350 kcmill service & two - 200 amp sub panels

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mechanical

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I'm a newbie to the forum and from my username you can safely assume that I'm a mechanical engineer with a very basic understanding of electricity. Recently we built a detached garage and I wanted a 200 amp panel in the new building, welders, compressors, car lift, heat and AC, etc..... The power company discouraged me from adding a new service and meter, and suggested coming off of the existing service as it would be cheaper. The house already had a 200 amp breaker panel served by an Alcan W 350 kcmil AL 600v 90c xlpe underground feeder (not in conduit and not a concentric ground) from a transformer located about 350 feet from the house. When the house was built 20 years ago the power company did not want to install a new transformer on our lot, so they buried a larger conductor instead to compensate for the distance (all utilities are under ground.) The new electrician has installed a 320 class meter base to replace the old one, and has run two separate 200 amp services off of it. One to feed the existing panel, and one to feed a new 200 amp disconnect, which now feeds the new 200 amp panel in the detached garage. My question is, is the power company's 350 kcmil feeder cable capable of supplying the total 400 amp load? I read that you really only get 80% or 320 amps from 400 amp service, but what can I truely expect from this installation as far as current draw at each panel? The new 200 amp panel is approximately 150 feet from the new discount and is fed by 4/0 AL conductor.
 
Re: 350 kcmill service & two - 200 amp sub panels

Yes . . . sort of. What the electric utility has the ability to do is to install the size of cable that will work for the demand that they expect you to have and not to the service size. Tomorrow, next year, or 20 years from now, if the cable is undersized, the electric utility is obligated to replace the cable. It does not matter what the reason is as long as it is undersized or fails because of the load.

What I am saying is, do not be concerned about the service cable. Through experience, the cable is sized for the expected load.

The only thing I am concerned about is if you have been in contact with the electric utility? If they installed the original cable for the existing service and you have now increased the service size without their knowledge, they have no obligation to upgrade their cable in the future without billing you for the replacement. There may be costs or obligation associated with a service equipment upgrade such as, a flat fee for replacing the cable, you supply the trench and backfill and we will replace the cable, etc. You need to be talking to the serving electric utility now! :D
 
Re: 350 kcmill service & two - 200 amp sub panels

Charlie is exactly correct. Start talking to your power utility company now. My only concern would be whether the meter pan is the correct size for this size service. I'm not sure I'm familiar with a 320 class meter pan. Can the lugs accept 350 kc mil wire on the incoming side and two 4/0 on each of the outside lugs ? The last 400 amp service I installed required a CT cabinet for POCO metering. If the 320 class meter pan is acceptable to the POCO then I guess you're set.

As far as the POCO goes, and as Charlie mentioned, they size their feeders to what they expect your demand will be, not to the connected load. There are examples of load calculations for residential services in annex D of the NEC code book. Take a look at them when you get a chance, follow the guide lines and see if you can calculate the demand load for your residence. Remember, you're not going to be running all of your lighting and appliances at the same time.........unless, of course, you have a mid-summer party with the pool going, all 4 air conditioners, all the lights on, 2 dishwashers going and the microwave, doing 2 loads of laundry, and if that wasn't enough you take your friends out to the garage to show them how your welder works !!! Even with that scenario, I doubt you'll be drawing anything near 400 amps
 
Re: 350 kcmill service & two - 200 amp sub panels

Let me offer a softer version of this advice. You had an electrician do this job for you. Very good. That person was responsible for contacting the utility, as the other Charlie and Goldstar have described. I suggest that you obtain some assurance that your electrician had properly coordinated this installation with the utility company.
 
Re: 350 kcmill service & two - 200 amp sub panels

Goldstar, most (all I have seen) of the 320 ampere meter fittings have studs for line and load and you provide the lugs for landing your service entrance conductors. They are rated for 320 amperes continuous load so that a 400 ampere single phase service may be fed with one and you will have no problems. Additionally, with the load profile of a residence, most electric utilities have no problem with a 320 ampere meter fitting. In fact, we still furnish self contained meter fittings and the 320 ampere fitting is one that we furnish. :D
 
Re: 350 kcmill service & two - 200 amp sub panels

Thanks for everyones input. I spoke to the power company before we started construction to discuss options for getting power to the new building. They suggested coming off the existing service as the least expensive method. A completely new service at the garage would have required a second meter, trenching across a road, and a new account with a minimum monthly bill regardless of usage. They sent out a lineman to confirm the conductor size because I indicated that we wanted a 200 amp panel in the garage as well as the original 200 amp service for the house. They stated that the 350 kc mil AL cable should be adequate and said that we would need to upgrade to a 320 class meter base, which we have done. The power company was present to shut off service during the meter base swap and then inspected the new hook up before re-energizing the service. There was also a local electrical inspector present, in addition to the master electrician who did the work. I did some checking on the ampacity of the 350 kc mil underground AL conductor and it does not appear to be capable of 400 amps, or even 320. I was just wondering what are the consequences if the load were to exceed the power companies conductor capacity.
 
Re: 350 kcmill service & two - 200 amp sub panels

If the amp load from the two 200 amp panels combined exceeds the amapcity of the power company's underground feeder, what happens? Is the feeder protected at the transformer?
 
Re: 350 kcmill service & two - 200 amp sub panels

Is the feeder protected at the transformer
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Not likely as they only protect the line feeding the transformer But some do size there fuses to do just that.

If the amp load from the two 200 amp panels combined exceeds the ampacity of the power company's underground feeder, what happens?
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Can be any one of many:
a large voltage drop (Likely)
the underground wires over heat and loose there insulation then electrolysis off. (remote)
Transformer over heats and opens (not very likely)
 
Re: 350 kcmill service & two - 200 amp sub panels

So, if the power company doesn't protect the feeder to the residence, then voltage drop would be becuase of heat and increased resistance on the feeder?

If the insulation melts wouldn't there be a dead short to the transformer?

Their feeder is connented to my meter base. Is there any danger to the house from a failure of the feeder due to undersizing? Thanks again for your comments.
 
Re: 350 kcmill service & two - 200 amp sub panels

Originally posted by mechanical:
So, if the power company doesn't protect the feeder to the residence, then voltage drop would be because of heat and increased resistance on the feeder?
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Two separate issues, you are going to have voltage drop with or without overload protection.

Originally posted by mechanical:
If the insulation melts wouldn't there be a dead short to the transformer?
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Yes that could happen, if it does the power company will be there to fix it.

Originally posted by mechanical:
Their feeder is connented to my meter base. Is there any danger to the house from a failure of the feeder due to undersizing? Thanks again for your comments.
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IMO it is a very remote danger to your house but than again the same can be said for most all service conductors.

I would sleep fine with the setup you describe, the power company has a much better idea of what the service for your house needs than any of us do.

The size of the service conductors can be based on the load, not the rating of two or more OCPDs

It would be surprising if you ever draw over 250 amps, the NEC rating of 350 AL.

Unless you can provide that actual load we can not determine the voltage drop.

If you did draw 250 amps, voltage drop at the main house would still be less than 5%.
 
Re: 350 kcmill service & two - 200 amp sub panels

It is not likely that anything will happen except for excessive voltage drop. In my opinion, that will not be a problem either with that size of cable. We use 4/0 Al for 400 ampere services where voltage drop is not a consideration. 350 kcmil will certainly carry the load and take care of the voltage drop as well.

I highly recommend that you relax and let go of this issue, the serving electric utility is doing a good job. :D
 
Re: 350 kcmill service & two - 200 amp sub panels

I appreciate every ones comments. I was just concerned that the power provider was just taking the easy way out again. We have had previous problems with a shorted underground, concentric ground conductor that the power company did not want to dig up. It wasn't until the kids got shocks off of the outdoor water faucet, the phone company refused to work on the low voltage wiring due to high voltage on their equipment, and I showed the power company's lineman an 16 volt potential between my water pipe and the soil that they acknowledged that they had a problem. You should have seen his face when I initially put one lead of my Radio Shack volt meter on the outdoor faucet and stuck the other lead in the soil and got 12V. Then he asked me to flip the main off and the potential shot up to 16v. He put his meter on and got 18v. I told him I was not generating any electricity in my basement. The next day they were digging up the cable. The particular conductor coming to my residence is not a concentric ground and this is why I had the highest voltage readings to the soil than the other residents who had the concentric ground conductor. I am just concerned that they are unwilling to bury the correct size conductor due to expense as in the past.

If 350 kcm Al is only rated for 250 amps underground, how can it be legitimately used on a 400 amp service? Can they just say that they didn't expect the load to ever exceed 250 amp?

Thanks for you input,

Mark

[ November 19, 2004, 10:28 PM: Message edited by: mechanical ]
 
Re: 350 kcmill service & two - 200 amp sub panels

I am having a problem with the term "concentric ground". The only concentric I am familiar with is around primary cable. I understand you had a problem but I just don't understand your terminology.
If 350 kcmil Al is only rated for 250 amps underground, how can it be legitimately used on a 400 amp service?
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250 amperes is what you would rate it to carry since that is what is in the NEC. We rate it according to the manufacturer's recommendations and at 90?C so it will carry considerably more than table 310.16 states.
Can they just say that they didn't expect the load to ever exceed 250 amp?
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Cable is no different than transformers. We can overload cable for short periods of time because of the time it will take for the temperature to rise in the cable. This works well for residential services because of the load profiles. To directly answer your question though, the NEC will oversize services by a factor of two. Based on that statement, no, I would not expect you to have more than 160 to 200 amperes of maximum demand. Again, I highly recommend that you relax and let go of this issue, the serving electric utility is doing a good job. :D
 
Re: 350 kcmill service & two - 200 amp sub panels

I have to disagree that they are doing a fine job when it takes the kids getting shocked off the water faucet and the phone company refusing to work on their own equipment due to high voltage before they would correct the problem.

The concentric ground is an uninsulated stranded AL conductor wrappped around the outside of the two insulated conductors carrying the 240 volts to the house. The idea was to use the earth as the ground. We are in a rural area without the benifit of an underground water piping system for a grounding grid. Unfortunately, the grindings that they backfilled the trenches with were very acidic and attacked the insulated conductors causing shorts.

Charlie,

You have told me twice now to relax and that my power supplier is doing a good job. Do you per chance work for a utility? Ha Ha. I work for a company that provides the coal to the boilers for the utilities.


Mark

[ November 19, 2004, 11:20 PM: Message edited by: mechanical ]
 
Re: 350 kcmill service & two - 200 amp sub panels

Look at my first post and see what it says at the bottom, "Charlie Eldridge, Indianapolis, Utility Power Guy". I am not trying to hide anything. Also, no electric utility in the country attempts to use the earth as a return path and I don't believe a bare concentric neutral would be aluminum (a cable company wouldn't make it and a utility would not buy it). By the way, using the earth as the sole return path is a violation of the NESC (except under certain conditions), the same as it is in the NEC.

You will notice that I have not addressed the "concentric neutral" problem. I can understand a particular individual not wanting to pursue the problem or not knowing what to do but I can not imagine the company trying to skirt the issue when there is really a problem that needed to be fixed. I do understand that the individual that you see is the company and anything he does is all you know about the company. I don't know how many of the posts you have read in this forum but you will see several times where I have come down hard on other electric utilities (I have even recommended that people turn them in to their state's public service commissions).

Getting back to the points that I was addressing with you though. Your service cable is fine and in that respect, they are doing a fine job. :D
 
Re: 350 kcmill service & two - 200 amp sub panels

Charlie,

I didn't mean to indicate that you were hiding anything. I was just needling you. Again I want to thank everyone for their reassurance on the sizing of my service installation and their comments. This is a very good site that you have here and I am glad that I found it. I have passed the site address onto the electrical engineers in our office. While my background is not electrical, I have read through many of the threads on this forum and have gotten a better understanding and appreciation for the code and requirements in this field.

Regarding the concentric conductor, here is an earlier post from this site addressing some of the problems that can occur:

http://www.mikeholt.com/mojonewsarchive/SV-HTML/HTML/Unjacketed-Concentric-Neutral-Cable-and-Stray-Voltage~20040128.php

Thanks again,

Mark
 
Re: 350 kcmill service & two - 200 amp sub panels

The concentric ground is an uninsulated stranded AL conductor wrapped around the outside of the two insulated conductors carrying the 240 volts to the house.
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Notice the Scott Yeazell article is talking about concentric neutral primary cable and that would be a concentric neutral around a single conductor most of the time. The problem they are having with that cable is part of the reason we went to a polyethylene jacketed concentric neutral cable.

Take care Mark,
Charlie
 
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