Service conductors for 1600A service at 480V

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mshields

mshields

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A contractor has asked me if he can use 3 - 750 per phase for a 1600A service in order to take advantage of existing conduit he'd like to reuse.

At first glace I see that 3 - 750's at 475A each doesn't get you to 1600A under the 75 deg column of Table 310.15(B)16. I note that it would work if the conductors were rated for 90 degrees as 750's in that column are good for 535A (which times 3 is 1605A)

Under what conditions may I use the 90 deg column. I assume the conductor itself and the terminations would need to be rated for 90 degrees. Is it as simple as that?

Thanks,

Mike
 
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GoldDigger

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mshields said:
A contractor has asked me if he can use 3 - 750 per phase for a 1600A service in order to take advantage of existing conduit he'd like to reuse.

At first glace I see that 3 - 750's at 475A each doesn't get you to 1600A under the 75 deg column of Table 310.15(B)16. I note that it would work if the conductors were rated for 90 degrees as 750's in that column are good for 535A (which times 3 is 1605A)

Under what conditions may I use the 90 deg column. I assume the conductor itself and the terminations would need to be rated for 90 degrees. Is it as simple as that?

Thanks,

Mike
Click to expand...

You would have to make sure that, at each end, the conductors terminate on connectors that are rated for 90 degrees and arguably are not in the same box with connected devices which have a lower rating.
In other words, if you plan to terminate each 750 to a larger conductor (or terminate the three 750s to a bus from which they exit as four 750s, for example, then the higher ampacity using lower temperature rating wires must travel some undefined distance before arriving at a device with only 75 degree terminations.
 
charlie b

charlie b

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It has long been my understanding, right or wrong, and probably wrong, that you can't use the 90C column because nobody makes 90C terminations. The only solution I can offer is to set the trip on the main breaker at or below 1425 amps. Can the owner live with that?
 
don_resqcapt19

don_resqcapt19

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If he wants to use 3 750kcmil per raceway, that would fit in 3" rigid conduit. You can fit 9 4/0s in that conduit. The 4/0 has a 90° C ampacity of 260 amps and 9 nine current carrying conductors would have a .7 derating factor. That would give you 260 x 3 x .7 or 546 amps in each of the 3 raceways for a total ampacity of 1638 amps.

What is the calculated load for this service? Assuming that it is 1475 amps or less, he could just add a second service disconnect. As soon as you have more than one service disconnect the service conductors only have to be sized for the calculated load and not for the size of the service OCPD. 230.90(A) Exception #3.
 
petersonra

petersonra

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charlie b said:
It has long been my understanding, right or wrong, and probably wrong, that you can't use the 90C column because nobody makes 90C terminations. The only solution I can offer is to set the trip on the main breaker at or below 1425 amps. Can the owner live with that?
Click to expand...

Polaris connectors are rated for 90 deg C.

Presumably you could terminate the wires at polaris connectors and come out with more conductors then you went in with and terminate them at 75 deg C rated device terminals.

I have never seen anything that suggests how long the tail would need to be between the polaris connectors and the 75 degree C rated device terminals.
 
Smart $

Smart $

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charlie b said:
It has long been my understanding, right or wrong, and probably wrong, that you can't use the 90C column because nobody makes 90C terminations. The only solution I can offer is to set the trip on the main breaker at or below 1425 amps. Can the owner live with that?
Click to expand...
Highlighted would be the wrong part. There are plenty 90°C-rated termination methods. The right part is that no NRTL listed "equipment", i.e. an enclosure containing devices and terminations, is evaluated for 90°C operation. This is where the terminating means suggested by GoldDigger comes in. All parts in the additional termination enclosures (2 required; one at each end) are evaluated for 90°C operation.
 
don_resqcapt19

don_resqcapt19

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petersonra said:
...
I have never seen anything that suggests how long the tail would need to be between the polaris connectors and the 75 degree C rated device terminals.
Click to expand...
I have seen suggestions that the conductor be at least 4' because the breakers are tested with 4' of cable attached. (do not have any documents that say this)
 
Smart $

Smart $

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petersonra said:
...
I have never seen anything that suggests how long the tail would need to be between the polaris connectors and the 75 degree C rated device terminals.
Click to expand...
There's a new addition as it stands now coming in the 2017 edition to conductor sizing [e.g. 215.2(A)(1)] that makes it seem as long as it not in the same enclosure as the 75°C equipment, there will be no required length.

EDIT: Here's how 215.2(A)(1)(a) Exception No. 2 reads now...
Where a portion of a feeder is connected at both its supply and load ends to separately installed pressure connections as covered in 110.14(C)(2), it shall be permitted to have an allowable ampacity not less than the sum of the continuous load plus the noncontinuous load. No portion of a feeder installed under the provisions of this exception shall extend into an enclosure containing either the feeder supply or the feeder load terminations, as covered in 110.14(C)(1).
Click to expand...
It doesn't specifically mention 90°C terminals, but I think one can speculate...
 
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gadfly56

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iwire said:
Most likely the neutral would not be considered a current carrying conductor for the purposes of derating
Click to expand...

I would think so too, since in a perfectly balanced world the neutral should carry nothing.
 
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M

mjmike

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iwire said:
Most likely the neutral would not be considered a current carrying conductor for the purposes of derating
Click to expand...

I'm sure you seen this coming... Why not?

This is a 3-phase 4-wire system (I assume) and unless a majority of the load is incandescent lamps and resistive heating elements, a majority will be non-linear loads. (310.15.B.5.c).
 
Smart $

Smart $

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mjmike said:
I'm sure you seen this coming... Why not?

This is a 3-phase 4-wire system (I assume) and unless a majority of the load is incandescent lamps and resistive heating elements, a majority will be non-linear loads. (310.15.B.5.c).
Click to expand...
That's far from the extent of linear loads. Fluorescent and gas-discharge lighting is linear where using magnetic ballasts. Motors are commonly a big power draw which are linear by themselves. We'll gradually get to the point where they count as non-linear as more become vfd or electronically powered...
 
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