how do you size service conductors for a 2 family house?

[wirenut3]

Article 100 states that a multifamily dwelling is a building that contains 3 or more dwelling units.So none of the reductions of table 220.84 would apply to a two family dwelling.The two family house is a gray area to me.
My question: If you wanted say 2- 200 amp panels for a two family house, what size cu wire would you need from the weatherhead down to the double meter pan? from the meter pan to the panel board I would use table 310.15(b)(6) which states 2/0 copper.
Can I use 2/0 copper from the weatherhead to the meter pan as well, even though I'm feeding 2 meters?

[Dennis Alwon]

If you are using 2 risers than I see no reason why you can't use Table 310.15(B)(6) for the conductors in the risers.

[infinity]

You should perform a load calculation to determine the size required for the service entrance conductors feeding the two meters. Or size the conductors based on your maximum possible connected load which in this case would be 400 amps.

[iwire]

If you are using 2 risers than I see no reason why you can't use Table 310.15(B)(6) for the conductors in the risers.

I agree but have to wonder why you would run two risers.

Use a two gang meter socket with one riser sized for only the calculated load of both units chosen from 310.16.

For example each unit may have a calculated load of 140 amps meaning the one riser is only required to have an ampacity of 280 amps regardless of both units having 200 amp mains.

[Dennis Alwon]

I agree but have to wonder why you would run two risers.


Since the OP's question was worded using 2/0 I assumed he had 2 risers and wanted to keep the risers with the maximum for the 200 amp panels.

[bjp_ne_elec]

Have to go with load calc (already stated by iwire and infinity), and I guess the question I have, is do you really need 200A to each side of the two family? If it doesn't have electric heat, you won't need over 150A.

When you go above four units you start to pick up some demand factors (ex. 220.61) that will become effective. For a two family, the demand factor for things like ranges, etc. is still at 100%.

[Dennis Alwon]

If it doesn't have electric heat, you won't need over 150A.



A 150 amp main breaker panel is almost unheard of around here and it would cost a considerably more than a 200 amp MB panel.

We have no idea what the units have in them nor do we know about possible future loads. This is why I suggest full amperage for the riser.

Perhaps the op didn't realize he could calculate the load-- It is all conjecture since we haven't heard back from the OP.

It is possible the OP has one riser and wants to pull 2/0 in it. If the load calculates low enough then he can. I just think it is wiser to size the riser with future loads in mind

[bjp_ne_elec]

Dennis - I think one riser is the way to go.

Like to hear from the OP, after he reads through the posts. Does he actually have a situation coming up, or is he just doing some research - either way wirenut3 - can you provide some feedback?

[wirenut3]

This is my first service job (other than when I did my own house as a home owner). The house is in Rutherford NJ and has two old Federal Pacific 100 amp panels in the basement.We definitely want to replace the panels, but everythings else is 40 years old as well. The job was originally done in Rigid Metal Conduit and I'll have to call the town on that because that seems to be the way most- but not all the homes in that area are done.

I agree with you guys about not needing 200 amp panels as the house is gas heat and each panel has only about six or seven mixed 20 and 15 amp circuits,but was wondering how to size the one riser if I wanted to upgrade the amps on the panelboards.

The way I see it the original service has neared the end of it's life expectancy- with the conduits and meter pan showing signs of rust. I don't think I want to just replace the panelboards because I'm concerned about what I'll find when I pull the meters.

I have some pictures but have to figure out how to post them.

Let's say I keep the two panels at 100 amps each, and I do a load calculation that shows I only need say 70 amps in each panel. Then the riser size would be based on that? 140 amps coming down to the meter pan?

By the way in the original job the pipe sizes seem to all be the same. 1 1/2" coming down from the weatherhead and 2- 1 1/2" pipes out of the meter pans 90'ing into the concrete foundation directly into the back of the panelboards.

[LarryFine]

Let's say I keep the two panels at 100 amps each, and I do a load calculation that shows I only need say 70 amps in each panel. Then the riser size would be based on that? 140 amps coming down to the meter pan?Correctamundo.

[wirenut3]

http://www.msnusers.com/h0op9n53t08r2je9fjc41pvu71/documents.msnw?fc_p=%2FPictures&fc_a=0

I believe if you copy and paste this will show pictures.

Note the romexes with the plug ends for air conditioning circuits in picture 5

[Dennis Alwon]

http://www.msnusers.com/h0op9n53t08r2je9fjc41pvu71/documents.msnw?fc_p=%2FPictures&fc_a=0

I believe if you copy and paste this will show pictures.

Note the romexes with the plug ends for air conditioning circuits in picture 5


cannot access the site. It asks for email address and password. While you are at it just paste your social security number also:grin:

Just kidding but I really can't access it.

[Houston Master]

NEC article 220.85....Two Dwelling units

Where two dwelling units are supplied from a single feeder and the calculated load under Part III of this article exceeds that of three identical units calculated under 220.84, the lesser of the two loads shall be permitted to be used.

What this article says is you go in and do a standard calculation on the two units...then you go in and do an optional calculation for three units of the same identical loads and apply the derate provided for multi-family dwellings you can use the lesser of the two.

Biggie:D

[wirenut3]

The only things that I think will be added are some dedicated outlets for a/c's and a line out to the garage. None of which will be done by me, so really I don't know what the home owner has planned for the future.

To be 100% safe Ill probably run a 2" down from the weatherhead with 2/0 copper - good for 200 amps and #4's to two 100 amp panels.

Sound feasible?

Thanks for all your comments for someone who's a little green at this, but I gotta start somewhere right?

[wirenut3]

Yeah Dennis I know, I'm not so up on this hiding my info stuff. Hope I don't expose too much....

[wirenut3]

I'm giving up for now on how to post the pictures, sorry I'm not more up on this.

[skeffibew1]

Simple You combine the two loads if you were going to go with 2 200 amp panels you would need to run 500 kc mil wire. If you were going to go with 2 100 amp panels or even 125 amp ( more common) I would run 4/0 copper down in a 2 1/2 conduit. I would then set a 2 meter combo with the main disconnects in it. Then the existing conduit to the panels would not have to be upgraded. I have installed many of these in old homes in St. Louis were they converted old large homes into 2 living spaces.

[infinity]

Simple You combine the two loads if you were going to go with 2 200 amp panels you would need to run 500 kc mil wire. If you were going to go with 2 100 amp panels or even 125 amp ( more common) I would run 4/0 copper down in a 2 1/2 conduit. I would then set a 2 meter combo with the main disconnects in it. Then the existing conduit to the panels would not have to be upgraded. I have installed many of these in old homes in St. Louis were they converted old large homes into 2 living spaces.


Although you may be correct that the #4/0's are large enough without a load calc you would simply be guessing. This may or may not get past the inspector. The original poster should ask him what he requires. I've had some ask for load calc.'s or for the service entrance conductors feeding the two meter stack to be as large as the addition of the two service main OCPD's.

[satcom]

Although you may be correct that the #4/0's are large enough without a load calc you would simply be guessing. This may or may not get past the inspector. The original poster should ask him what he requires. I've had some ask for load calc.'s or for the service entrance conductors feeding the two meter stack to be as large as the addition of the two service main OCPD's.

Good post!

[wirenut3]

Thanks for your thoughts guys....I guess I'll make another phone call to the town and simply ask if my 2/0 copper in a 2" pipe is sufficient. He did caution me to watch out for the size of my LB's if I use them, in my first call to them.

[infinity]

He did caution me to watch out for the size of my LB's if I use them, in my first call to them.


That's a good point. Typically LB's have conductor size restrictions that are smaller than the allowable fill of the conduit.

[wirenut3]

Although you may be correct that the #4/0's are large enough without a load calc you would simply be guessing. This may or may not get past the inspector. The original poster should ask him what he requires. I've had some ask for load calc.'s or for the service entrance conductors feeding the two meter stack to be as large as the addition of the two service main OCPD's.


Did you mean that the inspector could ask that the conductors feeding the panels have to be as large as the conductors coming down to the meters?

Wouldn't skeffibew1's use of the mains at the meterpan preclude that possibility? As they would be SER's at that point?

[wirenut3]

http://share.shutterfly.com/action/welcome?sid=0CcOWjVuzbsXJw&notag=1

I think this link will now work to show pictures. Pretty standard stuff I guess. note the violation in picture 5.

[infinity]

Did you mean that the inspector could ask that the conductors feeding the panels have to be as large as the conductors coming down to the meters?

Wouldn't skeffibew1's use of the mains at the meterpan preclude that possibility? As they would be SER's at that point?


The inspector needs to know if the conductors feeding the two meter stack are large enough to handle the connected load. One way to do this is to perform a load calculation. The other is to simply install conductors that are large enough for the worst possible case scenario which would be the total of the two loads.

For example, a fairly standard installation if you had a two family, would be a two meter stack with two 100 amp panels one for each apartment. You could simply use a set of 200 amp conductors to feed the meters and everyone is happy. Or go the longer route and perform a load calculation and maybe save some money by using smaller conductors to feed the meter stack.

[wirenut3]

Ok...I'm clear on this now....Thanks everyone for your help, input and comments.