When using cable tray ampacity for conductors installed in cable tray [free air rating as allowed in section 392.80[A][2][d]] when you transition from tray to conduit at what point are you required to use the ampacity for cables in conduit as opposed to free air rating in tray? For example if the tray installation is in the truss space and the final 10-15? to the panel is in pipe [RMC] are you required to use the ampacity rating for the pipe installation for the entire length? I thought there was information on this [maximum length of pipe as a percentage of the entire run] but cannot seem to locate it..Thanks?
conductor ampacity
- Thread starter Mike01
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luckylerado
Senior Member
- Location
- Lake of the Ozarks
Take a look at 310.15(A)(2) and the exception
(2) Selection of Ampacity.
Where more than one ampacity
applies for a given circuit length, the lowest value shall be used.
Exception: Where two different ampacities apply to adjacent
portions of a circuit, the higher ampacity shall be permitted
to be used beyond the point of transition, a distance equal to
3.0 m (10 ft) or 10 percent of the circuit length figured at the
higher ampacity, whichever is less.
(2) Selection of Ampacity.
Where more than one ampacity
applies for a given circuit length, the lowest value shall be used.
Exception: Where two different ampacities apply to adjacent
portions of a circuit, the higher ampacity shall be permitted
to be used beyond the point of transition, a distance equal to
3.0 m (10 ft) or 10 percent of the circuit length figured at the
higher ampacity, whichever is less.
Julius Right
Senior Member
- Occupation
- Electrical Engineer Power Station Physical Design Retired
So, it is a dilemma. According to 310.15(A)(2) one may take the higher ampacity into consideration if the conduit is less than 10 ft. or 10%.On the other hand according to 110.14 Electrical Connections. (C) Temperature Limitations.[Less than 100 A load 60oC and for more 75oC.]
?Unless the equipment is listed and marked otherwise?. You have to be Smart.
?Unless the equipment is listed and marked otherwise?. You have to be Smart.
Carultch
Senior Member
- Location
- Massachusetts
So, it is a dilemma. According to 310.15(A)(2) one may take the higher ampacity into consideration if the conduit is less than 10 ft. or 10%.On the other hand according to 110.14 Electrical Connections. (C) Temperature Limitations.[Less than 100 A load 60oC and for more 75oC.]
?Unless the equipment is listed and marked otherwise?. You have to be Smart.
The termination rule trumps all other sizing rules. Even though a termination is obviously less than 10 ft / 10% of the remaining length.
For what it is worth, most of the terminals you find on new equipment today are "listed and labeled otherwise" for 75C. It is more of an academic rule to have 60C being the default. It is very rare that you have terminations AND EQUIPMENT rated for 90C. Field-installed terminations, separate from manufactured equipment, can take credit for the 90C rating. Examples are MC4 connectors, Polaris insulated tap connectors, insulation piercers, and similar.
For your conduit of wiring emerging from equipment to cable tray, there are two rules you can likely take credit for, in order to ignore the bundling derates in the conduit:
1. the 24 inches and less "nipple rule". No derates need to apply for conductors this short.
2. the less than 10%/10ft of remaining length, "stub rule" as discussed above.
Note that the NEC is silent on how to calculate cable tray conductor ampacity.
It is not silent. In fact, it has a about 1-1/4 page on the matter of conductor ampacity [392.80].
ggunn
PE (Electrical), NABCEP certified
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- Austin, TX, USA
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- Consulting Electrical Engineer - Photovoltaic Systems
You can also add Polaris blocks in line to convert to lower temperature conductors before landing on terminals that require them.
Carultch
Senior Member
- Location
- Massachusetts
When you use that method, does the tap connector need to be in a separate enclosure from manufactured equipment as this drawing shows?
One reason I can see one would do this, is if they screw up, and put in a conduit that is too small for the 75C rated conductors, and maybe already have the 90C conductors pulled. Then they use this clever method to adapt as required by code.
However, adding in separate enclosures means major surgery on re-doing the raceway, so it is obviously something to avoid.
ggunn
PE (Electrical), NABCEP certified
- Location
- Austin, TX, USA
- Occupation
- Consulting Electrical Engineer - Photovoltaic Systems
I am pretty sure you can put a Polaris block in the panelboard enclosure. I don't know about the switch.
Cable tray installations are the only wiring method I'm aware of that don't require an enclosure for a splice/tap.
For other installations, it is said the splice/tap must be in a separate enclosure, after "X" distance, to obtain the 90?C rating. Doing it in an enclosure which is already limited to 75?C terminations does not get you approved for using the 90?C ampacity rating. The "X" distance is usually either 4' or 10'. The former because that is the length of conductor at which UL tests equipment with 75?C terminations, while the latter seems to be based on 310.15(A)(2) Exception.
Carultch
Senior Member
- Location
- Massachusetts
So it sounds like, the best solution is to anticipate this in advance.
Another place you might see this is when you have 75C equipment on one side, and 60C equipment on the other side. A case of the academic rule about 60C terminals 100A and less.
And the 75C equipment traps you in the situation where only 75C sizing will fit in the termination.
Anticipate if possible... knowledge of the pertinent details do not always present themselves in advance. :happysad:
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