in your existing enclosure if the factory KOs are in the wrong place (although that may be trouble if the desired location partially covers a factory KO, you may need to reinforce the remaining partial factory KO). You can drill your own mounting holes. It may be wise to plug any unused mounting holes, certainly any of significant size. If you knockout a factory KO and don't use it, a KO plug to fill it is required.

Cheers, Wayne

Once again, thank you so much for helping me out with these questions. I know I will have more at some point but I feel much better hearing another opinion. I try to research as best I can but some subjects have very little documentation. I will plug the factory holes with tape as I recall my sub panel for my pool actually came with little cut out circles of tape to cover any unused holes on the back side.