Nec 455.6(a)(2)

[erickench]

Okay I've just read the above referenced section and I find it confusing. It starts:

"Where the phase converter supplies specific fixed loads, and the conductor ampacity is less than 125 percent of the phase converter nameplate single-phase input full-load amperes,"

We are given that the ampacity of already installed conductors is below 125% of the nameplate amps.

"the conductors shall have an ampacity not less than 250 percent of the sum of the full-load, 3-phase current rating of the motors and other loads served where the input and output voltages of the phase converter are identical'"

Now we are told that the ampacity is to be 250% of full load and that the input and output voltages are identical.

"Where the input and output voltages of the phase converter are different, the current as determined by this section shall be multiplied by the ratio of output to input voltage."

Well if the input and output voltages are identical then the ratio is equal to one and the currents are therefore identical. So how do we go from 125% to 250%? Do we just increase the size of the conductors after they've been improperly installed? What is the purpose of this section?

 

[don_resqcapt19]

The 125% applies to the phase converter nameplate input current and the 250% applies to the sum of the fixed loads. It appears that this is intended to give you some relief on the input wire size when the phase converter is oversized.

 

[erickench]

Yes but does that mean that they have to increase the size of the existing input conductors? Does an electrician or engineer come in and say that the already installed input conductors are too small and that they have to be replaced with larger ones?

 

[don_resqcapt19]

If they are too small to meet the code rule and you are working on the system, then they should be replaced. What is the input rating of the phase converter? What are the motor load currents?