Need to increase feeder conductors to accommodate fuse coordination?

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When fuse coordination (black-out prevention) is required between a control panel and its bus plug, even when putting FRS fuses in the plug and Js in the panel disconnect, the fuse in the bus plug should be 1.5 times the setting of the one in the panel. Does that mean that the feeder conductors need to be sized to the bus plug fuse or can they be sized for the load? For example: A panel with a 80 amp load might have a LPJ100 fuse in the main disconnect to allow for future expansion. If an FRS150 fuse is put in the bus plug, do the feeders need to go to 1/0?
 

charlie b

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I don't know much about fuse types or numbers. But I do know that all conductors must be protected at their ampacity, and that most of the time (tap rules being an exception) that protection must happen at the point they get their power. So if you put a 150 amp fuse in a bus plug, then the wires leading downstream from that point must have an ampacity of 150 amps.
 
Thanks Charlie.

I need to make sure that this could not also fall under the same principle as tap conductors (because these feeders also terminate in an over current device like taps do) or like motor conductors where a fuse which could be considered too large to protect the conductors from an overload can be placed ahead of those conductors with the understanding that the thermal overload relay downstream provides that protection.
 

augie47

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Rich,
I have only been an "observer" on some coordination studies, but on the few I've seen there was no "1.5 rule" used. They seemed to address it more with fuse Class/ current curves.
 

charlie b

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Gus, I think it is more of a rule of thumb. It just turns out that, without having to do any other thinking or calculating, you can conclude that if the upstream fuse has a rating that much higher than the downstream fuse, then their fuse curves will generally give you a successful coordination.
 
The 1.5:1 ratio was taken from table 5.2.1 provided by Bussmann in their Selecting Protective Devices Handbook and was referring to RK5s in the bus plug and LPJs in the panel. There are many other combinations for other classes of fuses.
 

petersonra

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Northern illinois
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engineer
This sounds like a tap to me. Does it even need a fuse at the bus plug?

If that bothers you why not just remove the fuses in the control panel?

I am not a big fan of fuses in series for no good reason.
 
It kinda sounds like a tap to me too, but I'm pretty sure it is not.

Replacing a fuse in a bus 40 feet up can take hours in these plants. They have to get a JLG, safety harness, protective suits, move production workers, etc. We do not want these to open, ever if possible.

Cannot just do away with the fuses in the panel main disconnect.
 

jim dungar

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It kinda sounds like a tap to me too, but I'm pretty sure it is not.

Replacing a fuse in a bus 40 feet up can take hours in these plants. They have to get a JLG, safety harness, protective suits, move production workers, etc. We do not want these to open, ever if possible.

Cannot just do away with the fuses in the panel main disconnect.
It is actually very simple because you are dealing with feeder circuits to a control panel. If the conductors from the bus plug are smaller than the fuse, you are dealing with the tap rules of 240.21(B).

Because of the hassle in changing fuses, many of my customers always size fuses and conductors to the maximum possible for the bus plug. Other only use breakers because they can be reset from a distance. In either case coordination with the floor level equipment is a concern.
 
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