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A motor has two forms of protection. One for the over load and one for the short circuit-ground fault. The overload could be an integral part of the motor or starter. If not you must use the service factor or temperature rise rating to determine the overload separately. The short circuit-ground fault is based on the full load current multplied by a factor depending on what type of overcurrent protection device that you use. The full load current is based on the phases and horsepower using the the tables in artilce 430 of the national electrical code.
Here's a URL address to a website that describes polyfuses. They're used mainly for electronic equipment.2011/07/26 at 3:59 pm in reply to: How to minimize the phase difference to installation of equipments #12358
If you mean load balamcing then you would have to make a chart. See below.
A B C N
You then add up the full load amps for each phase. The idea is to get the phase amps as close to being equal as possible. For eample:
If a phase has too many amps(alot more than the other phases) then you must move the load from that phase to another phase until the total amps for each phase is closely equal to each other. This could be done by moving the circuit breakers around in a paneboard/switchboard.
Remember: certain loads do not have a neutral connection and therefore the neutral amps for that load is zero.
NEC 352 give's rules for PVC installation. Also NEC 362 for nonmetallic tubing which is in essence PVC. There doesn't seem to be any provision forbidding the use of PVC conduit in high buildings however, there are rules for spacing the supports.2011/07/26 at 3:42 pm in reply to: How can the Transfrmers be Protected from Surge, Short Circuits, Lightning etc. #12356
NEC 450.3 gives rules for sizing transformer overcurrent protection. There is a table in that article that gives the percentages for the overcurrent devices.
The mechanical input of a generator is always constant therefore the real power output is constant. However, a large amout of reactive power will cause the apparent power to increase with the real power held constant. This means more amps and therefore higher copper losses in the transmission lines.2011/07/11 at 5:40 pm in reply to: how to minimise voltage drop problem in PLC wiring? #12303
Use a larger wire gauge.
The earth should never be used as a ground fault current return path. I's resistance is too high and may therefore be unable to operate the overcurrent device in case of a fault.
NEC Table 220.12 gives you 2 VAs per square foot to calculate the minimum lighting load. Recepatacles are counted as 180 VA for simplex, duplex and triplex as per NEC 220.14(I)&(L). Motor loads are 125% of the largest motor plus the sum of the other motor loads. As for HVAC you take the highest of the air conditioning or heating leatings since they are not expected to be operating at the same time.
The KVA per phase would be 315/3=105KVA
The current per phase would be 105,000/220=477.27Amps
What is the voltage?
Is the power factor lagging or leading?
Is the power being taken from a generator or a transformer?2011/05/12 at 2:57 am in reply to: How I can calculate the Power (KVA) from the meter #12087
If you're just looking to size the cable then why not use the existing overcurrent protection rating for the service? I t would either be a fused disconnect or a main circuit breaker.
Well first you have to determine the current. What's the voltage? Once you determine the current you could use NEC Table 310.16 to determine the ampacity. Use the 75' column.
Power/Voltage = Current
How can you have 4 single phase circuits? If it's a three phase circuit and your trying to setup an ATS you would need a four-pole contactor so that the neutral would also be switched. Otherwise the two power sources would not be separately derived.
Article 424 of the National Electric Code gives rules for these installations.