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Why this is legal (code compliant) but you won't like it

jar546

jar546

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If you have a situation where let's say there is a gutter above a group of subfed panels that comes from an MDP (Main Distribution Panel) that is full of 4/0 copper wire (rated at 230A) to feed down to the sub-panels.

Coming out of the gutter you see a conduit that takes #4 copper (85A) runs to an enclosure 20' away with a single circuit breaker installed that is rated at 125A. The #4's are Polaris tapped from the 3/0 wire inside the gutter to feed the 125A breaker.

From the 125A breaker there is another conduit with #4's that goes an additional 50' away to a motor controller with an integral overload device, which then, in turn feeds a motor. The motor is a 20A 230V squirrel cage motor, 3 phase motor which means it is good for 54A.

How is this possible?
How can we tap off of a feeder rated at 230A to a wire rated at 85A without OCPD to a single breaker rated at 125A to a motor controller then motor with wires only rated at 85A and be legal?
 
Basically because you can tap the service and only protect it at the PV power levels....Or at least that is how it is done around here....
Marvin Hamon, PE: With a supply-side interconnection, several things dictate conductor sizing. Most of it comes out of NEC Article 230. I assume you are using a 100 A fused disconnect between the inverter and the tap point.

The conductors between the disconnect and the tap are unprotected service-entrance conductors. You size them so they will not be overloaded, since you can’t size them to be protected from a fault on the supply side. Start with 230.42(A) and (B). The conductors must be sized for the inverter output and for the disconnect size. So in this case, you size the conductors for the 100 A disconnect.

On the load side of the disconnect, you size the conductors so the 100 A fuses protect them.



Grounding is a little more difficult. Since your ac disconnect is a service entrance, it has the neutral-to-ground bond and has to be connected to the grounding-electrode system. You size the grounding-electrode conductor from Table 250.66 based on the size of the conductor from the tap to the disconnect. You size the equipment grounding conductor from the disconnect to the inverter using Table 250.122. If there is a ground conductor from the utility, you should run it to the disconnect.
 
As you know, I am not an electrician, but I do try. I understand the allowability of taps and "overprotecting" a conductor for motor starting and such, it takes me a while to hammer through he NEC to figure it out...
 
Yeah, again, this has nothing to do with PV systems. This is as described. I don't want to get thread drift on this subject. Right idea, wrong code sections/chapter.
 
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