TAP Question- 800 Amp Service

[aelectricalman]

I have two buildings (on one property) that currenty have 2 services (500 amp and 200 amp). We are combining both services in the courtyard of the two buildings, and making it an 800 amp CT Cabinet and meter located on a new concrete pad. Immediately following is a new 800amp Service Entry Rated transfer switch. The transfer switch has only 1 Main circuit breaker. The problem is, I am delivering power to two seperate buildings. One building is a 500 amp panel, the other is a 200 amp panel. I am trying to stop short of installing an 800amp main lug Nema 3R $$$$$$ panel next to the transfer switch to house two breakers (500 amd 200 amp). I would land the 800amp feeders on the buss and distribute the power accordingly to both existing panels in the buildings (mains currently exist at both panels). I don't know everything about tap rules but my distributor is telling me that I can take the 800 amp feeder out of the transfer switch to a trough and tap the proper sized wiring for each panel according from the lugs, therefore alleviating the need to have a main lug panel next to the transfer switch. He put a couple people in touch with me that both told me it was ok to do. Tell me I'm not crazy! I can not see me taking an 800 amp rated wire(s) to a lug where I split it into a 500 amp rated wire(s) and a 200 amp rated wire(s), thus distributing these wires to the existing panels in the building. I do not buy that I can place a 3/0 copper for a 200 amp service and 2 sets of 4/0 copper for a 500 amp service on the same lugs with the 2 sets of 350kcmil for the 800amps service without having an overcurrent means between them. Granted both existing panels have overcurrent protection at the appropriate size, the wires from the trough feeding the two panels will not be rated at 800amps (the size of the main in the transfer switch). How is this ok? Does anyone have a code reference I can piggyback on? Thanks again.

 

[augie47]

It appears you may be able to hang your hat on 240.21(B)(5), Outside taps, but you might want to read the section as the devil is in the details.

 

[curt swartz]

How are you comming up with your wire sizes? Parallel 4/0's are not large enough for 500 amp tap conductors and parallel 350's are not large enough for 800 amps.

 

[augie47]

How are you comming up with your wire sizes? Parallel 4/0's are not large enough for 500 amp tap conductors and parallel 350's are not large enough for 800 amps.

Unless his calculated load is greater than the ampacity or it's a transformer secondary, he should be good to go under 240.4

 

[RUWired]

Unless his calculated load is greater than the ampacity or it's a transformer secondary, he should be good to go under 240.4

See 240.21(B). 240.4 not allowed.

 

[aelectricalman]

Your right and your wrong all in the same sentence.

Your right and your wrong all in the same sentence.

Youre right on the service entry for the 800 amps. I meant to say 500kcmil copper. Youre wrong on the Parallel 4/0 though. Its good for 560 amps paralleled derated ampacity.

 

[RUWired]

Youre right on the service entry for the 800 amps. I meant to say 500kcmil copper. Youre wrong on the Parallel 4/0 though. Its good for 560 amps paralleled derated ampacity.

Parallel 4/0 conductors are only good for 460 amps at 75 degree c. The next higher is not permitted for tap conductors.

 

[Smart $]

Youre right on the service entry for the 800 amps. I meant to say 500kcmil copper. Youre wrong on the Parallel 4/0 though. Its good for 560 amps paralleled derated ampacity.

Parallel 4/0 conductors are only good for 460 amps at 75 degree c. The next higher is not permitted for tap conductors.

...and parallel 500's are only good for 760A max. [75?C termination temperature limitation per 110.14(C)]. Minimum size required to supply an 800A service disconnecting means is parallel 600's.

Getting back to the building feeders, they can land directly on the load lugs (i.e. three-hole) of the transfer switch, as long as the lugs are appropriate for the two different sizes of wires. No need to run out parallel 600's to a trough, eliminating all the extra materials and terminations...

 

[augie47]

See 240.21(B). 240.4 not allowed.

Thank you, Sir. Even ole dogs..........
I realized that was true for 240.21(C) but had overlooked (B)...
wonder how many times I've let that slip by....

 

[RUWired]

...and parallel 500's are only good for 760A max. [75?C termination temperature limitation per 110.14(C)]. Minimum size required to supply an 800A service disconnecting means is parallel 600's.

...

The load side of the transfer switch are standard "feeder conductors" which are protected by the next higher rule unless these conductors are being fed from a SDS.

Rick

 

[kwired]

The load side of the transfer switch are standard "feeder conductors" which are protected by the next higher rule unless these conductors are being fed from a SDS.

Rick

240.4 does not allow next size higher overcurrent device if the overcurrent device is over 800 amps. (does not apply in this case)

240.21(B) does not allow the use of next size higher overcurrent device for tap conductors. Tap conductors in this case have to be 500 amps and 200 amps minimum. That would be either 900kcmil or parallel 250 copper for the 500 amp tap and 3/0 copper for the 200 amp tap.

aelectricalman:

240.21(B)(5) allows tap conductors to be unlimited length if installed outdoors.
They must comply with all these conditions:

(1) The conductors are protected from physical damage in an approved manner.
(2) The conductors terminate at a single circuit breaker or a single set of fuses that limit the load to the ampacity of the conductors. This single overcurrent device shall be permitted to supply any number of additional overcurrent devices on its load side.
(3) The overcurrent device for the conductors is an integral part of a disconnecting means or shall be located immediately adjacent thereto.
(4) The disconnecting means for the conductors is installed at a readily accessible location complying with one of the following:
a. Outside of a building or structure
b. Inside, nearest the point of entrance of the conductors
c. Where installed in accordance with 230.6, nearest the point of entrance of the conductors

You could tap a 14 AWG from this 800 amp feeder if it meets all of these conditions.

 

[RUWired]

240.4 does not allow next size higher overcurrent device if the overcurrent device is over 800 amps. (does not apply in this case)

240.21(B) does not allow the use of next size higher overcurrent device for tap conductors. Tap conductors in this case have to be 500 amps and 200 amps minimum. That would be either 900kcmil or parallel 250 copper for the 500 amp tap and 3/0 copper for the 200 amp tap.

.

Kwired i was referring to the 500 kcmil conductors from the load side switch to the tap wireway. I already stated the 4/0 conductors were inadequate.

Rick

 

[Smart $]

The load side of the transfer switch are standard "feeder conductors" which are protected by the next higher rule unless these conductors are being fed from a SDS.

Rick

The 500's vs. 600's mentioned are on the line side of the ATS—the Service Disconnecting Means from what I gather per OP. That makes them service conductors.

I have two buildings (on one property) that currenty have 2 services (500 amp and 200 amp). We are combining both services in the courtyard of the two buildings, and making it an 800 amp CT Cabinet and meter located on a new concrete pad. Immediately following is a new 800amp Service Entry Rated transfer switch. The transfer switch has only 1 Main circuit breaker.

Additionally, the panel feeders are actually feeder taps. The SDM rating has no bearing on their sizing other than it being greater than normally rated for the size of conductor. That is, the next-size-up does not apply here on either side of the ATS.

 

[Smart $]

Kwired i was referring to the 500 kcmil conductors from the load side switch to the tap wireway. ...

This is probably where we are getting issues crossed. I am picturing the installation without wireway, where the feeder tap point is the load side lugs of the ATS.