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safety switch sizing
- Thread starter svaelec
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I don't know what kind of transformer it is. Since you mentioned the output current of the transformer is 25 amps, I am assuming that it is rated 9 KVA with primary 480 volts (10.8 amps) 3 phase and secondary 208 volt (25 amps) 3 phase. There are two methods to protect the transformer, primary only protection and primary and secondary protection. Check Table 450.3(B). For primary only protection, the maximum rating of overcurrent protection for the transformer is 125% of the primary current (10.8 amps). For primary and secondary protection, the maximum rating of overcurrent protection for the primary protection is 250% of the primary current (10.8 amps), the maximum rating of overcurrent protection for secondary protection is 125% of secondary current (25 amps). The non-fused safety disconnect switch will not be able to protect the transformer.
megloff11x
Senior Member
Just as 4WD drive doesn't help you stop any better... When you want to remove power the switch has to be able to open up. Even a switch well over the margin may have its contacts welded. I used to keep a few souvenirs at my desk. Follow the code for your application.
Matt
Matt
The transformer is actually a PV inverter so the requirements of 450 do not apply. Let me restate the question: 110-9 refers to the nominal voltage and current ratings for interrupting equipment. The nominal amperage of the circuit in question is 25 amps. Would a 30 amp non-fused safety switch meet the requirements of the code although the amperage rating of the circuit breaker is 40 amps due to derating factors.
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- Wisconsin
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- PE (Retired) - Power Systems
A switch rated as 30A is designed to carry 30A continuously.
But, the actual application depends on your switch and the load. Switches should always be sized based on their Make, Break, and Continuous ratings. Most times we only refer to the continuous when discussing switches, but it is not uncommon to have the Break rating of the switch significantly lower than its continuous rating (for example a toggle switch might be rated 60/6/10A) . This is why it is important to have devices rated for their load characteristics, whether it is tungsten lighting, horsepower, or resistance heating.
Most power switches (including, wall switches safety switches and contactors) are rated to break their continuous rating, however most "control" switches and relays are not.
But, the actual application depends on your switch and the load. Switches should always be sized based on their Make, Break, and Continuous ratings. Most times we only refer to the continuous when discussing switches, but it is not uncommon to have the Break rating of the switch significantly lower than its continuous rating (for example a toggle switch might be rated 60/6/10A) . This is why it is important to have devices rated for their load characteristics, whether it is tungsten lighting, horsepower, or resistance heating.
Most power switches (including, wall switches safety switches and contactors) are rated to break their continuous rating, however most "control" switches and relays are not.
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