bergren
New member
- Location
- Los Alamos, New Mexico
I am working on modifying an existing installation. Let's set the scene. There are two generators, each rated at 975kW (1465A), each feeding an individual 1600A breaker before being combined on a 4000A switchgear bus. From there we are feeding, via cables, a 2500A circuit breaker. There are four 1/C 535.5 MCM cables per phase, each rated at 644A at 75C (interpolated from NEC Table 310.15(B)(17)). They are installed in 2 x 24" wide ladder style trays (2 of the 4 parallel runs per tray) with the maintained spacing that allows us to use 392.80(A)(2)(c). There is a single 535.5MCM ground in one of the two trays, and no ground in the other tray. There is no neutral (HRG at each generator).
The goal of the upgrade is to be able to utilize the full output capacity of both generators in parallel. Thus we will be increasing the size of the power circuit breaker from 2500A to 3200A (which will be 100% rated allowing me to use 215.2(A)(1) Exception 1 and size this feeder to the combined generator output with no multiplication factor: 1465x2=2930A). To get adequate ampacity for a 3200A breaker, I need to add one more parallel run of the identical cable to one of the two trays. My question is three-fold.
1. I believe that, according to 250.122(F), I need one EGC per tray. I can't use the tray as the EGC since the system is over 2000A, but since the parallel feeds aren't all in the same tray, each tray needs an EGC. Is this a correct assessment?
2. When sizing the ground according to 250.122 I know you're supposed to use the overcurrent device ahead of the cable. However sizing it to a 1600A breaker seems wrong, requiring only a 4/0. Since there are two 1600A breakers in parallel, do I add them together? That would then put me up to a 500 MCM ground per tray. I can't seem to find a straight answer on this in the code.
3. Let's assume we need an EGC in each tray (this question is moot if we don't). In one tray you will have 3 parallel runs of 1/C 535.5 MCM plus 1 EGC = 10 cables. Do I need to maintain the spacing between the EGC and the current carrying conductors? Trust my math on this that with spacing between all 10 cables, I'd need 25" of tray and that by eliminating one of those spaces I can use the existing 24" tray. So my proposed installation would be from left to right (- represents a space): |A-B-C-A-B-C-A-B-CG|
Thanks in advance for your replies!
The goal of the upgrade is to be able to utilize the full output capacity of both generators in parallel. Thus we will be increasing the size of the power circuit breaker from 2500A to 3200A (which will be 100% rated allowing me to use 215.2(A)(1) Exception 1 and size this feeder to the combined generator output with no multiplication factor: 1465x2=2930A). To get adequate ampacity for a 3200A breaker, I need to add one more parallel run of the identical cable to one of the two trays. My question is three-fold.
1. I believe that, according to 250.122(F), I need one EGC per tray. I can't use the tray as the EGC since the system is over 2000A, but since the parallel feeds aren't all in the same tray, each tray needs an EGC. Is this a correct assessment?
2. When sizing the ground according to 250.122 I know you're supposed to use the overcurrent device ahead of the cable. However sizing it to a 1600A breaker seems wrong, requiring only a 4/0. Since there are two 1600A breakers in parallel, do I add them together? That would then put me up to a 500 MCM ground per tray. I can't seem to find a straight answer on this in the code.
3. Let's assume we need an EGC in each tray (this question is moot if we don't). In one tray you will have 3 parallel runs of 1/C 535.5 MCM plus 1 EGC = 10 cables. Do I need to maintain the spacing between the EGC and the current carrying conductors? Trust my math on this that with spacing between all 10 cables, I'd need 25" of tray and that by eliminating one of those spaces I can use the existing 24" tray. So my proposed installation would be from left to right (- represents a space): |A-B-C-A-B-C-A-B-CG|
Thanks in advance for your replies!