Navigation of 240.21(C) options

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tortuga

tortuga

Code Historian
Location
Oregon
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Electrical Design
Greetings all I have a 15kVA 480/240 to 120/240 single phase transformer going indoors at a non-supervised location.
The primary side feeder will have a 40A 480V breaker with #8 THHN.
The secondary will feed a standard residential style 100A main breaker loadcenter.
Everything will be in raceway.

T450.3(B) says I don't need secondary protection on this transformer.
However per 240.4 and the definition of a feeder in art 100 I need to protect the secondary feeder.
The 120/240 loadcenter is a few feet away and has a rating of 100A (main factory installed).
I was thinking of just feeding the 100A breaker with #4 THHN as that matches the secondary nameplate A.
That sails me into the reef of 240.21(C)
Scanning the 6 options under (C):
1 does not apply its dual voltage
2 (1) (b) is not met with #4 @85 Amps
3 is out its not supplying a switchboard or switch gear.
4 Its inside so thats out.
5 is out its not feeder tapped.
6 gets me off the reef but it looks like 240.21(C)(6)(2) will require #3 THHN since they will be protected at 100A.

kinda an academic question at this point but was curious if there was a way to use #4 any of you come up with?
Thanks in advance.

Nameplate:

transformer.jpeg
 
If your panel has a 100 amp main you need 100 amp or larger conductors which is a minimum of #3. You would need to change to 100 amp CB to something smaller if you want to use smaller secondary conductors.
 
tortuga said:
Greetings all I have a 15kVA 480/240 to 120/240 single phase transformer going indoors at a non-supervised location.
The primary side feeder will have a 40A 480V breaker with #8 THHN.
The secondary will feed a standard residential style 100A main breaker loadcenter.
Click to expand...

In my opinion people start at the wrong point in a circuit. They start with their source breaker and then try to make their load fit.

1. Start with the end of your conductor, because it needs to handle what ever is connected to it. You have a 100A breaker so you should feed it with 100A of conductors (ignoring lots of other details)
2. Protect the conductor you have chosen in step 1. Either use the end protective device, in which case you will likely be applying tap rules, or put a different protective device at its beginning.
3. Finally go to the transformer rules in 450, and see if you can make adjustments to the primary side protective device, based on the secondary the size and location of the secondary conductor protection.

Notice how the first two steps might apply for almost every circuit you protect, even the feed to the transformer.
 
Thanks for the feedback.
jim dungar said:
In my opinion people start at the wrong point in a circuit. They start with their source breaker and then try to make their load fit.

1. Start with the end of your conductor, because it needs to handle what ever is connected to it. You have a 100A breaker so you should feed it with 100A of conductors (ignoring lots of other details)
Click to expand...
Good point, the calculated load is assumed to be 15kVA or less.
I just like the 100A factory main as opposed to back feeding a 80A plug in breaker.

jim dungar said:
2. Protect the conductor you have chosen in step 1. Either use the end protective device, in which case you will likely be applying tap rules, or put a different protective device at its beginning.
Click to expand...
Well I am required to protect the transformer, thats the bottle neck NEC 450.3

jim dungar said:
3. Finally go to the transformer rules in 450, and see if you can make adjustments to the primary side protective device, based on the secondary the size and location of the secondary conductor protection.

Notice how the first two steps might apply for almost every circuit you protect, even the feed to the transformer.
Click to expand...

T450.3(B) allows two setups. primary only and primary and secondary. I can have a 40A primary and no secondary or 80A primary and 80A secondary.
I dont know why one setup would be better than another? Perhaps if you were using tap rules to tap a primary off a 80A 480V large feeder?
I went with protecting the transformer and its primary feeder with a 40, no secondary transformer protection.
Secondary feeder is protected as per 240.21(C)(6)(2)
 
Without secondary protection you will violate 408.36.

408.36 Overcurrent Protection. In addition to the requirement of 408.30, a panelboard shall be protected by an overcurrent protective device having a rating not greater than that of the panelboard. This overcurrent protective device shall be located within or at any point on the supply side of the panelboard.
Click to expand...
 
tortuga said:
Well I am required to protect the transformer, thats the bottle neck NEC 450.3
Click to expand...

How do you know you need secondary protection?

The panelboard needs a main and the transformer primary needs to be protected.
My point is that for the most part 450.3 only comes into play when you want to exceed a base primary device rating, other than that the table is pretty useless.
 
Why not buy a main lug loadcenter and backfeed a 2p 60? We've done that a zillion times for these smaller transformers.
 
Cow said:
Why not buy a main lug loadcenter and backfeed a 2p 60? We've done that a zillion times for these smaller transformers.
Click to expand...
No reason other than my co-worker doesn't like back fed breakers along with funny little reatiner clips that you need, and he put a 100A main breaker panel on the wall already. 😁
 
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