Wire size after disco fuse

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Dmc16

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Sparta,nj,United states
Hello all it’s been a while..
So have to replace roof top ac.
3 phase 480 volt 60 amp. Fed from old federal pacific gear and don’t want to change fuses etc.
max fuse on new is 25 amps. I know it’s possible to fuse it down once it hits disconnect.
My question is on the load side does code allow me to size wires per the 25 amp fuses or do I size according to feeder.
Any code references are appreciated. Having trouble finding answer.
Thanks
 
Dmc16 said:
Hello all it’s been a while..
So have to replace roof top ac.
3 phase 480 volt 60 amp. Fed from old federal pacific gear and don’t want to change fuses etc.
max fuse on new is 25 amps. I know it’s possible to fuse it down once it hits disconnect.
My question is on the load side does code allow me to size wires per the 25 amp fuses or do I size according to feeder.
Any code references are appreciated. Having trouble finding answer.
Thanks
Click to expand...

You would fuse according to the nameplate not the feeder. The feeder should be protected at the panel.
 
Dmc16 said:
And yes I agree on feeder. It’s protected at panel. So my thought is treat it like a “branch circuit” and use wire based on fuse size at disconnect.
Click to expand...
As long as your fuse is not over 25 amps and your wire is good for 22.1 amps you're gold.
 
Dmc16 said:
Max is 25
Min 22.1
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In addition to your original question, I don't think you fully understand OCPD and conductor sizing for equipment such as this. The data plate is not telling you the max and min of the OCPD. The data label on the unit will give you an MCA value and you use this for conductor sizing. The label will also tell you the max OCPD rating and you use that to determine the OCPD size. In other words, the conductors don't have to have the same ampacity as the OCPD.
If your label states a max OCPD of 25 then you should use that vale and if you are using a breaker, yes, that is a standard value. And no, you can't use a 30 because you supplier does not stock 25 amp breakers. The conductors only need an ampacity of of 22.1, so you could use 12 gauge 75 degree rated conductors.
 
texie said:
In addition to your original question, I don't think you fully understand OCPD and conductor sizing for equipment such as this. The data plate is not telling you the max and min of the OCPD. The data label on the unit will give you an MCA value and you use this for conductor sizing. The label will also tell you the max OCPD rating and you use that to determine the OCPD size. In other words, the conductors don't have to have the same ampacity as the OCPD.
If your label states a max OCPD of 25 then you should use that vale and if you are using a breaker, yes, that is a standard value. And no, you can't use a 30 because you supplier does not stock 25 amp breakers. The conductors only need an ampacity of of 22.1, so you could use 12 gauge 75 degree rated conductors.
Click to expand...
Is MCA another measurement to kcmil? Or what is the meaning of MCA?


If you size the branch conductor (tapped?) from a feeder protected with an OCPD on its line side, then wouldn’t the “tapped” branch circuit have to terminate to the device with an OCPD on its load side (overload protection but not short circuit protection) with an ampacity less than or equal to the feeder OCPD regardless if the devices nameplate calls for a higher FLA full load ampere or current rating?
 
texie said:
In addition to your original question, I don't think you fully understand OCPD and conductor sizing for equipment such as this. The data plate is not telling you the max and min of the OCPD. The data label on the unit will give you an MCA value and you use this for conductor sizing. The label will also tell you the max OCPD rating and you use that to determine the OCPD size. In other words, the conductors don't have to have the same ampacity as the OCPD.
If your label states a max OCPD of 25 then you should use that vale and if you are using a breaker, yes, that is a standard value. And no, you can't use a 30 because you supplier does not stock 25 amp breakers. The conductors only need an ampacity of of 22.1, so you could use 12 gauge 75 degree rated conductors.
Click to expand...
You have to be careful with saying #12. If it is THWN, then fine, if NM cable, then no, you would have to use 10-2. I know you said 75 Deg conductors, but just in case someone reads this and thinks all #12 is good for 22.1A.
 
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