QUICK CALCULATION LESSON

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Alwayslearningelec

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When you have a disconnect/safety switch feeding a panel or another switch what amperage do you size the feeder off of. THe size of the switch etc. THAT IS FEEDING the other switch or panel, correct? Not the size of the equipment beinf fed?? So if you have a 12000A switch feeding a 200 or 400a switch the feeder would have to be rated for 1200a no matter what size the equipment being fed is?? I know there are other factors such as voltage drop(3%) type of conductor, in/out of conduit etc. which 310.16 gives info on.

Looking for a just a guideline....Thanks.

I am going to buy MH calculations book....
 
Think about this. If you have a 200 amp panel would you run 200 amp feeders to a 60 amp furnace?

The feeder is sized for the equipment you are supplying.
 
horsegoer said:
...

Looking for a just a guideline...
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Sounds to me like you are mixing the various ratings involved in circuit design. Usually conductors are sized, at a minimum for the load they must carry... but they can be larger. Then the ocpd rating is determined so as to protect the conductors and perhaps the connected equipment. Then last is the determination of equipment ratings, which is what I believe you are referring to...

For example, a 200A switch/disconnect rating only means it can withstand a current of 200A without fault or degrading. If it is an unfused disconnect, the circuit feeding it cannot have a rating greater than 200A. If it is a fused disconnect, the circuit it feeds could be rated 200A or less, but never more than 200A. The supplied circuit rating is dermined by the conductor and fuse size, not the switch rating. Regarding feeder and feeder taps, the fused disconnect could be supplied from a feeder circuit having any rating. The fuse size would limit the current to 200A or less and sized to protect the circuit and, if applicable, feeder tap conductors.
 
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Dennis Alwon said:
Think about this. If you have a 200 amp panel would you run 200 amp feeders to a 60 amp furnace?

The feeder is sized for the equipment you are supplying.
Click to expand...

No... you would be feeding from the breaker in the 200a panel...so the size of that breaker feeding the equip would be the feeder size...right?
 
ptonsparky said:
12000, 1200 whatever it takes. Try getting 1200 amp conductors into a 200 amp switch.

Look up the section for taps in the NEC. Generally 240.21 but there are others for branch circuits & motors.
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So are the conductors sized per the size of the supply or the load?
 
horsegoer said:
So are the conductors sized per the size of the supply or the load?
Click to expand...

Conductors are sized to the load. With Voltage Drop, distance, ambient tempature that you

take into account. Look at the panel in your apartment. The wires serving microwave, GFCI's

etc, are more than likely 12 AWG. on a 20 amp breaker.
 
horsegoer said:
So are the conductors sized per the size of the supply or the load?
Click to expand...

A combination of both. Distance will be one of the factors. Ten feet or less and you can just about tap off the 1200 with any size as long as you are feeding a properly sized overcurrent/short circuit protective device. Say 30 amp for #10s. Twenty five feet puts a more restrictive size limit to the wire before fusing, and 100' in some locations changes things again.

The conductors still need to be sized at minimum for the load, but the location of the feeder fusing (CB) will also determine the size of the the feeder tap conductors.

Hope this helps and not just confuses more, fluency is not one of my attributes. Only looks.
 
Depending on the load you may have to add 125% to the nameplate or the load being served. For instance if a motor runs at 45 amps we would have to add 125% to that load. Now instead of 45 amps we need conductors rated 56.25. I could have a 100 amp disconnect at the equipment and only a 60 amp breaker with #6 conductors feeding the disco.
 
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