NEC 240.86, Section C, provision 1 ?

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intern

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Greetings,
I'm having trouble interpreting this part if the code:


"C) Motor Contribution Series ratings shall not be used where

1) Motors are connected on the load side of the higher-rated overcurrent device and on the line side of the lower-rated overcurrent device..."[/​
INDENT]

Could some one refer me to an illustration of this? Is it insinuating that one should not connect a motor to a bus with out an OCPD? Who would do such a thing!?!
:confused:​
 
Not a motor _directly_ connected to the bus without an OCPD, but a motor which contributes to the available short circuit current, which is connected to the _same_ bus via its _own_ OCPD.

With a series rating, you have two circuit breakers, electrically in series, where the 'downstream' breaker has a lower short circuit current rating than the 'upstream' breaker, and the 'upstream' provides protection for extreme short circuits. The problem is that the bus in the panel is a common tie point that is downstream of the main breaker and upstream of all the other breakers. This means that any short circuit current contributed by any motors connected (via OCPD) to the bus will bypass the main breaker and go directly to the short.

-Jon
 
Section 240.86 is titled "Series Rating", This section addresses the application of ocpd's (breakers) and their adequacy in relation to the systems available fault current.

See 110.9 and 110.10 for a start. If a series system is installed the theory is that the down stream breakers are coordinated with the upstream OCPD to clear short circuits with out violating 110.9. These systems are rated by the manufacture (usually) and consider the short circuit interrupting rating of the OCPD's and the point to point availble fault current. If a down stream device where to try to clear a short in a series rated system that exceeds that individual breakers rating, and the up stream device reacts, you will have two contacts opening at the same time. This creates a situation referred to as dynamic impedance. The upstream device will take longer to clear because the additional resistance will reduce the peak fault current. The increased time to clear essentially violates 110.9 and may in fact never clear prior to burn off.

Why all this info.

If a breaker starts to clear to protect a down stream device that has also begun to clear and a motor is in between the two, then the motor will coast and act as a generator and possibly add to the available fault current. Violates the series rating

So section (C) was added (I think in 99) to cover this situation.

Thats how I see it and hope this helps

Charlie
 
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Thanks for clearing this up. I understand the concept of motor contribution but I'm clear as to what measures I'd have to take to be able to power the motor from a the bus.
Thanks,
intern
 
intern are you an engineer whose disipline deals primarly with series rated system??

If not, you would need one or consult the manufacture with the motor data to determine if it will create an issue.
 
EE Co-Op

EE Co-Op

Cpal,
In answer to your question, I am an electrical engineering intern who has been asked to write something and give a brief presentation on series rating. (For a company who does employ series rated systems in their designs.) The problem is that I have no practical engineering experience... In school I do lots of calculus all day.
So, in conclusion, I don't want to look like an idiot.
Regards,
Intern
 
Intern,
While the link that Charlie has provided has a lot of good infomation, you have to remember that it was written by a fuse manufacturer and does not always show circuit breakers in their best light.
Don
 
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