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Discussion Starter · #1 ·
Hello everyone. I need to replace the cable that connects a 200 amp main shutoff on the back of the house to the 200 amp electrical panel in the utility room that is about 12' away from the main shutoff. The existing cables are 2/0 aluminum and need to be updated to 4/0 AL.

It looks like I need 4/0-4/0-2/0 AL cable with maybe a 6 ground wire? I'm less sure about the size the ground wire needs to be than I am the other wires.

I am also having a hard time figuring out exactly which cable to get. There is conduit that runs up the outside of the house from the main shutoff, then through the attic, and down to the box in the utility room.

Would you go with SER cable? Or MHF? Or something else? I know the SER does not need conduit, but what if conduit is already there?

Thank you in advance for any advice. I appreciate it.
 

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4/0 aluminum XHHW compact conductors or 2/0 copper THWN conductors plus #4 bare grounding conductor. Disconnect also needs a grounding conductor to the grounding electrode system. Do not attempt to use SER in the conduit as it will be nearly impossible to install in such a small conduit.
 

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Discussion Starter · #4 ·
Thank you.

Would you use 4/0 for both hots and neutral? The neutral lug in the new panel is smaller than the lugs for the hots. Made me think it must take 2/0. Here's the interior of the new panel. It's a Square D QO 42 space plug on neutral panel.


Also, this is what the full arrangement on the back of the house looks like. The shutoff is the box barely pictured, on the far right.


They grounded everything from the box where the service cables enter from the ground.


I was thinking to tie the ground from the new panel and the shutoff into that. Is that a good strategy, or would something else be better?
 

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Discussion Starter · #6 ·
Thanks.

What would you change about the arrangement on the back of the house? Newer, nicer equipment, or something more fundamental?

I'm a little confused why the meter on the left doesn't have a shutoff like meter on the right. But I had not planned on doing anything at this time with the side of the house fed by that meter.

Thanks again.
 

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It looks like I need 4/0-4/0-2/0 AL cable with maybe a 6 ground wire? I'm less sure about the size the ground wire needs to be than I am the other wires.
#6 copper or #4 aluminum.

Would you use 4/0 for both hots and neutral?
This decision may already be made for you.

What size and wall schedule is the conduit between the disconnect and the panel? Look for this printed on the conduit itself rather than trying to measure. If you can't see it then measure the inner diameter as best you can.

The neutral lug in the new panel is smaller than the lugs for the hots. Made me think it must take 2/0.
One of the panel stickers will show the maximum wire size for the lug. I would be very surprised if it's less than 250 MCM.

I was thinking to tie the ground from the new panel and the shutoff into that. Is that a good strategy, or would something else be better?
The grounding electrode conductor in that enclosure is for grounding the service, not for connecting an equipment grounding conductor. You won't interact with the GEC in any way.

You will need to add a main bonding jumper inside the disconnect enclosure to derive the equipment grounding conductor. Clean away the paint around the unused hole on the left side, then install a double lug using a screw and nut. A little anti-oxidant paste on the bare steel wouldn't hurt if you have some available. Connect one terminal of the new lug to the unused terminal of the service neutral using a #4 copper wire. If #4 does not fit into that unused terminal you're going to be in some trouble. The equipment grounding conductor of the feeder then connects to the other terminal of the new lug.

This is something which should have already been done when the disconnect was put in place. The installation was not compliant.

What would you change about the arrangement on the back of the house? Newer, nicer equipment, or something more fundamental?
A new disconnect would be service rated and wouldn't need any of the hoopla I wrote above. It would already contain the necessary provisions to accomplish the same thing.

I'm a little confused why the meter on the left doesn't have a shutoff like meter on the right.
If the service entrance conductors don't extend far into the home on the left side then an outside disconnect wouldn't be necessary. It's a less expensive configuration.

But I had not planned on doing anything at this time with the side of the house fed by that meter.
Until you bring both sides under a single service disconnect you're going to have to be careful about changes you make to either side. You'll need to make a total load calculation of both sides to ensure you don't overload the service and update it each time changes are made.
 

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Discussion Starter · #8 ·
Thank you for the guidance. I appreciate that a lot.

The conduit is 2". The segment of the conduit that says what schedule it is is obscured. It's wall is thicker than schedule 40. So does that make it schedule 80?

The sticker inside the panel says the main and neutral lugs are 4-300 CU/AL.

I think I understand what you are describing with the jumper and the bonding. You'd run the ground out from the panel to the new lug by the shutoff, and have a jumper from that lug to that unused terminal right beside the neutral on the righthand side of the shutoff. Did I get that right? It does not get tied to the ground rods. That whole arrangement is separate and is left alone.

I see what you are saying, too, about how it complicates matters to have both meters fed by a single service. The left side is 100 amp and nothing is changing on that side. The side I am working on, the righthand side, was already 200 amp, but there are some changes. I will have to tally things up, but I doubt the changes I'm making will tax the system more than it previously was. It previously was heated by baseboard heaters, and I've removed those. The plan is to use a ductless mini split instead. It's a very small place. The mini split will use less electricity than the totality of baseboard heaters did. But in the other direction, we are planning to go with a tankless water heater, and it will use more electricity than the old water heater---but it isn't a big tankless heater. There will be a few new circuits smaller circuits as well. But most stuff stays the same.
 

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The neutral at the meterbase and disconnect should be mechanically attached to the enclosure. The grounding electrode conductors from water pipe and ground rods can be landed in either the meterbase or disconnect switch if they are bonded (attached) to the enclosure which they should be. Some power companies don't permit grounding in the meterbase. The equipment grounding conductor to the sub-panel also originates here. 4 wires to sub- panel 4/0 -4/0-/2/0-4 copper or 2 Aluminum.
 

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The conduit is 2". The segment of the conduit that says what schedule it is is obscured. It's wall is thicker than schedule 40. So does that make it schedule 80?
Most likely. If it was heavier than 80 you would have noticed it immediately.

That does mean that 4/0-4/0-4/0-2/0 MHF isn't going to fit. You can either go with 4/0-4/0-2/0-4 MHF and work with the reduced neutral capacity or you can do as brric said and use individual XHHW-2 conductors. Four 4/0 and one #4 will still be acceptable in the schedule 80 conduit. You'll need to mark the ends of one of the 4/0 conductors white and mark the #4 green.

I think I understand what you are describing with the jumper and the bonding. You'd run the ground out from the panel to the new lug by the shutoff, and have a jumper from that lug to that unused terminal right beside the neutral on the righthand side of the shutoff. Did I get that right?
Correct. You're connecting the service neutral to the cabinet of the disconnect. The equipment grounding conductor then also connects to the cabinet.

It does not get tied to the ground rods. That whole arrangement is separate and is left alone.
Yes. The way the service grounding is configured now is exactly how you want it.

But in the other direction, we are planning to go with a tankless water heater, and it will use more electricity than the old water heater---but it isn't a big tankless heater.
Even a small tankless electric water heater uses an enormous amount of current. Those are the kinds of things you need to be careful with.
 

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Discussion Starter · #11 ·
Thank you guys.

I have a couple of questions.

That does mean that 4/0-4/0-4/0-2/0 MHF isn't going to fit. You can either go with 4/0-4/0-2/0-4 MHF and work with the reduced neutral capacity or you can do as brric said and use individual XHHW-2 conductors. Four 4/0 and one #4 will still be acceptable in the schedule 80 conduit. You'll need to mark the ends of one of the 4/0 conductors white and mark the #4 green.
I'm a little confused by this. Is 4/0-4/0-4/0-2/0 a typo with respect to the size of the ground? I thought I needed a 4 or 6 bare copper wire.

So you could use 4/0-4/0-4/0-4 MHF or XHHW-2 in that conduit? Or does the neutral need to be 2/0? I didn't quite follow, sorry.

Also, with the jumper in the main shutoff, the more I think about it, I'm not sure I understand why you don't directly connect the neutral from the sub panel inside to the unused terminal beside the neutral in the panel? Why the jumper? I can think of a couple of possible reasons, but I'm not exactly sure----is it because the neutral/unused terminal is not connected to the cabinet And the lug with the jumper would be? Or is it because the wire coming from in the house could be 6, and the wire to the unused terminal should be 4?

Thanks a lot. I appreciate it.
 

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You will need a separate equipment grounding buss for the new panel. Neutrals and equipment grounding conductors must be separated and isolated at the panel. Do not install the bonding screw in the new panel.
 

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I asked once before and was told MHF is mobile home feeder wire. This designation is not recognized as a type of wire in the NEC. It is probably Type USE. Underground Service Entrance conductors. Typically this is not 90 degree rated wire.
 

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I'm a little confused by this. Is 4/0-4/0-4/0-2/0 a typo with respect to the size of the ground? I thought I needed a 4 or 6 bare copper wire.

So you could use 4/0-4/0-4/0-4 MHF or XHHW-2 in that conduit? Or does the neutral need to be 2/0? I didn't quite follow, sorry.
It's not a typo. MHF is only available in a few standard configurations. The two which would be of interest to you are 4/0-4/0-2/0-4 and 4/0-4/0-4/0-2/0. The larger size will not fit with the existing conduit which is installed, leaving only the 4/0-4/0-2/0-4. That's fine to use as long as you keep the neutral current within the 2/0 ampacity.

If you want the full sized neutral you will need to purchase and use the conductors individually. Three 4/0 and one #4 XHHW-2 conductors will fit into the conduit.

4/0-4/0-4/0-4 MHF would be perfect for you if it existed. It unfortunately does not.

Also, with the jumper in the main shutoff, the more I think about it, I'm not sure I understand why you don't directly connect the neutral from the sub panel inside to the unused terminal beside the neutral in the panel?
Did you mean to say neutral or ground here?

Why the jumper? I can think of a couple of possible reasons, but I'm not exactly sure----is it because the neutral/unused terminal is not connected to the cabinet And the lug with the jumper would be?
Correct. The #4 copper jumper will be the main bonding jumper for that half of the service. It connects the service neutral to the cabinet. On small equipment this is usually done with a specific screw but your disconnect does not have this.

You have only the single unused terminal and this jumper can't be spliced, otherwise it would be fine to have the feeder's equipment grounding conductor on that same terminal block. Instead the feeder's equipment grounding conductor is connecting to the cabinet of the disconnect.

It might help to see a diagram of how the service and feeder are configured. Ignore the grounding electrode system connection, what you have now is what is needed.

 

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If the entire length of the run is conduit, putting cable in there is not a good plan. It's extremely hard to pull cable (like you'll be forced to give up and bring in a professional basically for his truck full of pulling tools, and the pro will want to finish the whole job). Also, cable requires larger conduit, because it counts as a single wire of the widest dimension. That big MH cable is 1.54". Your conduit must be 138% of that (assuming you have no other wires in there) so 2.13" ID. Whoops!

Whereas if you pull individual wires (aren't the existing wire individual wires?) then you get to do normal conduit fill calcs. Even if you went 4x 4/0 THHN... that is 37.92% fill in Sched 80 PVC, you're allowed 40%. Now, realistically one will be quite small (e.g. a bare #6 Cu ground) so you'll be more like 29% fill. That's good for ease of pulling.
 

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Also, cable requires larger conduit, because it counts as a single wire of the widest dimension.
Read the entirety of Note 9 of Chapter 9 Table 1. I've underlined the relevant portion.

(9) A multiconductor cable, optical fiber cable, or flexible cord of two or more conductors shall be treated as a single conductor for calculating percentage conduit or tubing fill area. For cables that have elliptical cross sections, the cross-sectional area calculation shall be based on using the major diameter of the ellipse as a circle diameter. Assemblies of single insulated conductors without an overall covering shall not be considered a cable when determining conduit or tubing fill area. The conduit or tubing fill for the assemblies shall be calculated based upon the individual conductors.
 

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Again 2-4/0 phases, 1- 2/0 neutral, and ,1-#2,Al. or#4 cu. equipment grounding conductor. All this debating is ridiculous . This forum is to help educate people that are doing projects. Not to confuse them or try to prove how smart you are.
 

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Read the entirety of Note 9 of Chapter 9 Table 1. I've underlined the relevant portion.
Errrrrr... I'll need a little more convincing that MHF counts as an assembly by that meaning, though I see merit to the argument.

Regardless, it'll be a murderpull. OP is much better off with individual conductors.
 
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