# sizing lateral for 400-amp residential service



## mikewillnot

I have a disagreement between the POCO and a sub-contractor on this, and as I've never done a service of this size and type, would appreciate some 2014 NEC feedback. 120/240 dwelling. 

Existing house has 200-amp underground service, with 4/0 aluminum lateral and SEU from meter to 200-amp panel. An outbuilding is being "upfitted" with an additional 200-amp panel, and a 4/0 underground line from the house meter location to the outbuilding, to a new 200-amp panel. see my so-called diagram below. The meter enclosure is being upgraded to 320-amp. We will be replacing the existing 4/0 AL lateral, currently in 2-1/2" PVC, with larger copper. The question is, what size? 

This has been approved by the POCO, with a vague nod to "whatever the code requires" for lateral sizing, "probably 250 or 350". 

I contacted a larger contractor, with tuggers, winches, and the like, for a quote on replacing the lateral. He said, based on a reading of 2011 NEC table 310.15(B)(7), that the new lateral needed to have ungrounded conductors of 400 copper. 

Threads I've read are all over the place on this. Suggestions or code interpretations would be greatly appreciated.


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## jw0445

This is what it says. He is correct unless you can find 350
When I do them I run 500 AL and that's in a 4" conduit with a 4" spare conduit
Your 2 1/2" conduit is a no go.

Put an 24" bull wheel on the pole and pull it with your truck.


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## RePhase277

Doesn't the POCO run the conductors in your area?


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## telsa

You're coming at this all the wrong way.

You need to do a Load Calculation for these buildings FIRST.

The meter sizes are _quantized_. [ They come in but a few 'frame sizes.' ]

That is, they jump all the way from 200A to 320A ( aka 400A) in a single bound.

This means NOTHING WRT the actual current involved.

The hefty meters can be 'backed off' by OCPD that are less than 200A times two.

YOU have to figure out what the TRUE load is... and slap in the right size fuses// C/B mains.

THEN you can back-calculate what the conductors ought to be.

Odds are strong that you are w a a a y over designing this baby.

However we can't possibly help you out with our head in a bag. 

We have absolutely no idea WRT the true nature of the loads to be fed.


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## lighterup

telsa said:


> You're coming at this all the wrong way.
> 
> You need to do a Load Calculation for these buildings FIRST.
> 
> The meter sizes are _quantized_. [ They come in but a few 'frame sizes.' ]
> 
> That is, they jump all the way from 200A to 320A ( aka 400A) in a single bound.
> 
> This means NOTHING WRT the actual current involved.
> 
> The hefty meters can be 'backed off' by OCPD that are less than 200A times two.
> 
> YOU have to figure out what the TRUE load is... and slap in the right size fuses// C/B mains.
> 
> THEN you can back-calculate what the conductors ought to be.
> 
> Odds are strong that you are w a a a y over designing this baby.
> 
> However we can't possibly help you out with our head in a bag.
> 
> We have absolutely no idea WRT the true nature of the loads to be fed.


Telsa. 
If I may add my 2 cents.
Yes , he may very well be over killing it , BUT BUT BUT , the given info
by the HO for figuring the total connected load may not be the truth...
he would be relying on the HO to be forthright while assuming all the
liability. (This is not like a pre-engineered / approved commercial plan 
where the EC can walk off with his own set of plans (for any future
reference) and proof that he/she followed the electrical engineers specs.
I think this particular electrician should throw caution to the wind and 
feed the service to its rating which is 320 amps.
As Bill O'Reilly would say .."what say you..where am I going wrong here?"


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## lighterup

mikewillnot said:


> I have a disagreement between the POCO and a sub-contractor on this, and as I've never done a service of this size and type, would appreciate some 2014 NEC feedback. 120/240 dwelling.
> 
> Existing house has 200-amp underground service, with 4/0 aluminum lateral and SEU from meter to 200-amp panel. An outbuilding is being "upfitted" with an additional 200-amp panel, and a 4/0 underground line from the house meter location to the outbuilding, to a new 200-amp panel. see my so-called diagram below. The meter enclosure is being upgraded to 320-amp. We will be replacing the existing 4/0 AL lateral, currently in 2-1/2" PVC, with larger copper. The question is, what size?
> 
> This has been approved by the POCO, with a vague nod to "whatever the code requires" for lateral sizing, "probably 250 or 350".
> 
> I contacted a larger contractor, with tuggers, winches, and the like, for a quote on replacing the lateral. He said, based on a reading of 2011 NEC table 310.15(B)(7), that the new lateral needed to have ungrounded conductors of 400 copper.
> 
> Threads I've read are all over the place on this. Suggestions or code interpretations would be greatly appreciated.


Mikewillnot..

Why don't you just add a 2nd set of 4/0 ALURD , keeping the
first underground laterals in use and set up the service in 4/0
parrallels...either way you will be trenching because as the other
gent said , the 2-1/2" conduit will be a no go for new 400's?
Put double lugs in the new 320 amp meter socket enclosure...

Or ask public utility company & AHJ if you can set a 2nd 200 amp
service on the outbuilding?


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## active1

With this type of work is there can be requirements / defaults that apply.
Such as the utility or local amendments that by default you may need to have X size conduit, conductor size, and material.

The problem I had was in the past some of the local codes wanted RMC the entire run, buried 24", only copper, over sized conduits, and conductors sized per OCP using 310.15. 

But if the utility or their authorized contractor ran it they didn't have the local amendments or NEC. So the smaller AL conductors without a conduit always came in way way less money.


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## Southeast Power

telsa said:


> You're coming at this all the wrong way.
> 
> You need to do a Load Calculation for these buildings FIRST.
> 
> The meter sizes are _quantized_. [ They come in but a few 'frame sizes.' ]
> 
> That is, they jump all the way from 200A to 320A ( aka 400A) in a single bound.
> 
> This means NOTHING WRT the actual current involved.
> 
> The hefty meters can be 'backed off' by OCPD that are less than 200A times two.
> 
> YOU have to figure out what the TRUE load is... and slap in the right size fuses// C/B mains.
> 
> THEN you can back-calculate what the conductors ought to be.
> 
> Odds are strong that you are w a a a y over designing this baby.
> 
> However we can't possibly help you out with our head in a bag.
> 
> We have absolutely no idea WRT the true nature of the loads to be fed.


Agreed!
That Lat should be sized for the load not the breaker math.


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## RePhase277

I am of the opinion, over the past few years, that NEC services are oversized. And becoming more so every day.


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## mikewillnot

The lateral crosses under a busy roadway, total run @170ft pole to house. POCO suggested upsizing in the existing conduit, and if possible, it makes a lot of sense.


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## jw0445

mikewillnot said:


> The lateral crosses under a busy roadway, total run @170ft pole to house. POCO suggested upsizing in the existing conduit, and if possible, it makes a lot of sense.


Missed all of that info in your sketch. Why won't POCO set a pole on your side of the street?


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## sbrn33

I would honestly fit whatever size copper I can put in that 2.5 inch conduit. Then if I had to I would come up with a load calc to match it. 
Just being honest and using common sense.

Edit, now that I looked at it I would run 500 aluminum in that bad boy.


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## telsa

sbrn33 said:


> I would honestly fit whatever size copper I can put in that 2.5 inch conduit. Then if I had to I would come up with a load calc to match it.
> Just being honest and using common sense.
> 
> Edit, now that I looked at it I would run 500 aluminum in that bad boy.


The new structure is a BARN.

So, I rather lean that way, myself.

I still gag at any EC that does not work up a Load Calc.


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## mikewillnot

I am working on the load calcs tonight. Just gathered the information today. The so-called barn is being upscaled in directions yet to be determined. Currently it is getting a sizable addition with an exercise studio, bathroom, and laundry. There is a inground pool adjacent. Who knows? Someday there may be a small apartment in there.


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## Cow

mikewillnot said:


> The question is, what size?
> 
> This has been approved by the POCO, with a vague nod to "whatever the code requires" for lateral sizing, "probably 250 or 350".


The code only requires you size the wire to the load. 230.42.

Anything more than that, is up to you.


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## telsa

RePhase277 said:


> I am of the opinion, over the past few years, that NEC services are oversized. And becoming more so every day.


Out here EUSERC rules the roost... and there is no question but that EUSERC has seriously up-sized Service Lateral standards. ( ~2001 ish )

What had been 4" PVC became 5"...

PG&E even bumped the Service Lateral standard to 3" in GRC (where it rises from the trench up into the all-in--one)... for single family homes at 200A. 

That's some seriously over spec'd stuff.

They're pretty free when spending your money.

They are tight wads with their money -- on your conductors.


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## Cow

telsa said:


> Out here EUSERC rules the roost... and there is no question but that EUSERC has seriously up-sized Service Lateral standards. ( ~2001 ish )
> 
> What had been 4" PVC became 5"...
> 
> PG&E even bumped the Service Lateral standard to 3" in GRC (where it rises from the trench up into the all-in--one)... for single family homes at 200A.
> 
> That's some seriously over spec'd stuff.
> 
> They're pretty free when spending your money.
> 
> They are tight wads with their money -- on your conductors.


I wonder if living in California has anything to do with it.....?

We don't have those crazy rules where I'm at.


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## mikewillnot

Ran the load calculations, finally. 
Main house - 105 amps
Barn & Addition, with future loads - 125 amps

I'm thinking 350 copper for the ungrounded conductors would fit in the 2-1/2, and also be about 20% to spare. 

Now I'm wondering about how much I can undersize the neutral / ungrounded conductor. I'm thinking 250 copper -- although I imagine that will open up a can of worms here.


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## Southeast Power

mikewillnot said:


> Ran the load calculations, finally.
> Main house - 105 amps
> Barn & Addition, with future loads - 125 amps
> 
> I'm thinking 350 copper for the ungrounded conductors would fit in the 2-1/2, and also be about 20% to spare.
> 
> Now I'm wondering about how much I can undersize the neutral / ungrounded conductor. I'm thinking 250 copper -- although I imagine that will open up a can of worms here.


You can just use the same calculation sample to find the neutral load.
Easy guess would be 70% of lighting and appliance loads and back out all 240 volt loads.
I think I would get my calculations to about 350 al with a 4/0 neutral.

You are going to install everything, turn it all on and be amused that you can only get about 60 to 80 amps after all of this.


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## RePhase277

Three 350s can fit in a 2.5", but you're gonna find out what the term "cuss like a sailor" means.


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## Southeast Power

RePhase277 said:


> Three 350s can fit in a 2.5", but you're gonna find out what the term "cuss like a sailor" means.


2-350 Al and 1- 4/0?


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## RePhase277

Suncoast Power said:


> 2-350 Al and 1- 4/0?


350 aluminum is compact stranded. The OP said copper. Me personally, I don't like to go over 250 in 2.5".

EDIT: Never mind. I was thinking of 3-phase for some reason, even though it was perfectly clear it was single. Carry on.


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## sbrn33

I can tell you my DU would put 4/0 aluminum and call it a day.


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## Southeast Power

sbrn33 said:


> I can tell you my DU would put 4/0 aluminum and call it a day.


Here too.


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## mikewillnot

sbrn33 said:


> I can tell you my DU would put 4/0 aluminum and call it a day.


What's a DU?


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## sbrn33

mikewillnot said:


> What's a DU?


Dept of utility. I forgot you guys use POCO


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