Can 1200A frame electronic circuit breaker be allowed for 630A service entrance?

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dissonant

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So I was looking at the Siemens SND 1200A frame adjustable trip circuit breaker to be used as the main breaker for a 630A service for a small apartment complex. The continuous current rating (Ir) has a 630A trip setting available but I'm concerned about effects of the long time delay and instantaneous pickup. The lowest time delay would allow 6 x Ir , or 3780A, for 2.5 seconds, but the instantaneous pickup can reduce this further if set to the lowest setting of 1500A. Is there a certain duration you can exceed the conductor ampacity? I'm not sure where to find this in the NEC.

My particular design is for 225kVA, 3 phase overhead service @ 208Y/120V. Looking at using AL SER cable (4-4/0 with 2/0 GND) with 3 - 4/0 conductors per phase.
 
Given the type of load your plan seems overly complicated with adjustable trip circuit breakers. Personally with a calculated load of 630 amps I would just install an 800 amp service.
 
This piqued my interest - to look at the thermal damage curve for one versus three 4/0 conductors used as a parallel assembly, so I plotted it out - attached.

I think the answer to your question is Yes, the 1200-amp frame adjustable trip breaker can provide protection provided there is restricted access to the settings per 240.6(C). On the attached TCC I plotted an 800-amp trip unit with an adjustment of 80% for the LTPU = 640 amps since I don' have the 630-amp IEC model trip unit in my device library. I think this is also permitted because of the roundup rule for devices under 800 amps.

I've shown thermal withstand limits for 1-4/0 and for 3-4/0s, and I've also shown the Siemens 640-amp trip breaker adjusted to maximum, and adjusted to protect the single 4/0 through most of it's damage curve. Your responsibility is only to the NEC such that overcurrent protection is met for the assembly, so from that standpoint it works.

But, the correct way to develop the settings for the 640-amp trip breaker is to provide adjustments that allow coordination with the primary fuse for the transformer. Given this is powering an apartment complex there shouldn't be significant inrush which could affect the settings recommendation.

So I suggest getting that information from the Utility and having someone plot the devices to assure that the primary fuse only trips for internal transformer faults, ahead of the secondary main breaker. If you give me the primary fuse part number and voltage I'll be happy to plot it out.

✌️ & 👣
 

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infinity said:
How do you get 630 amps out of 3 sets of #4/0 aluminum?
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I used a conservative SKM library for Aluminum conductors and since the OP used 630 amps that's what I selected. The OP stated free-air such that if 310.17 were used that number could easily be 900 amps.
 
dissonant said:
Using Table 310.15(B)(16) for 90 Deg AL, 4/0 = 205A x 3 conductors = 615A. I am planning on 3 separate 3" RMC conduits for entrance into the electrical room from the weatherhead.
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You're limited to the 75° C ampacity which is 180 amps. If your calculated load is 630 amps then that is your minimum conductor size at 75° C.
 
petersonra said:
can you actually use the 90 deg column?
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Hmm, I am having trouble finding if its listed for 90 deg. Most of their other molded case CBs are rated for 75 deg, and the spec sheet only mentions 75deg rated lugs, so I will probably switch to parallel 350MCM CU as they don't have AL in larger than 4/0 stocked here. Since its rated for 620A and less than 800A, I know I'm allowed to specify the next higher breaker of 700A for a thermal magnetic breaker but was looking at the electronic breaker to be closer to the cable rating. I don't know the fuse rating for the utility pole mounted transformer bank yet. My actual calculated load is 544A, but thats with a 35% demand factor for the multi-family dwelling. If I change one assumption of the range from 7500W to 9200W, then it jumps to 583A, so wanted to provide as much additional capacity as possible, within the limitation of the 225kVA utility transformer.
 
dissonant said:
Hmm, I am having trouble finding if its listed for 90 deg. Most of their other molded case CBs are rated for 75 deg, and the spec sheet only mentions 75deg rated lugs, so I will probably switch to parallel 350MCM CU as they don't have AL in larger than 4/0 stocked here. Since its rated for 620A and less than 800A, I know I'm allowed to specify the next higher breaker of 700A for a thermal magnetic breaker but was looking at the electronic breaker to be closer to the cable rating. I don't know the fuse rating for the utility pole mounted transformer bank yet. My actual calculated load is 544A, but thats with a 35% demand factor for the multi-family dwelling. If I change one assumption of the range from 7500W to 9200W, then it jumps to 583A, so wanted to provide as much additional capacity as possible, within the limitation of the 225kVA utility transformer.
Click to expand...
see post 2
 
dissonant said:
Hmm, I am having trouble finding if its listed for 90 deg. Most of their other molded case CBs are rated for 75 deg, and the spec sheet only mentions 75deg rated lugs, so I will probably switch to parallel 350MCM CU as they don't have AL in larger than 4/0 stocked here. Since its rated for 620A and less than 800A, I know I'm allowed to specify the next higher breaker of 700A for a thermal magnetic breaker but was looking at the electronic breaker to be closer to the cable rating. I don't know the fuse rating for the utility pole mounted transformer bank yet. My actual calculated load is 544A, but thats with a 35% demand factor for the multi-family dwelling. If I change one assumption of the range from 7500W to 9200W, then it jumps to 583A, so wanted to provide as much additional capacity as possible, within the limitation of the 225kVA utility transformer.
Click to expand...
That Sentron CB will work fine with your application. There is a 630A continuous current setting on the CB options on a 1200A frame Siemens.
 
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