Wire Sizing

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Re: Wire Sizing

Originally posted by dvmont:
We dug down and measured the temperature at the outside of the conduits. The temperatures were about 195 deg Farenheit (90.5 deg Centigrade) on the outside of the conduits. The current was about 1950A in the circuit.
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Yikes! :eek: That is much hotter than I would have expected!

With actual measured temperatures like that, I can see why you were having problems.

How do you determine the thermal characteristics of soil, as it applies to Annex B?

How did you solve the problem at this particular site?
 
Re: Wire Sizing

Tx2step,

There are tables in Annex B that give you spacing and ampacities. Without testing, I would use the lowest available ampacity ? that is, the one shown for the lowest earth thermal conductivity. If the tables don?t show a duct configuration you are using, I would either ask someone to take a look at the installation or use the ampacities for a duct configuration that would be worse than the one you are installing.

When you look at the load factor, use 100%. Ideally, an industry operates at 100% capacity. If you know there are cycling loads or know how the work will be staged, you may be able to use a lower capacity factor.

I don?t believe the code requires any particular spacing. The tables in Annex B show spacing similar to what is commonly used. The tables in Annex B are a short version of the tables provided in the ICEA standards.

At the particular site noted above, the solution was to install a tray system and new feeders.

Another tactic you can use when installing cable underground is to call the cable supplier, tell him what you are doing, and ask him what the cable rating would be. You may or may not get an answer. They don?t like the liability associated with giving a value.
 
Re: Wire Sizing

tx2step, dvmont.

I am not going to lose any sleep over this for the reasons Don stated.

If I start doing industrial work I will have to change my thinking, as long as I am in commercial / light industrial I will go with the 310.16 ratings.

The company I work for has installed literally 100s of large services for commercial / light industrial / multi dwelling units etc. without trouble using the 310.16 values.

There is no question in my mind that Don is correct that the services to these types of occupancies are oversized by the calculations in 220.
 
Re: Wire Sizing

Bob --

I agree with you. I've done a lot of UG installations based on the ratings of 310.16 and have never had a single problem.

If the installation is based on a load calc from 220, I wouldn't give it another thought, for the reasons Don described -- lots of oversized safety factor built in.

My question is from a different direction.

I've had quite a few times when a light-industrial application asks for a specific capacity that's much larger than what a basic load calc would give you. This was to accommodate the addition of machinery in the future that I wasn't given any load data on.

In that case, if I were asked to install a service or feeder that would provide, say, a 1200 A capacity, then Don and dvmont are telling me that using 310.16 won't provide the actual capacity that the customer has asked for if their load factor is very high. In that case, if that feeder burns up, the customer may ask some hard questions or try to hold me liable?

I haven't had a problem with this yet, but I'd like to get a better handle on it.

My guess is that in most cases these light-industrial applications didn't come very close to a 100% load factor, and/or may not even apply an actual load that approaches 100% of the wire's rated capacity, and that's the reason there hasn't been a problem so far. Most light-industrial operations that I've seen don't operate all loads on a 100% continuous basis, and most don't operate 24 hours per day.

It looks like I should add a few more questions to those that I usually ask, mainly to try to determine how close to a 100% load factor that they might possibly have in the future.

If they're a typical operation, then I probably don't need to consider Annex B and can just stick with 310.16. But if they are atypical, and might really approach the full 310.16 rated ampacity and also approach 100% load factor as well, then I may need to discuss derating with them.

I'd like to get a better handle on how many hours per day at what load factor will overheating start to become a problem. Any thoughts on that Don or dvmont?
 
Re: Wire Sizing

tx2step, iwire -

I think iwire has a good point. When you're shooting in the dark, it's a good thing to have a conservative rating.

In the installation I described above, the intent was to start with a low power consumption and continue building with an expected peak of about 4000 Amps. All this was figured out ahead of time. The load did grow at the expected rate. It just couldn't get past the reality roadblock.

I would guess that when someone wants to have a 1200A service, they expect to use it. I'd size the underground for the full 1200A. There's a big difference between a 1200A service and a service for a given location.

If they just want a service, then what does anyone know. Using a code evaluation may be conservative (probably is most of the time). Basically, if you use the code evaluation, you did the best you could with no information. If I had to do that, I'd want to make sure I was legal.

I've never been in a spot where I used code load calculations. All I've done is add up the loads, use some load factors (I call them utilization factors instead of diversity factors), and size the installation accordingly. Basically, I size the installation for the load. I only missed once - had to add a new transformer. Typically, the load I estimate has been within about 5% of the actual load and I expect it to grow - it usually does.

In general, I oversize the primary feeders (usually 13.8KV or so) to allow some additional capacity. That way, the user can add some more switchgear if it's needed for expansion.

In one case, there was a 200A service at 13.8KV (about 5 MW of connected capacity). I installed two 4/0 CU underground. A part of that was for voltage drop and a part of that was for general conservatism. I didn't want another 3 MW added on the assumption that all was fine. With the nature of the user, they might want an additional 3MW.

Along with doing what the customer wants, you have to know your customer.

Hope this helped,

Dave
 
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