Bus Rating Question

cdcengineer

Senior Member
I ran into a situation with an existing installation today. Odd service that has definitely been modified since it was first constructed.

The service consists of a 1,200 A, 208Y/120V, 3-phase main service disconnect which is hot sequence metered - all good so far. There are (3) sets of 500 kcmil CU conductors running into a tap box where there is a parallel set of 500kcmil AL feeders serving a 600 A switchboard (SWBD). The other tapped set consists of 500 kcmil CU which serves a 400 A panelboard. All of this equipment is less than 5' away from each other.

The first red flag I noted was that the (3) sets of 500kcmil CU were run in two conduits (violating 300.5(I) & 300.20). Not all phase and neutral run in same raceway. This got us looking closer. We then realized that the (3) 500 kcmil CU were not large enough for the 1200 A main. We couldn't identify this at first because the main was not accessible.

The calculated load for the building is just over 400 Amps. I proposed a solution that we remove one set of the (3) 500 kcmil CU leaving 760 Amps. We eliminate the tap situation serving only the 600 Amp SWBD. We would then re-feed the 400 A panelboard from a spare 400 A bucket in the SWBD.

All seemed good-to-go. However, I would never specify a 1200 A main ahead of a 600 A rated bus. The loads are less than 600 A, but if this was a new installation, we would buy a 600 A main. FYI - the SWBD consists of (2) 200A/3P & (1) 400A/3P fusible buckets.

Throwing out the extraneous info in the questions, the real question boils down to whether it is allowed to serve a 600 Amp bus from a 1200 A main? I know per exhibit 230.26 the sum of the OCP can exceed the ampacity of the service conductors. I first thought that we would violate 705.12 (120% rule), but this is not applicable.

I'm wondering if anyone can massage this brain cramp? I would never try and do something like this from the get-go, but because it's existing and they're trying to leave as much existing as possible, what is the gotchas I'm not seeing? I know there would be potential for loads to exceed the 600A bus which would not be protected against via OCP, but that could potentially happen in a tap scenario if we had two 600 A SWBD tapping say (3) SETS OF 600 kcmil CU feeders served from a single 1200 A main.

Any input is greatly appreciated as always.

DC
 
panelboards must be protected at their rating, see 408.36. Also I am not clear on which conductors are on the line side of the service disconnect and which are on the load side, but note that the conductors on the load side could be taps and thus have an ampacity less than the 1200 main. The conductors on the line side of the main must have a rating equal or greater that the main - no next size up rule over 800 amps....
 

cdcengineer

Senior Member
The (3) sets of 500 CU are on the load side of the main service disconnect. They run into a junction box which is mounted back to back from the main. In the junction box is where the 500's are then tapped. I would say that the (3) 500 CU are undersized for the 1200 A main because they are not taped at the load side of the main, but tapped downstream (although less than 10').

Good call on 408.36. I felt like there was something I wasn't finding. Does the same requirement hold true for switchboards?

This service has been cobbled together. I'm thinking the way to make it right is to remove the 1200 A main and replace with a 600 A, leave only two of the three sets of 500 CU feeding only the 600 A switchboard. Form the SWBD we re-feed the MDP.
 

kwired

Electron manager
Location
NE Nebraska
The (3) sets of 500 CU are on the load side of the main service disconnect. They run into a junction box which is mounted back to back from the main. In the junction box is where the 500's are then tapped. I would say that the (3) 500 CU are undersized for the 1200 A main because they are not taped at the load side of the main, but tapped downstream (although less than 10').

Good call on 408.36. I felt like there was something I wasn't finding. Does the same requirement hold true for switchboards?

This service has been cobbled together. I'm thinking the way to make it right is to remove the 1200 A main and replace with a 600 A, leave only two of the three sets of 500 CU feeding only the 600 A switchboard. Form the SWBD we re-feed the MDP.
Is this main a circuit breaker or fused switch? If fused why not replace with 1000 amp fuses and then conductor sizes are fine.

Not quite sure I understand what you have for feeders and feeder taps though. You say they are tapped less then 10 feet downstream. A feeder tap starts at the point where the conductors are smaller then the size required by the overcurrent protection, so you can have hundreds of feet of 1200 amp feeder but the feeder taps do not start until you connect smaller conductors to the 1200 amp feeder. But no matter which tap rule applies the tap must end in a single overcurrent device so if hitting a main lug switchboard or panelboard you either need a main or reduced overcurrent protection on the feeder if load calculations still work out for it.
 
Is the main gear a switchboard or panelboard? There doesnt appear to be a requirement to protect a switchboard at its rating - interesting, that never occurred to me.

Ok so it appears you have a feeder coming off the 1200 main (that is tapped down the line a few feet) so it would need to have an ampacity of 1200 or more. It cant be considered a tap because you cant tap a tap. However, could you just rearrange things a bit and run two taps from the load side of the main to each panelboard? Note that it has been istabished that a tap can start on the load side terminals. That might be the cheapest option. Are the panelboards main breaker (they should be even as is)?
 

kwired

Electron manager
Location
NE Nebraska
Can you post a one line diagram of what you have to help clarify what you have?

I think you are saying the 1200 amp service is fine, but the feeder it supplies has too small of conductors, and some of the taps from the feeder supply main lug panelboards and/or switchboards that are not protected at their rating.
 

Smart $

Esteemed Member
Location
Ohio
... Does the same requirement hold true for switchboards?
No.

This service has been cobbled together. I'm thinking the way to make it right is to remove the 1200 A main and replace with a 600 A, leave only two of the three sets of 500 CU feeding only the 600 A switchboard. Form the SWBD we re-feed the MDP.
For 5', pull all existing conductors and replace with 2 sets of 350 CU or 500 AL to the SWBD. Complies with 240.21 tap rule, not over 10'.

What are the conduit sizes? I'm wondering if you can get in 2 sets of 500's to the SWBD and 2 sets of 250's to the MDP and be done with it. Still refeeding, but it will be a more robust solution.
 

cdcengineer

Senior Member
No.


For 5', pull all existing conductors and replace with 2 sets of 350 CU or 500 AL to the SWBD. Complies with 240.21 tap rule, not over 10'.

What are the conduit sizes? I'm wondering if you can get in 2 sets of 500's to the SWBD and 2 sets of 250's to the MDP and be done with it. Still refeeding, but it will be a more robust solution.
Conduits are (2) 4" from main to tap box. From tap box we have two 3-1/2" to SWBD and (1) 3" to MDP.

I tried to upload a one-line sketch but it appears I am limited to file size. How would I scan a jpg and keep it small enough to submit as attachment?

Thanks to all for the feedback
 

kingpb

Senior Member
Why not run another conduit for the third set of conductors and leave the 1200A main? I don't think there is an issue with having a 1200A breaker ahead of the 600A board, but you will need 1200A worth of cable, IMO anyway.
 

Smart $

Esteemed Member
Location
Ohio
Conduits are (2) 4" from main to tap box. From tap box we have two 3-1/2" to SWBD and (1) 3" to MDP.

I tried to upload a one-line sketch but it appears I am limited to file size. How would I scan a jpg and keep it small enough to submit as attachment?

Thanks to all for the feedback
Sounds like double two sets will work... but you haven't mentioned whether a 3- or 4-wire system or whether any neutrals, so I didn't crunch the fill numbers.

From SDM to JB, each 4" conduit (2)
(1) set 500kcmil
(1) set 250kcmil
(1) 3/0 EGC

From JB to SWBD, each 3.5" conduit (2)
(1) set 500kcmil (pulled through unspliced)
(1) 3/0 EGC

From JB to MDP, 3" conduit
(2) sets 250kcmil (pulled through unspliced)
(1) 3/0 EGC

All copper. Each set derated 80% (assumed 6 CCC's). EGC's will requiring splicing in tap box since you'll have two coming in and three leaving.

Additionally assumed not more than 10' of conductor. You said equipment was no more than 5', but you have to go by conductor length.
 
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kwired

Electron manager
Location
NE Nebraska
Conduits are (2) 4" from main to tap box. From tap box we have two 3-1/2" to SWBD and (1) 3" to MDP.

I tried to upload a one-line sketch but it appears I am limited to file size. How would I scan a jpg and keep it small enough to submit as attachment?

Thanks to all for the feedback
Attachments in this site stink, use a third party site like photobucket and link to it in your post and you can post all kinds of things not allowed by attachments.
 
Why not run another conduit for the third set of conductors and leave the 1200A main? I don't think there is an issue with having a 1200A breaker ahead of the 600A board, but you will need 1200A worth of cable, IMO anyway.
As long as those panels are main breaker to protect them at their rating - the OP hasnt said whether they are MB or MLO.
 

cdcengineer

Senior Member
Sorry for the delayed response. Long day today. Had a childhood (thru college) buddy pass away. He was 42. Woulda been 43 in June. I haven't seen him in years, probably 15, but old friendships (at least the closest ones I have) always seem strong. Conversations pick up where they left off and there is no effort to BS'ing with old buddies. Don;t know the details, but he left work feeling sick and never showed up the next day. They sent someone to check on hime since he never missed work. He was gone - I'm guessing heart attack, and he wasn't in bad shape. Wow! Makes a guy think and appreciate what he has. It can be all for nothing in a blink of an eye.

Back on point... It's a 4-wire system with neutrals.

As for the SWBD and MDP, they are MLO. Therefore I think we're SOL. I'll try and post a photo (skethc of one-line) and link later this evening.
 
Sorry for the delayed response. Long day today. Had a childhood (thru college) buddy pass away. He was 42. Woulda been 43 in June. I haven't seen him in years, probably 15, but old friendships (at least the closest ones I have) always seem strong. Conversations pick up where they left off and there is no effort to BS'ing with old buddies. Don;t know the details, but he left work feeling sick and never showed up the next day. They sent someone to check on hime since he never missed work. He was gone - I'm guessing heart attack, and he wasn't in bad shape. Wow! Makes a guy think and appreciate what he has. It can be all for nothing in a blink of an eye.

Back on point... It's a 4-wire system with neutrals.

As for the SWBD and MDP, they are MLO. Therefore I think we're SOL. I'll try and post a photo (skethc of one-line) and link later this evening.
Yeah so of they are MLO, not only is it not compliant now, but nixes the idea of running taps off the main to make it compliant. As you probably know, buying components and larger breakers for panelboards is typically ridiculously expensive, so your best option may be to just rip everything after the main out, buy 2 250 amp or 3 200 main breaker panels (they shouldnt be too pricey) and feed them with taps off the load side terminal of the main breaker. You would fix your 300.3(B) issue in the process. Maybe cost a grand in materials.
 

Smart $

Esteemed Member
Location
Ohio
Yeah so of they are MLO, not only is it not compliant now, but nixes the idea of running taps off the main to make it compliant. As you probably know, buying components and larger breakers for panelboards is typically ridiculously expensive, so your best option may be to just rip everything after the main out, buy 2 250 amp or 3 200 main breaker panels (they shouldnt be too pricey) and feed them with taps off the load side terminal of the main breaker. You would fix your 300.3(B) issue in the process. Maybe cost a grand in materials.
Switchboards and switchgear do not have the supply protection requirement like panelboards.

The simplest fix may be to wire as I suggested earlier but install a 400A fused disco off the J-box and route the 2 sets of 250's through it.

OP'er still needs to crunch the numbers for conduit fill and confirm derating for number of conductors.
 
Switchboards and switchgear do not have the supply protection requirement like panelboards.

The simplest fix may be to wire as I suggested earlier but install a 400A fused disco off the J-box and route the 2 sets of 250's through it.

OP'er still needs to crunch the numbers for conduit fill and confirm derating for number of conductors.
Right, missed that the 600 A was a switchboard.

OR depending on the layout and whats going to what, just replace the 400 board with a 200 A MB panel - OP said the whole load is just over 400......
 

kwired

Electron manager
Location
NE Nebraska
Switchboards and switchgear do not have the supply protection requirement like panelboards.

The simplest fix may be to wire as I suggested earlier but install a 400A fused disco off the J-box and route the 2 sets of 250's through it.

OP'er still needs to crunch the numbers for conduit fill and confirm derating for number of conductors.
But after reading the OP again, I think he has a switchboard and a panelboard both fed with feeder taps - if so the taps must end in a single overcurrent device.

Maybe with 10 foot tap rule there could be exception -

Not less than the rating of the device supplied by
the tap conductors or not less than the rating of the
overcurrent protective device at the termination of
the tap conductors.
What exactly does the bold part mean - I'm assuming a switchboard bus would be a "device" that fits the purpose here - but that still means a 600 amp bus would need minimum of 600 amps worth of tap conductors supplying them to comply.
 
But after reading the OP again, I think he has a switchboard and a panelboard both fed with feeder taps - if so the taps must end in a single overcurrent device.

Maybe with 10 foot tap rule there could be exception -

What exactly does the bold part mean - I'm assuming a switchboard bus would be a "device" that fits the purpose here - but that still means a 600 amp bus would need minimum of 600 amps worth of tap conductors supplying them to comply.
Good point. I usually think of 408.36 being the thing that requires a MB. I have frequently contemplated those words you boldened and wondered exactly what situation that was meant to cover.
 

Smart $

Esteemed Member
Location
Ohio
But after reading the OP again, I think he has a switchboard and a panelboard both fed with feeder taps - if so the taps must end in a single overcurrent device.

Maybe with 10 foot tap rule there could be exception -

What exactly does the bold part mean - I'm assuming a switchboard bus would be a "device" that fits the purpose here - but that still means a 600 amp bus would need minimum of 600 amps worth of tap conductors supplying them to comply.
Aligns with my thinking. The 2014 cycle clarifies the 10' tap rule in this regard.

Not less than the rating of the equipment containing
an overcurrent device(s) supplied by the tap conductors

or not less than the rating of the overcurrent
protective device at the termination of the tap conductors.
...and 2 sets of 500kcmil copper, derated 80%, have an ampacity of ____ .
 
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