Conductor Ampacity

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youngbuck

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Location
Blue Bell, PA
I have a generator and Load bank that is used to test the generator up to 200% loading. The generator should be tested at 150% for 5 minutes and 200% for 5 seconds. When sizing the conductors, if I were to size based on the 200% loaded it will virutally double my conductor size. Is there a way around this? Different insulation levels able to handle higher ampacity ratings, etc?
 

Dennis Alwon

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Chapel Hill, NC
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Not sure what would be required but I doubt a 5 second 200% load will affect a conductor that is rated 150% to begin with.
 

G._S._Ohm

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DC area
I have a generator and Load bank that is used to test the generator up to 200% loading. The generator should be tested at 150% for 5 minutes and 200% for 5 seconds. When sizing the conductors, if I were to size based on the 200% loaded it will virutally double my conductor size. Is there a way around this? Different insulation levels able to handle higher ampacity ratings, etc?
Conductors, and fuse wire, have thermal inertia.
It might take 15 minutes for a #14 conductor to stabilize at its final temperature so 5 seconds may result in no noticeable temp rise for any conductor of the sizes used in power circuitry.
 

youngbuck

Member
Location
Blue Bell, PA
Thanks for your help. I just spoke with a cable manufacturer and he stated that they have short circuit ratings on the conductors in which the conductor would be able to handle the 200% loaded for 5 seconds as long as it was sized for 150%.

He was not sure if it would protect the conductors if sized for 100%.
 

don_resqcapt19

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Illinois
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retired electrician
Table 1 in this document gives the current that will result in a conductor temperature of 150?C in five seconds. The current required to do that is far in excess of 200% of the conductor ampacity.
 

G._S._Ohm

Senior Member
Location
DC area
Table 1 in this document gives the current that will result in a conductor temperature of 150?C in five seconds. The current required to do that is far in excess of 200% of the conductor ampacity.
Thanks for the link.
The Onderdonk melting is better than the Preece formula for wire fusing currents.

This'll take me a week to digest. :lol:
IIRC, Gar measured wire temp vs. time but when I did it I noticed only a 1C rise with passing 20A from a dryer outlet through #14 copper Romex and it took a quarter hour to stabilize.
 

mayanees

Senior Member
Location
Westminster, MD
Occupation
Electrical Engineer and Master Electrician
OP,
You still need to comply with the NEC's ampacity limitations, such that the only way you could do what you're proposing is if the system is over 600 Volts. (240.100)
If it is over 600 Volts, plot the cable's thermal damage curve, then examine the magnitude and expected duration of your load and see where it lands.

John M
 
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