The Electrical Code

215.2 Minimum Rating and Size. So to begin this does not apply to anything over 600 volts. Then it goes along and says that the wire size needs to be enough to supply the loads that were calculated for those feeders to handle. This is great now we are talking. Because loads are some times constant, the code makers want the wire size to sustain the heat that this produces so by multiplying by 1.25 or 125% you get a wire size bigger which is designed to handle the additional heat from what ever its feeding.

And then there are exceptions to this rule and there are three of them one if the breaker is designed to operate at 100% of its name plate suggest. Two says if the feeder is feeding a noncontinuous and a continuous load then there is no exception because your adding the noncontinuous load to the amp amount there for the wire and breaker will see a lower amount of amps and will have less heat to handle. Three goes by says pretty much the same thing but they throw in the word grounded and are not connected to an over current device, so yes no breaker. Right to the wires feeding the panel you see this in situations were there can be no chance of a breaker interrupting the electrical power to the device, much like a fire alarm system, a oxygen machine supplying oxygen to a human that twill die from not having it. In other words the design of the device has redundant alarms telling the user if there is a possible device failure and if so attention is required to sustain operation or replacement of the device.

So then (b) says that the wire size should correct for voltage drop. By the way it does not say this but that is what they mean by correction factors. So say how long it is or if its aluminum, etc.

Then is goes on to refer to 250 which refers to ground wire size.

So it goes on to say that the service wires should be bigger then the feeder wires. That calculation should be a no brainer.

Then again if you have not installed 600 volt + cabinets or devices that need that size voltage and are trying to figure it out with out any prior knowledge I warn heavily against this. Don’t do it. Transformers that require 600+ volts also need special attention to shut off power. Then we have situation that need power to continuous loads that as I said before needs to be calculated by 125%.

The final thing to say about this article is that if the system is deigned and is being installed under the supervision of a licensed electrical engineer, then you have my blessing to install something over 600+ volts.

“(A) Feeders Not More Than 600 Volts.

(1) General. Feeder conductors shall have an ampacity not less than required to supply the load as calculated in Parts III, IV, and V of Article 220. Conductors shall be sized to carry not less than the larger of 215.2(A)(1)(a) or (b).

(a) Where a feeder supplies continuous loads or any combination of continuous and noncontinuous loads, the minimum feeder conductor size shall have an allowable ampacity not less than the noncontinuous load plus 125 percent of the continuous load.

Exception No. 1: If the assembly, including the overcurrent devices protecting the feeder(s), is listed for operation at 100 percent of its rating, the allowable ampacity of the feeder conductors shall be permit‐ ted to be not less than the sum of the continuous load plus the noncontinuous load.

Exception No. 2: 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).

Exception No. 3: Grounded conductors that are not connected to an overcurrent device shall be permitted to be sized at 100 percent of the continuous and noncontinuous load.

(b) The minimum feeder conductor size shall have an allowable ampacity not less than the maximum load to be served after the application of any adjustment or correction factors.

Informational Note No. 1: See Examples D1 through D11 in Informative Annex D.

Informational Note No. 2: Conductors for feeders, as defined in Article 100, sized to prevent a voltage drop exceeding 3 percent at the farthest outlet of power, heating, and lighting loads, or combinations of such loads, and where the maximum total volt‐ age drop on both feeders and branch circuits to the farthest outlet does not exceed 5 percent, will provide reasonable efficiency of operation.

Informational Note No. 3: See 210.19(A), Informational Note No. 4, for voltage drop for branch circuits.

(2) Grounded Conductor. The size of the feeder circuit grounded conductor shall not be smaller than that required by 250.122, except that 250.122(F) shall not apply where grounded conductors are run in parallel.

Additional minimum sizes shall be as specified in 215.2(A)(3) under the conditions stipulated.

(3) Ampacity Relative to Service Conductors. The feeder conductor ampacity shall not be less than that of the service conductors where the feeder conductors carry the total load supplied by service conductors with an ampacity of 55 amperes or less.

(B) Feeders over 600 Volts. The ampacity of conductors shall be in accordance with 310.15 and 310.60 as applicable. Where installed, the size of the feeder-circuit grounded conductor shall not be smaller than that required by 250.122, except that 250.122(F) shall not apply where grounded conductors are run in parallel. Feeder conductors over 600 volts shall be sized in accordance with 215.2(B)(1), (B)(2), or (B)(3).

(1) Feeders Supplying Transformers. The ampacity of feeder conductors shall not be less than the sum of the nameplate ratings of the transformers supplied when only transformers are supplied.

(2) Feeders Supplying Transformers and Utilization Equipment. The ampacity of feeders supplying a combination of transformers and utilization equipment shall not be less than the sum of the nameplate ratings of the transformers and 125 percent of the designed potential load of the utilization equipment that will be operated simultaneously.

(3) Supervised Installations. For supervised installations, feeder conductor sizing shall be permitted to be determined by qualified persons under engineering supervision. Supervised installations are defined as those portions of a facility where all of the following conditions are met:

(1) Conditions of design and installation are provided under engineering supervision.

(2) Qualified persons with documented training and experience in over 600-volt systems provide maintenance, monitoring, and servicing of the system.”