Sizing 2-family services

[goldstar]

There aren't too many 2-family houses in my area so I don't come across this that often but, aside from load calculations, is there a rule of thumb for sizing 2-family and multi-family services. For example, if you are upgrading the service to a 2-family house that already has a 100 amp SEU cable into a 2-socket meter pan with two 60 amp main breaker panels and you change out the breaker panels to 100 amp mains, is it acceptable to change the SEU to 150 amp or do you have to bump it up to 200 amp ? I guess what I'm asking is whether there is a ratio between the size of the breaker panels and the size of the service entrance cable. Thanks in advance.

Phil
Gold Star Electric
New Jersey

 

[Jerseydaze]

I can tell you the way we do it is use 200 Amp Seu down to a dual meter socket then enter with 100 amp Seu to two hundred amp main breaker panels.

 

[bphgravity]

There are no special rules. Treat the service entrance as if it is serving one total load consisting of both occupancies as calculated by Article 220. The feeders need only be sized for the individual load they will be expected to carry. Review 230.90(A) and especially Exception #3 in regard to the overcurrent protection.

 

[jeff43222]

I don't think there's a ratio. I'd just do a load calc and size accordingly. Last time I did a duplex I used #1 Cu from the service point to the dual socket, then #4 Cu from there to the 100A panelboards.

If you want to be totally covered, you'd have to size the service entrance by adding up all the sizes of the panelboards. I've always wondered about OCP for the wiring between the service point and the sockets, since theoretically that wiring could be overloaded but the individual mains in the panelboards wouldn't trip, but OCP for the service isn't required for some reason.

 

[infinity]

I can tell you the way we do it is use 200 Amp Seu down to a dual meter socket then enter with 100 amp Seu to two hundred amp main breaker panels.

This is typically how we would install a two family service also. If you wanted to perform a service calculation you may be able to reduce the size of the service entrance conductors to the meters.

 

[celtic]

We typically just bump it to 200A and forget the load calcs.
The difference in price is minimal and the difference in time is non-existant...the benefit to the HO is that the 200A is possible the max the POCO will allow - when they goto sell, it's a built in return of their investment....or for when that "bonus apartment" mysterisly gets built

 

[goldstar]

We typically just bump it to 200A and forget the load calcs.

Thanks for the resaponses . I figured this would probably be the norm. The reason I asked this was in looking at the original service there was a 100 amp SEU riser cable into the meter enclosure and serving two 60 amp main breaker panels. If you were the original installer and sized the service based on the face value of the main breakers you'd be 20 amps over the rating of the SEU riser cable. However, if you figured on a worst case scenario of say 80% of the face value of the mains you'd come up with 96 amps and would be safe with the 100 amp SEU. However, in my scenario, if I sized it to the face value of the main breakers, I would have to use 200 amp SEU cable. If I figured on 80% of the face value I'd still come up with 160 amps and the 150 amp SEU cable wouldn't be sufficient. The problem is I don't think anyone makes a 175 amp SEU cable so I would have to either change the service to PVC and install 3/0 aluminum or bump it up to 200 amps using SEU cable.

Ever wonder why the POCO isn't required to change their triplex but we have to worry about being 10 or 20 amps over on the size of our SE cables ?

 

[JES2727]

If you were the original installer and sized the service based on the face value of the main breakers....

Thats backwards, and thats the mistake in the way you're looking at this. The service is based on the calculation for the building as a whole, not for each unit added together. The way you're looking at it is just like adding up the values of all the breakers in a panel and saying that a 200A service is too small because the breakers add up to more.

John

 

[celtic]

If you were the original installer and sized the service based on the face value of the main breakers you'd be 20 amps over the rating of the SEU riser cable.

W/O even breaking out a code book, you must know that line of thinking is wrong.
Think about it....
a typical 30 Ckt panel and a 200A feed
30 x 20A CBs = 600A ???

I don't think so

Sometimes, you have to do the math....in the case of bumping a service from 100A (with 2x60A mains) to a 200A (w 2x100A mains), there is no need to. You have already almost doubled the available amperage to each unit w/o changing the existing load.

Ever wonder why the POCO isn't required to change their triplex but we have to worry about being 10 or 20 amps over on the size of our SE cables ?

No, not really.
The POCO is not covered by the NEC ~ 90.2(B)(5)(A).

 

[goldstar]

Thats backwards, and thats the mistake in the way you're looking at this. The service is based on the calculation for the building as a whole, not for each unit added together. The way you're looking at it is just like adding up the values of all the breakers in a panel and saying that a 200A service is too small because the breakers add up to more.

John,

I guess that's the answer I was looking for but wanted to know why. And, I guess there's no shortcut around doing the load calcs and sizing the service and separate panels accordingly. Thanks for enlightening me.

Think about it....
a typical 30 Ckt panel and a 200A feed
30 x 20A CBs = 600A ???

I don't think so

Celtic,

Maybe I didn't state my case correctly. As we all know, circuit breakers are there to protect the wire. In your scenario, each of the 20 amp breakers is protecting the wire associated with it and no, I don't believe that there would ever be a case where all 30 breakers in your panel would be drawing anywhere near their maximum allowable currents. However, if each were drawing say 33.3% of their respective allowable loads (again another far fetched assumption), now we're pushing the envelope with the panel's main breaker. In my scenario there were two 100 amp main breaker panels with no combined common protection for the main riser. If each of their respective total draws was 50% or greater and if I don't change the 100 amp SEU to 150 amp SEU don't I run the risk of overheating the SEU cable ?

The POCO is not covered by the NEC ~ 90.2(B)(5)(A).

I know. Don't you hate that ? If they chose to run lamp cord from the pole to a house who governs what they can and cannot do ?

 

[infinity]

It seems that if the existing service entrance to the meters is 100 amps and you do not anticipate any new loads than the new service entrance conductors can also be 100 amps. Obviously to be sure of this you would need to perform a load calculation, but if the current 100 amp conductors haven't burnt up, and you're not adding any additional load you could anticipate that the new 100 amps conductors be adequate.

 

[goldstar]

Thanks Trevor. After reading all the responses including yours I can see now why my thinking was a bit backwards. The amount of load we added was minimal and in all reality there isn't much chance that this particular residence would be adding anything substantial (with respect to amperage) in the future. We bumped up the SEU to 150 amps because other 2-family houses on the same block were done the same way. One part of the house needed a few more circuits but not necessarily more amperage than the original 60 amps. But, they're renovating gradually so who knows what the needs will be.

Thanks to all again.

Regards,
Phil

 

[iwire]

I know. Don't you hate that ? If they chose to run lamp cord from the pole to a house who governs what they can and cannot do ?

Usually there is a public utility commission and many times they hold the POCO to the NESC.

The thing to remember is an NEC calculated load is just that....it is a 'calculated load' it has no relationship to the actual load that residence will require.

The power companies have been supplying power to homes since the beginning they have all the historical data they need to size the drop in a way that will work fine.

If there is a problem with the system it is with the NECs unrealistically high calculated loads not with the power company's methods.

A power company engineer that used to post here said they generally find the actual service load from a dwelling is 45% to 50% of the NEC calculated load.

One other thing to consider that this POCO man would point out is that the POCO 'warranties' the work forever and is available 24/7 if there work fails.

 

[goldstar]

Good info Bob. Thanks for posting.

Regards,

Phil