Feeder Sizes

[mstrlucky74]

So this is not making sense to me. MC-4 is being fed with4-4” w/ 4 #600 & 1 #4/0 ground from a 1600A switch in the main service gear. MC-7 & 10 are being fed with 4-3-1/2” w/ 4 #500 & 1 #4/0. Seems way oversized or too many parallel setsfor 800amps(MC-4 & MC-7. Agree? Conductors are THHN & Copper. Thanks.

 

[infinity]

Is that one riser with taps to the three MC boards? If so the 4-500 Kcmil protected at 1600 amps is no good unless the 1600 amp switch has smaller fuses in it. For whatever reason I cannot read your first attachment.

 

[petersonra]

I can't read your diagrams either.

It seems to me though that the conductors going to MC7 and MC10 are tap conductors and don't meet the tap rules since they don't terminate on a single set of OCPD.

 

[infinity]

I'm guessing that they reduced the conductor size since the load above the 4th floor is diminished. This will work if they use a 1500 amp OCPD in the main service gear.

 

[mstrlucky74]

Is that one riser with taps to the three MC boards? If so the 4-500 Kcmil protected at 1600 amps is no good unless the 1600 amp switch has smaller fuses in it. For whatever reason I cannot read your first attachment.

Yes it is. Why is it no good?

 

[mstrlucky74]

I can't read your diagrams either.

It seems to me though that the conductors going to MC7 and MC10 are tap conductors and don't meet the tap rules since they don't terminate on a single set of OCPD.

They are tap conductors. Explain a little further what you mean about not terminating on a single OCPD. Thanks.

 

[mstrlucky74]

I'm guessing that they reduced the conductor size since the load above the 4th floor is diminished. This will work if they use a 1500 amp OCPD in the main service gear.

I'm still confused. Why do we need 4 sets of 3-1/2" with #500's for 800A dist. board? Isn't that way over sized?

 

[infinity]

Yes it is. Why is it no good?

The riser portion consisting of 4 sets of 500 Kcmil does not meet the conditions of the tap rules so the use of a 1600 amp OCPD with conductors that only have an ampacity of 1520 amps is no good.

 

[mstrlucky74]

The riser portion consisting of 4 sets of 500 Kcmil does not meet the conditions of the tap rules so the use of a 1600 amp OCPD with conductors that only have an ampacity of 1520 amps is no good.

What are the tap rule conditions?

 

[infinity]

What are the tap rule conditions?

For your installation the total length of the tap conductors cannot exceed 25'.

 

[mstrlucky74]

For your installation the total length of the tap conductors cannot exceed 25'.

What part of the code can I reference this? Thanks,

 

[infinity]

240.21(B)(2)

240.21(B)(2) Taps Not over 7.5 m (25 ft) Long. Where the length of the tap conductors does not exceed 7.5 m (25 ft) and the tap conductors comply with all the following:
(1) The ampacity of the tap conductors is not less than one-third of the rating of the overcurrent device protecting the feeder conductors.
(2) The tap conductors terminate in a single circuit breaker or a single set of fuses that limit the load to the ampacity of the tap conductors. This device shall be permitted to supply any number of additional overcurrent devices on its load side.
(3) The tap conductors are protected from physical damage by being enclosed in an approved raceway or by other approved means.

 

[mstrlucky74]

240.21(B)(2)

Thanks. So back to my other question...aren't those feeders oversized to feed an 800A board?

 

[infinity]

Thanks. So back to my other question...aren't those feeders oversized to feed an 800A board?

Something is missing from this equation. On the riser drawing it appears that the riser is being tapped to feed the 800 amp boards but there must be some missing information. :?

How are the boards connected to the riser?

 

[mstrlucky74]

Something is missing from this equation. On the riser drawing it appears that the riser is being tapped to feed the 800 amp boards but there must be some missing information. :?

How are the boards connected to the riser?

What do you mean how are they connected? Off the taps they are being fed?

 

[infinity]

What do you mean how are they connected? Off the taps they are being fed?

You have a 1600 amp riser and 800 amp boards, how are the 800 amp boards connected to the riser?