Brain Check

[infinity]

Simple question, 225 kva transformer 480-208/120 indoors, secondary condcutor length 40', I say no good limited to 25'. {240.21(C)(6)}

 

[RUWired]

Simple question, 225 kva transformer 480-208/120 indoors, secondary condcutor length 40', I say no good limited to 25'. {240.21(C)(6)}

Could it have been installed with 240.92(C) in mind?

 

[texie]

Simple question, 225 kva transformer 480-208/120 indoors, secondary condcutor length 40', I say no good limited to 25'. {240.21(C)(6)}

In general, I think you are correct unless there are other unknowns here.

 

[dicklaxt]

Simple question, 225 kva transformer 480-208/120 indoors, secondary condcutor length 40', I say no good limited to 25'. {240.21(C)(6)}

What is the reason for a limit to conductor length??????????????
Who cares if it's 10 feet or 200 feet? It's still voltage,current etc and the bark and bite are the same too

dick

 

[Dennis Alwon]

I guess I agree unless the conductors are outside the building

 

[jumper]

What is the reason for a limit to conductor length??????????????
Who cares if it's 10 feet or 200 feet? It's still voltage,current etc and the bark and bite are the same too

dick

I do not know the technical reason for the length limitation, but this is a summary of 240.21(C)(1-6) from the NECHB.

The six applications covered in 240.21(C)(1) through (6)
permit transformer secondary conductors without an over current
protective device at the point the secondary conductors
receive their supply under one of the following
conditions:
1. The primary over current protective device, as described
in 240.21(C)(1), can protect single-phase (2-wire) and
3-phase (delta-delta) transformer secondary conductors.
2. The transformer secondary conductors do not exceed
10 ft.
3. The transformer secondary conductors do not exceed
25 ft (two applications).
4. The transformer primary plus the secondary conductors
do not exceed 25 ft.
5. The transformer secondary conductors are located outdoors.
Like 240.21(B) for conductors tapped to a feeder,
240.21(C) specifically prohibits application of 240.4(B)
with transformer secondary conductors covered by the requirements
of 240.21(C)(1) through (C)(6). See the commentary
for 240.21(B).
The secondary terminals of a transformer are permitted
to supply one or more than one set of secondary conductors.
The first sentence of 240.21(C) specifies that the requirements
apply to ?a set of conductors supplying a single load?

 

[charlie b]

What is the reason for a limit to conductor length?

The basic reason is that the alternative would have been to require protection right at the secondary terminals of the transformer. The important concept is that all conductors must be protected against overcurrent conditions. If you connect too much load to the transformer and wind up overloading the secondary conductors, a breaker at the load end (e.g., the main circuit breaker on the first panel downstream) can open, thereby terminating the event, and protecting the seconday conductors. And that would be true no matter how long the secondary conductors are.

However, consider what would happen if the secondary conductors experience a short circuit between the Phase A wire and the Phase B wire. The transformer will pump a large amount of current through the portion of the secondary conductors that lie between the transformer and the fault point. The breaker downstream will not see that current, and therefore cannot provide any protection for the secondary conductors. The result will be some serious damage, and perhaps a fire. If there were a breaker at the transformer secondary, it would provide the protection. But the code allows us to not put a breaker right at the transformer.

So to directly answer your question, the reason for limiting the conductor length is to limit the probability that a short circuit can take place in that limited length of wire. The longer the wire, the more chances there would be for someone to find a way to run into the wire with a forklift or other heavy object. We keep the run short, and we protect it against damage by enclosing it in conduit, and we accept the (limited) risk that comes from not having an breaker right at the transformer.

 

[charlie b]

Simple question, 225 kva transformer 480-208/120 indoors, secondary condcutor length 40', I say no good limited to 25'. {240.21(C)(6)}

Agreed.

 

[dicklaxt]

So to directly answer your question, the reason for limiting the conductor length is to limit the probability that a short circuit can take place in that limited length of wire. The longer the wire, the more chances there would be for someone to find a way to run into the wire with a forklift or other heavy object. We keep the run short, and we protect it against damage by enclosing it in conduit, and we accept the (limited) risk that comes from not having an breaker right at the transformer.

I'll buy that

dick

 

[jap]

I'll buy it too.

Charlie along with several of the others are very good at explaining things and allowing us all the privelage of learning from them.