Multi family water pipe

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blueheels2

Senior Member
Location
Raleigh, NC
Occupation
Electrical contractor
Hey I’ve got a multi family service, 4 200 amp meter/mains over 1 wireway. Largest ungrounded conductor to each is 4/0 so I size the the gec to the metal water pipe off of the kill area of the 4/0 x 4 wires to get the size. 4/0 is 211,600, so 211,600 x 4 = 846,400 KC mil.

In 250.66 if I use AL wire the size would be 4/0 correct.

That’s the first question. If I’m wrong can you lead me to the right answer.

Second question. The water lines are 1/2 or 3/4 inch copper. Do they make a water bond clamp for that small of a pipe that will accept 4/0 wire. I’m not finding it. I don’t think split bolting to the 4/0 and reducing at each pipe to a wire size they would accept would be correct?


Thanks
 
1. I would use the service-conductor size, which should be less than 4 x 4/0.

2. How about a pipe clamp with a built-in lug? (The water main is only 3/4"?)

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See 250.104(A)(2) if you are simply bonding the metallic water piping
 
If it's an electrode then you are back to 250.53 and 250.66.
IMO, you can size it based on the service conductors supplying your wireway which will likely be smaller than your (4) 4/0s
 
blueheels2 said:
I used 4/0 Al for the service conductors so I think that means my original math is correct?
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Your math is correct. From your OP I assumed the 4/0 conductors to your disconnects were tapped from a POCO service in the wireway supplying all four. If so, that conductor is likely a lot smaller than your 211 kcmil combined number.
 
augie47 said:
Your math is correct. From your OP I assumed the 4/0 conductors to your disconnects were tapped from a POCO service in the wireway supplying all four. If so, that conductor is likely a lot smaller than your 211 kcmil combined number.
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Ah. I see what you are saying. I thought I had to size it off of my conductors. The power company has a single set of 250 mmm Al feeding the wireway that ties on to my 4/0 service conductors.
 
augie47 said:
IMO, you can size your GEC based on POCOs 250s
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If the poco changed their wire size, lets say in like 5 years, would you then have to increase the GEC size?

Seems risky to size it based on their size when they can change it at any time.
 
Elect117 said:
I think they are talking about bonding all the pipes together with one conductor and bringing that single conductor to the wire way and bonding it there.
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The building would have to be strangely shaped for that to be more sensible or economical, IMO.
 
4/0 Al is 1/2 what the equivalent copper runs are 3/0 or #4 cost wise. But it’s harder to work with. And they are about 100’ from the service average.
 
It is, to me, an interesting scenario as I see some grey areas. Can we size the GEC by the 250s being one, Another being can we treat the supply as one system and connect to it only once and at the closest point.
Hopefully you will get some other opinions here but in the end it would probably be a good idea to run t by your inspector ahead of time as there are variables.
If I were inspecting it I would likely treat it as a 250.104 install to assure I had each section bonded in the event the systems were not mechanically connected at some point but call could certainly be questioned.
 
blueheels2 said:
4/0 Al is 1/2 what the equivalent copper runs are 3/0 or #4 cost wise. But it’s harder to work with. And they are about 100’ from the service average.
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Careful with Al. It requires more care when using as a GEC. See 250.62/250.64.

I do not think you can bond aluminum to a water pipe since it the Al will be exposed to earth.

But reading it back, it looks like they changed the NEC in 2020 to allow Al to be exposed in earth as long as the environment isn't corrosive? Isn't in earth a corrosive environment due to presence of water?
 
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