As we all are a little unclear on areas of the trade, I have always been unclear about certain times to derate. Can someone explain to me in a simple way, with code references, when to derate. I shouldn't sya I am totally lost on when and how to derate, but when I see 400 amp switches fused at 400Amps with 400A rated wire. Where is the derating in that? This has been one of the thinhgs I never can grasp. Probably because I have never really asked for clarification.

Just typed up a long reply and ended up clicking mouse and lost everything, so here goes another shot.

Your question will take a few posts to answer, so I will get it started.
If you refer to CEC 4-004, there are a number of rules that refer to derating and correction values applied to conductors. Generally, when you have excess heat, you will have to apply correction factors to the current carrying capacity of the conductors.
For example, excess heat could come from higher than normal ambient temperatures that may be found in some industrial environments. Also, excess heat can come from other conductors such as in a raceway or cable tray. In those cases you would refer to the tables 5A,B,C or D for the appropriate value to derate the conductors by.
Keep in mind, if you are encountering a retrofit or addition to existing, you may have to apply correction factors to the existing work such as pulling another cable in a cable tray. Then you would have to review the ampacity of the existing conductors if you wanted to use the same tray.
In the example you mentioned, there may have been no need at all for derating, or the original install may have been incorrect.

I have a better understanding now with respect to derating and loading on a distribution. I called our local inspector and gave me a one on one lesson. Basically to find your calculated load, take your largest one, multiply by 125%, and add the rest of the loads to get your demand. Usually ESA has workshops for section 8. I asked why they don't offer them anymore and his reply was no body is wanting to take it, which boggles his mind. He said it is one of the most misunderstood sections of the code and figured more people would want to understand it better.

Demand factors and derating are two separate things, you may still have to apply section 4 rules to conductors you have sized based on section 8 calculations.

To keep it easy...in general you derate for the following:

When conductors (more then 3) are in contact with each other;
When wires are in contact with each other;
When the ambient temp is excessive;
When distance is a factor; and,
When calculating fuse coord issues.

I think that is about it...

Cheers

I totally understand about derating. I think the original question I posted is being confused. My question basically was how do you know if an existing distribution is becoming overloaded and how do you determine if you can continue adding loads without the risk of blowing the main fuses feeding that switchgear?

Navyguy - derating for overcurrent protection coordination is a new one to me, is there reference to this in the CEC? Do you have any more info on it? thanks.

What about taking a load check with as much equiptment on as possible, knowing what the load is you are installing and looking at your main to determin if you can handle the total loads?

Wanted to toss in some added examples to this thread - just for fun.

DISCLAIMER: Although I deal with the NEC, the basics should be similar...



There is no need to derate this scenario ***under normal circumstances***, since everything equals out to be 400 Amp Capacity.

Where things could change - thus becoming "NOT NORMAL CIRCUMSTANCES" would be:

• Ambient Temperature is always higher than 30°C,
• Load Amperes is >320 Amperes continuously for 3 Hours or more (LCL)

First off - and let me point out that I am not being a smart***, but if the Main OCPD often trips / blows a fuse, then there is a good indicator that either:

• The Service Capacity is being exceeded for an extended period of time,
• There is something faulting out in the Service Equipment,
• A large connected load is drawing excessive starting current.

The way to determine if an existing Service's capacity is, or will become inadequate, is similar to what COOKCC has mentioned in the above post.

Determine the existing loads and apply it to the new designed loads.
Coincidental loads need to be considered, along with heavy starting loads & LCL (Long Continuous Loads).

When your calculated loads exceed the existing capacity - figured either by a Load Calc, Panel Schedules - or both, then a Service upgrade is needed.

This should be ample to cover your queries.

Let me know if you have further questions.

Scott

That is pretty well the answer that the local inspector gave me. I totally understand now how to determine if I can add to an existing switchgear without risking the blowing of main fuses when the job is complete. Thanks to all who had there input.

The easiest/most common example I can use is when doing your splitter calculations. There are circumstances when you will derate (oversize) wire and other components to allow for the proper fuse selection/coordination. This has consequences right back to the service entrance possibly.

Other examples (not so common) will be for specialized equipment and high frequency items to allow for better conductor cooling with unique fuse requirements.

Hope this was helpful..

Cheers

thanks for the reply, funny, I just did that exact example a couple of weeks ago with splitter and disconnects but didnt really think of it as derating at the time.