800 Amp Buss Duct Maxed?

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d21x

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I have an 800 amp buss duct that pulls the following:

L1 425 amps avg, max draw is not much more.
L2 averages about 400 amps, but the max instantaneous looks like 600 amps.
L3 also averages 400 amps, but the max instantaneous is about 575.

I believe the general rule is not to go above 80% of what the buss duct is rated for. When should I be worried about getting another duct in due to the high current draw?
 
It almost sounds like you are misinterpreting the rating of 800 amps. Just to make sure we are talking about the same thing, the 800 amp rating means the bus is rated for 800 amps on line 1, 800 amps on line 2, and 800 amps on line 3. You don’t add the three phase average currents (475 plus 400 plus 400), and compare that to the rating. Each phase can have an average of 640 amps, and you will be no higher than 80% of the 800 amp rating. Also, the max instantaneous current being close to the 640 is not an issue.

Welcome to the forum.
 
d21x said:
I have an 800 amp buss duct that pulls the following:

L1 425 amps avg, max draw is not much more.
L2 averages about 400 amps, but the max instantaneous looks like 600 amps.
L3 also averages 400 amps, but the max instantaneous is about 575.

I believe the general rule is not to go above 80% of what the buss duct is rated for. When should I be worried about getting another duct in due to the high current draw?
Click to expand...

I believe the bus duct is usually 100% rated. However, the protective device feeding it is probably only rated "80% loading" for continuous loads.
 
d21x said:
I have an 800 amp buss duct that pulls the following:

L1 425 amps avg, max draw is not much more.
L2 averages about 400 amps, but the max instantaneous looks like 600 amps.
L3 also averages 400 amps, but the max instantaneous is about 575.

I believe the general rule is not to go above 80% of what the buss duct is rated for. When should I be worried about getting another duct in due to the high current draw?
Click to expand...

I do believe that you can load the duct to 100% as long as the loads are not continuous. If you have continuous loads then you need to stay at 80%. 80% of 800 = 640 amps continuous. Seems like you may be okay for a while.
 
Ragin Cajun said:
If the OC device protecting the bus is 100% rated, you're good for 800A continuous.

RC
Click to expand...

I don't believe that is true. The breaker is rated 100% for non-continuous loads and 80% for continuous loads. Art. 210.19
 
Dennis Alwon said:
I don't believe that is true. The breaker is rated 100% for non-continuous loads and 80% for continuous loads. Art. 210.19
Click to expand...
Some protective devices can operate at their rated amperage continuously and are so labeled. It is this type of OCPD that RC is referring to.
 
If you look, for example, in the Square D catalog, you will see the larger breakers are available in 100% ratings. They will always have electronic trip units that will walk the dog (lots of settings that come in VERY handy!) Of course, they will cost more as you are getting more. Your application will determine which you need.

If you have an older, standard molded case circuit breaker (MCCB) it's good for 80% continuous. Remember, you can still load it to 100% NON - continuously.

Panel boards, bus duct, etc. are rated for 100% IF the feeder breaker is rated for 100%.

RC
 
800 Amp rating is dependant on many factors.

The main one is ambient temperature and how hot it runs. In normal operating conditions assuming that all connections are done correctly it should be able to oprate at 800 Amps per phase. If in hot location or heat can not escape due to insulation the rating is reduced. On the other hand if in low ambient it can run hotter. The manufactures data sheet will state a temperature rise above ambient. Rule of thumb is for every 6 Degrees C above the designed temperature rating is it will halve the insulation life of the system. Now this temprature rating will apply to the hottest part of duct, where cooling my be limited.
 
.. keep the rule of thumbs coming...

.. keep the rule of thumbs coming...

SIMROX said:
800 Amp rating is dependant on many factors.

Rule of thumb is for every 6 Degrees C above the designed temperature rating is it will halve the insulation life of the system.
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I love a good rule of thumb, and that sounds like a good one!

Thanks.

JM
 
SIMROX said:
Rule of thumb is for every 6 Degrees C above the designed temperature rating is it will halve the insulation life of the system.
Click to expand...
At least it will never be zero. ;)
 
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