Solar interconnection tap space issue.

J

JP23

Member
Location
California
The scenario is a 277/480 3 ph. 4w. utility side bus tap to a 1200 amp disconnect for a solar installation. The available space
in the switch gear for the taps does not appear large enough for all of the parallel conductors
required. There is a single 10 ft. 3r gutter spanning over each termination point with myers hubs and nipples.
The project engineer is saying that any welding wire will suffice and 4/0 will carry 400 amps.
I have never seen this in any permanent building wiring scenario and am questioning it. I did find
a post where someone said if the welding wire had the RHH/RHW rating it
would be acceptable to use. Any advice on this would be appreciated.
 
First of all. 4/0 will not carry 400A. Double check your "engineer" is licensed and not some design group that pretends to do engineering.

You can use locomotive wire/welding wire/ cable with type b stranding, but will need to use the 75°C column for the bus terminations AND make sure you bought the right listed terminations. They can't use the regular pressure connectors. Technically. And if this is in a switchgear with bus taps, you don't want to start violating the switchgears listing / labeling. The switchgear manufacturer might have their own requirements for how you tap the bus. Insurance will probably also require a field evaluation for it.

My suggestion, without seeing the section myself, is to tap in copper or aluminum bussing and then wire or put in a new service with an extra section between the meter and main for a line side tap.
 
It must have a marking type showing one of the wire types in 310.4 to be used in an NEC installation. The 400 amp ratign is based on the 90°C column of 310.17. Those conditions are not likely to be met, so you will be limited to a lower ampacity.
 
Thanks for the input! I received the cut sheet from the wire manufacturer for locomotive cable this morning and inquired about the
termination requirements. Would need to upsize it. Agree with using the 75 degree column.
 
JP23 said:
Thanks for the input! I received the cut sheet from the wire manufacturer for locomotive cable this morning and inquired about the
termination requirements. Would need to upsize it. Agree with using the 75 degree column.
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Even if busbar rated 90C, equipment termination 75C only and so 75C column use
 
Deltaforce said:
Even if busbar rated 90C, equipment termination 75C only and so 75C column use
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No how it works. If I can terminate directly to bus bar on both ends, I can use the 90°C rating. This is how cable bus works when running cable bus from transformer adapter boxes to the bus bar of switchgear. It works the same for other installations where the terminations are made to bus bar at both ends.
 
don_resqcapt19 said:
Typically you can use the 90°C ampacities for a bus connection...just not for an equipment connection. The 405 amps is based on the 90°C column of Table 310.17.
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That isn't how UL words it. Bus ampacity and the ampacity of termination provisions are based on 75°C.

Termination can be tested for 90°C.

The equipment the terminations are installed on are not. UL 1558 talks about the test for the 75°C rise (to validate the wire termination) and on it's ampacity table, it says to use the 75°C ampacity for wires rated 90°C.

If the listing requires you use 75°C ampacity, I don't know who you could say you could terminate or consider terminating wire with the expectation that the equipment can handle the 90°C rating.

P.S. It also says the temperature ratings on the paperwork they give you. It says it on the door for panelboard and in the shop drawings for switchgear.

don_resqcapt19 said:
No how it works. If I can terminate directly to bus bar on both ends, I can use the 90°C rating. This is how cable bus works when running cable bus from transformer adapter boxes to the bus bar of switchgear. It works the same for other installations where the terminations are made to bus bar at both ends.
Click to expand...

Once anything is installed in equipment you have to use that equipment's temperature rating. Which is at most, 75°C. There isn't any manufacturer out there making equipment suitable for higher than that. I also have been unable to find a listing for a piece of equipment that says otherwise. I would also argue that any enclosed wiring / busbars would share in that rating limitation.

Bus bar at both ends would have been evaluated at the same temperature rating. 75°C when enclosed. And most flexible bus is a max of 75°C anyway per it's listing.

You can not use the 90°C wire ampacity, or the flexible cord ampacities for that matter, for anything other than splicing wire and cords together.

Just look at any of the documentation from UL on equipment terminations. For example, UL 891 pg. 27, section 6.2.7.23. UL 508 is the same. UL 857 page 37. I have looked for an exception to this and have never found one. I have also asked it at conferences with UL's reps. They all agree that there is no case (other than splices to the same type at the respective ends) where the 90°C could be used. If you can find a listing for it, let me know.

There is no rating or test for the use of 90°C ampacities for the termination in or on equipment. There is not even a listing for some systems/conductors that have 90°C ratings in their specifications.
 
Elect117 said:
That isn't how UL words it. Bus ampacity and the ampacity of termination provisions are based on 75°C.

Termination can be tested for 90°C.

The equipment the terminations are installed on are not. UL 1558 talks about the test for the 75°C rise (to validate the wire termination) and on it's ampacity table, it says to use the 75°C ampacity for wires rated 90°C.

If the listing requires you use 75°C ampacity, I don't know who you could say you could terminate or consider terminating wire with the expectation that the equipment can handle the 90°C rating.

P.S. It also says the temperature ratings on the paperwork they give you. It says it on the door for panelboard and in the shop drawings for switchgear.



Once anything is installed in equipment you have to use that equipment's temperature rating. Which is at most, 75°C. There isn't any manufacturer out there making equipment suitable for higher than that. I also have been unable to find a listing for a piece of equipment that says otherwise. I would also argue that any enclosed wiring / busbars would share in that rating limitation.

Bus bar at both ends would have been evaluated at the same temperature rating. 75°C when enclosed. And most flexible bus is a max of 75°C anyway per it's listing.

You can not use the 90°C wire ampacity, or the flexible cord ampacities for that matter, for anything other than splicing wire and cords together.

Just look at any of the documentation from UL on equipment terminations. For example, UL 891 pg. 27, section 6.2.7.23. UL 508 is the same. UL 857 page 37. I have looked for an exception to this and have never found one. I have also asked it at conferences with UL's reps. They all agree that there is no case (other than splices to the same type at the respective ends) where the 90°C could be used. If you can find a listing for it, let me know.

There is no rating or test for the use of 90°C ampacities for the termination in or on equipment. There is not even a listing for some systems/conductors that have 90°C ratings in their specifications.
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Ok, but that is not how it has been specified on stamped engineered drawings for the installation of cable bus. Where the conductors of the cable bus have been connected to the line side bus bars of switchgear, the design has been based on the 90°C ampacity and where the conductors of the cable bus have been connected to breaker or other equipment terminals the ampacity is based on the 75°C column. These installations have been in place for a long time at a location where they do yearly IR testing and there have been no issues.
 
don_resqcapt19 said:
Ok, but that is not how it has been specified on stamped engineered drawings for the installation of cable bus. Where the conductors of the cable bus have been connected to the line side bus bars of switchgear, the design has been based on the 90°C ampacity and where the conductors of the cable bus have been connected to breaker or other equipment terminals the ampacity is based on the 75°C column. These installations have been in place for a long time at a location where they do yearly IR testing and there have been no issues.
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I am not sure why they don't test for the 90°C rating. I bet there are a bunch of applications where it would work. Or could be used. Like with forced air / cooled equipment. It is the same issue I have with the free air / cable tray. I wish a manufacturer would just tell UL to modify their standard for it already. It is one of the rules that is unwritten in the NEC because it is in the listings. And I HATE that. I complain about it at those same conferences.

Any stamped set, or approved installation from the AHJ will supersede that stuff. I have no doubt that where it is installed is safe. And that they are doing proper maintenance.

Would I stamp a line side tap with 4/0 to be rated 400A. No. I would not. I have seen wire sized 90°C melt the bus of a 4000A switchgear that is fully loaded continuously. It was summer and they turned off the AC in the electrical room and the temp measured at the bus's connection point with the bolts was ~195°F (91°C). Other parts of the gear were ~313°F to 200°F. They called in because the main was tripping but nothing else was. When they opened it up the bolts were melting. More than likely the small gaps of space between the bus plates. There were beads of metal sitting above the insulation barrier above the main. Additional runs where added and the melting went away. The temp came down like 20°F, to about 157°F (~70°C). The bolt was also replaced and retorqued. The air-conditioning unit in the electrical room was put back in service. So it is hard to say what the one resolution was. It could have been the additional runs or it was torqued properly the second time around (manufacturer didn't send it torqued properly) or the air conditioning unit providing air flow / cooling. But I just wanted to provide an example of where a measured temp of ~90°C at the termination can degrade the performance / create a hazard.

Would I stamp cable bus used between the low side of a transformer to the line side of a switchgear line up in a data center or similar where there are measures for routine maintenance and they take electrical performance seriously, yea. I probably would. Just kind of depends.
 
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