does each hot conductor in the service cable have to support the full rated ampacity? for 200A 240V service with 2 hots and 1 neutral, does each hot have to be rated for 200A? if so, why? the load will be spilt among the 2 phases. maybe not perfectly equal, but surely not 0%/100%.

next question -- what ampacity will 4/0 aluminum underground service drop support? I've read & heard different stories. ComEd (the power co) said 3/0 is good for 260A. but they haven't given me a number for the 4/0 AL thats in the ground. they are insisting I get an electrician to do a load calculation/audit first.

 

It's not 2 phases. It's single phase split You have the potential of 200A @ 120V on each leg.

 

The Poco's follow their own set of rules.
Normally aluminum used in conduit underground is 3/0 = 175A and 4/0 = 205A.

I'm not sure why you are questioning the power company ?

 

When installing a new service, code typically dictates a demand load calculation. You can do your own, easiest way is to Google "Demand Load Calculation" and enter the details into the calculator. The POCO probably wants to ensure that you've confirmed that a 200A service is sufficient for your needs.

After that - they should be able to tell you exactly what you need in terms of wire, meter pans, etc. There should be an engineering handbook on their website that gives you all the details you need.

 

Each conductor is required to be able to carry the entire load, in your case, 200 amps. A neutral, on the other hand, that only carries the imbalance of the two (or three in three phase) ungrounded conductors (hots) may be smaller. Typically I do not undersized a neutral unless it is part of a cable. Ex: 2----4 triplex.

As PJ said, the power company follows their own set of rules when sizing their wires and are required to follow the NEC. 4/0 aluminum is good for 200 amps based on 310.15(B)(7)

 

The ampacity of the conductors are based on the insulation type and temperature ratings.

A 200 amp service is not two legs at up to 100 amps. Your could pull up to 400 amps at 120 or 200 amps at 240.

 

well the question to the power company is will my existing service support an additional 100A feeder to a detached garage? The person I talked to on the phone said their records showed 3/0 AL to my house which is good for 260 (I now question that, too). I opened the meter and saw that the cable is in fact 4/0 AL. So I called back and they told me to get an electricians audit.

I looked up a demand load calculator online. It gives me 143A existing load for the house. that's about right. The house has a 200A main panel. The meter is 200A. So I want to add 100A service to the garage on top of this.

so will 4/0 direct burried AL do this? if the power co has to abide by NEC2014 then it appears no, 4/0 AL will not support my garage plans. Unless I am missing something? frankly the NEC ampacity rules are a byzantine mess, at least for us laymen. for good measure I turned on every electrical appliance/light/pump/AC unit/fan/motor in my entire house all at the same time and measured the 4/0 conductor with a temp scanner. it never got above 37F (it was 50F outside, but the ground is still cold).

 

How did you determine that you need 100A available to the garage?

 

I want to have provisions for an EV charger

 

If you have a 200 amp service in the house, you will have no issues running a 100 amp feeder to your garage. Even with the addition of an EV charger.

 

What other loads will be in use in the garage when the EV charger is on to need 100A? Don't EV chargers use at most about 35A to 40A?

 

my uncle has a Tesla. he highly encourages me to get as much power as possible to the garage, with 100A being the minimum. 15 years from now when I'm trying to charge 3 electric vehicles in there at the same time, while also running my air compressor and a space heater I'll probably wish I put 200A to it. but that's not in the budget.

 

I just replaced my 40 year old 100amp panel with a 200amp panel. It has two underground 1/0 AL service entrance hots and a 1 gauge neutral. The power company said that should be no problem handling the load of a 200amp panel and the inspector didn't seem to care as long as Edison said it is ok. Now true, I am not really adding anything extra to the house but the Power company doesn't know that. I would think they would question why I wanted to upgrade to a 200amp panel.

skeeter

 

Oh my... they allowed you to up your service panel to 200A and kept the service feed as 1/0 Al. That's nuts.

Edit: Your service panel is outside, right? Here our panels are mainly inside and 1/0 Al for 200A wouldn't fly for the service entrance.

 

Power companies do not follow the NEC. They generally follow the NESC or a variant of it, but bottom line is that due to all of their wiring being outside and generally locked or inaccessible due to height, they use different types of wire insulation and use them at higher temperature ratings.

 

I don't see anything nuts about it. Most electric utilities size their lines and equipment by calculated load provided by the electrical contractor when the home is built and not the size of the service. It is the customer's responsibility to inform the utility of any significant increase in load.

The service lateral is not service entrance wiring. Service entrance wiring typically is required to be sized by the NEC; 4/0 aluminum or 2/0 copper for 200 amp service to a dwelling.

 

My 200amp panel is accessed from outside but it is flushed mounted in the wall. And even stranger is the 2" underground service entrance conduit comes up through the foundation into the wall. I replaced the panel because it was packed full with tandem breakers and none could be added. I am remodeling in stages and mostly wanted to be able to separate out the circuits with a lot more zones. Not really adding a lot more electrical load just distributing it differently.

skeeter

 

I know all of that. As to Skeeter's post I was thinking they allowed all the service wire to remain as 1/0 Al. I assume the original service entrance was also 1/0 Al.

 

yes, it was........

skeeter