I'm trying to understand an old engineers report for our apt building's electrical system.

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bgupta

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It says that we have a feed from ConEd that is 800A 3-phase 208V 4wd. It labels the conductors as 4/0 gauge, which looks about right.

Bearing in mind that this building is well over 100 years old, and probably only got electricity in the 1920s, and that the existing system is pretty old, is it even possible to push 800A over 4/0 4WD? Best as I can tell modern specs would limit this to around 400A? (The feed is below ground.)

Thanks,
Brian

P.S. - I'm not an electrician, but did go through Navy Fire Control training which was an 18-week program that did cover a lot of AC and DC electrical basics. (Caveat shipboard power was 440v 3-phase floating ground, and this was almost 30 years ago.)
 
No. (I'm aware that three of the 4 terminals are corroded and should be replaced.) The feed are the cables on the left.



uc
 
the utility companies have their own rules about what they will supply you as far as wire size. They have a lot of operational data from many decades to support what they do. But once it gets to wires covered by the NEC you have to abide by those rules.
 
I agree with post #4...
+ Thank God you were able to get the picture and walk away (erspecially if you had to remove a cover)
Its a testament to how much abuse a cable can take.
Professional help is definitely warranted.
 
As post 5 says, the utility company operates under a different set of rules than the NEC.

There is no way that 4/0 can carry 800A.

However it is very likely that your '800A service' is actually carrying much less than 800A of current. The utility probably knows the real usage, and sizes their conductors for what they expect the real load to be.

I absolutely agree with post 6: professional help is warranted.

-Jon
 
bgupta said:
P.S. - I'm not an electrician, but did go through Navy Fire Control training. . . .
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For the benefit of those of you who were not in the Navy (I am a retired Commander), "Navy Fire Control Training" has nothing to do with flames. It is related to the electronics that allow you to point and shoot on-board weapons.

That said, there is not much more we can offer you here. I trust you are not planning to do anything to "fix" this installation, but will use any information you learn here to convince the building's owner that repairs are warranted.
 
charlie b said:
For the benefit of those of you who were not in the Navy (I am a retired Commander), "Navy Fire Control Training" has nothing to do with flames. It is related to the electronics that allow you to point and shoot on-board weapons.

That said, there is not much more we can offer you here. I trust you are not planning to do anything to "fix" this installation, but will use any information you learn here to convince the building's owner that repairs are warranted.
Click to expand...

Absolutely. It would be a violation of code for me to touch this, and unwise to boot. I'm planning to make some recommendations for the next steps.

At the bare minimum, I plan to recommend that the terminals that connect the feed are all replaced (by a licensed electrician). We're getting conflicting information from an old engineering report, and two different electricians. I'm just trying to sort out the truth, as there are now three versions of it. (IFor better or worse, I live in this building, and am probably one of the more technical folks.)

I guess the only remaining open question I have is, what Amperage can 208V 3-phase 4WD 4/0 gauge cables actually carry? (And how could I have calculated this myself?)

Thanks,
Brian
 
bgupta said:
I guess the only remaining open question I have is, what Amperage can 208V 3-phase 4WD 4/0 gauge cables actually carry? (And how could I have calculated this myself?)
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There are charts and tables to tell you the answer to this question. It is a matter of how much copper (or aluminum) there is in the conductors and how hot the insulation can get before it fails. Also how hot the terminals can get before they fail.

So the answer is not as simple as you might like, but generally, 230 Amps for copper and 180 Amps for aluminum.

I would warn you though that the load calculated via the formulas found in the code will exceed the actual load. Most times by at least a factor of 2 or 3. So IF the load was calculated to be 800 Amps, it might well be more like 300 Amps, and while the conductors ratings might make it seem like this is a dangerous situation because of overload, the reality is it might well be fine.
 
Also, just because it is an 800 Amp service does not mean much. It is just the rating of the meter more than anything. 800 Amps at 208 V 3 phase is almost 300 kVA. If you look at the transformer nameplate the utility installed it is probably a lot smaller than that.
 
bgupta said:
I guess the only remaining open question I have is, what Amperage can 208V 3-phase 4WD 4/0 gauge cables actually carry? (And how could I have calculated this myself?)

Thanks,
Brian
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depends on a lot of things..
on the code cycle, the authority under which the building was wired years ago, or maybe the engineers approval was all that was needed.

here’s an example table from the 1897 NEC
B38B2A3D-DDE6-4056-8A99-3855A32EA86E.jpeg
 
winnie said:
As post 5 says, the utility company operates under a different set of rules than the NEC.

There is no way that 4/0 can carry 800A.

However it is very likely that your '800A service' is actually carrying much less than 800A of current. The utility probably knows the real usage, and sizes their conductors for what they expect the real load to be.

I absolutely agree with post 6: professional help is warranted.

-Jon
Click to expand...

Usually services end up drawing 1/2 to 1/3 of the their rating in practice.
 
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