Fuses/Breaker Voltage Sizing...

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rtbullfighter

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I have a motor control package that is a 3phase 460volt motor contactor w/overloads and currently we are fusing 2 wires for controls. The fuses that we are using are 600v/700v rated. My question is: Can we use either a breaker, resetable fuses, or even fuses themselves that are rated at 300 volt?
The ampacity of these controls is 2 amp fuses, but the FLC is less than that... We are trying to improve the overall control package for these motors, but the catch to using all 300 volt fuses is the 3ph 460v motors that we send out. Remember that the two phases will be only 277v to ground. And I did read 240.60(1) in the 2008 NEC that states that I cannot, due to the phase-to-phase voltage is over 300v. 240.60(2) states though a single phase that is fed from a 3ph 4 wire is acceptable if the phase-to-neutral is less than 300v. But I have no neutral at any of these motors that we sell. These are 50" fans, and the power to the control package is done by qualified electricians in the field all over the world. The control package is self contained at the bottom of the fan, and the only things these fuses actually run are a fractional oscillating motor and the motor contactor COIL ONLY! Thank you for your time to review and answer my question.

Randy
 
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Very simplistically.
The fuse/breaker needs to be rated for the voltage between the conductors which carry the current you are trying to break. So, because you are not using the neutral conductor all of your protective devices will be connected across 480V.
 
Thanks Jim, but as I am playing devil's advocate tonight, the code referance is intended for Fuses and on a branch circuit. This here is a Control circuit, using on of the tap rules. (which i can look there too, thanks). I will try to post another reply when i find out the answer a little more clearly that way if this question arrises again you can help with the findings and answers. Thanks Again.

Randy
 
On a 277v circuit you would not "see" over 300v, unless it was multi wire, but on a 480v you would.
Picture a 2 pole fuse holder, one of the fuse blows, meter across the blown fuse you get 480v. Thats one of the reasons why you cant use a 300v fuse.

The other reason is the design of the 480v fuse is made to extiguish a higher value arc when a fault occurs where as a 300v fuse could cause a destructive arc event.
 
read part 6 of article 430 it starts with sec. 430.71. Also see table 430.72(B). Make sure to read all of 430.71 and all of 430.72
 
What size of circuit is needed to power your equipment? Your control circuit may not need reduced overcurrent protection, or you could possibly use larger control circuit conductors, or get a listing for your equipment. I install pump panels that have 200 amp fuseholders and have 14 AWG control conductors with no additional overcurrent devices. The unit is listed as it is, but if I wanted to build basically the same thing I can't do it without additional overcurrent protection.
 
rtbullfighter said:
Thanks Jim, but as I am playing devil's advocate tonight, the code referance is intended for Fuses and on a branch circuit. This here is a Control circuit, using on of the tap rules. (which i can look there too, thanks). I will try to post another reply when i find out the answer a little more clearly that way if this question arrises again you can help with the findings and answers. Thanks Again.

Randy
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Try 110.3(b), or have you found a manufacturer that will allow this?

Ground is not a current carrying conductor so the voltage to ground rating is not important to the interruption of the protective device, what matters is the voltage across the device. The fact it is a control circuit does not matter.
 
kwired said:
What size of circuit is needed to power your equipment? Your control circuit may not need reduced overcurrent protection, or you could possibly use larger control circuit conductors, or get a listing for your equipment. I install pump panels that have 200 amp fuseholders and have 14 AWG control conductors with no additional overcurrent devices. The unit is listed as it is, but if I wanted to build basically the same thing I can't do it without additional overcurrent protection.
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kwired, take a look at 430.72(A), that would allow you to have a tapped control circuit and the values in table 430.72(B) would apply
 
*JUSME123*That is a good read, but I am looking for the Voltage values, and not the overcurrent.
I want to know if i can use 300 volt rated overcurrent protection on a 480 volt system, that is 277 volts to ground. Thank you for the help too, needed some of that information too.

Randy
 
rtbullfighter said:
*JUSME123*That is a good read, but I am looking for the Voltage values, and not the overcurrent.
I want to know if i can use 300 volt rated overcurrent protection on a 480 volt system, that is 277 volts to ground. Thank you for the help too, needed some of that information too.

Randy
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You can use a 300 volt fuse if your circuit is connected at 277 volts...that is line to neutral. If it is conneced line to line, you cannot use a 300 volt fuse.
240.60 General
(A) Maximum Voltage ? 300-Volt Type Cartridge fuses and fuseholders of the 300-volt type shall be permitted to be used in the following circuits:
(1) Circuits not exceeding 300 volts between conductors
(2) Single-phase line-to-neutral circuits supplied from a 3-phase, 4-wire, solidly grounded neutral source where the line-to-neutral voltage does not exceed 300 volts
Click to expand...
 
jusme123 said:
kwired, take a look at 430.72(A), that would allow you to have a tapped control circuit and the values in table 430.72(B) would apply
Click to expand...

I was aware of T430.72 when I posted. The example of a pump panel with 200 amp fuses and 14AWG control circuit would not be allowed by this table, I would need a 8AWG control circuit conductor minimum without additional overcurrent protection for the control circuit.

The manufacturer of the pump panels use 14AWG with no additional overcurrent protection, but they have a UL listing for it. I can't build my own in the same way without getting it listed.
 
Thanks to all....

Thanks to all....

I want to thank you all for repling to my post. And Don you are right. I talked to a manufacture of the fuses/breakers and that is what he was telling me, "that since i did not have a neutral wire, I could not use that rating". I also found out that the manufacture that I talked to does not design or make a 300 volt rated because it is not the most commonly used voltage ANYWHERE. He did state that it was very rare though when used. Thank you again for all the time and asweres here.

Randy
 
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