Voltage Drop, down sizing after upsizing for voltage drop.

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electricmason86

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I have a 40 hp 125vdc motor that is located 160 meters from the source, the wires have been sized for the voltage drop, these wires are much too large for the termintions in the motor termination box. Question is can i run the larger conducters into the the disconnect and than small wires to the motor.

Thank much
 
Yes you can, provided of course that the smaller wires are still of sufficient ampacity for the circuit.
The length of the smaller wires needs to be kept as short as possible or significant voltage drop will result.
Remember that just a single meter of say 4mm* wire will cause as much voltage drop as another about 12 meters of 50mm*.

*similar arguments apply to USA cable sizes.
 
broadgage said:
Yes you can, provided of course that the smaller wires are still of sufficient ampacity for the circuit.
The length of the smaller wires needs to be kept as short as possible or significant voltage drop will result.
Remember that just a single meter of say 4mm* wire will cause as much voltage drop as another about 12 meters of 50mm*.

*similar arguments apply to USA cable sizes.
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Can someone translate this into English? :D
 
weressl said:
I will play. What limit would you think be applicable in this case?
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Whereas the Indian NEC imposes a limit on the maximum size of EGC, the US NEC imposes none. I guess this has to do with higher fault level in USA. But it can not be infinite. So some upper limit may be applicable for the purpose of discussion here. This may be ascertained from the formula linking the fault level with the size of the EGC. But is some one willing to state that formula here?
 
T.M.Haja Sahib said:
Whereas the Indian NEC imposes a limit on the maximum size of EGC, the US NEC imposes none. I guess this has to do with higher fault level in USA. But it can not be infinite. So some upper limit may be applicable for the purpose of discussion here. This may be ascertained from the formula linking the fault level with the size of the EGC. But is some one willing to state that formula here?
Click to expand...
For the installation being discussed in this thread, EGC size when the ungrounded conductors are larger than normal, there is no limit to the increase in the code. The code rule requires that the EGC be increased in proportion to the increase in size of the ungrounded conductors. If the OCPD is 30 amps or less the ratio of the size of the EGC to the ungrounded conductor is 1:1, so they will have to be the same size no matter how large you make the ungrounded conductor. When the OCPD is larger than 30 amps, the EGC is not the same size and you do have to do some calculations to find the required size of the EGC if you upsize the ungrounded conductor.
 
T.M.Haja Sahib said:
Whereas the Indian NEC imposes a limit on the maximum size of EGC, the US NEC imposes none. I guess this has to do with higher fault level in USA. But it can not be infinite.
Click to expand...

We don't size circuit conductors nor equipment grounding conductors based on the available fault current. Both are based on the over current device and / or load.
 
T.M.Haja Sahib said:
Whereas the Indian NEC imposes a limit on the maximum size of EGC, the US NEC imposes none. I guess this has to do with higher fault level in USA. But it can not be infinite. So some upper limit may be applicable for the purpose of discussion here. This may be ascertained from the formula linking the fault level with the size of the EGC. But is some one willing to state that formula here?
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iwire said:
We don't size circuit conductors nor equipment grounding conductors based on the available fault current. Both are based on the over current device and / or load.
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T.M.Haja Sahib said:
Sorry, but you have no idea of it.
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Really?

Maybe you can explain it too us?
 
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iwire said:
Really?

Maybe you can explain it too us?
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After checking the suitability of a conductor size for current capacity and voltage drop,the last check is for its short circuit withstand capacity.
 
T.M.Haja Sahib said:
After checking the suitability of a conductor size for current capacity and voltage drop,the last check is for its short circuit withstand capacity.
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Not in the US, for <600V conductors.
The standard UL tests to verify the short-circuit rating of a circuit breaker and fusible switch are performed using 75?C cable. For the device to pass it must prevent damage to the cable as well as to itself.

So in cases where the EGC is the same size as the phase conductor no further checking is required.
 
don_resqcapt19 said:
For the installation being discussed in this thread, EGC size when the ungrounded conductors are larger than normal, there is no limit to the increase in the code. The code rule requires that the EGC be increased in proportion to the increase in size of the ungrounded conductors. If the OCPD is 30 amps or less the ratio of the size of the EGC to the ungrounded conductor is 1:1, so they will have to be the same size no matter how large you make the ungrounded conductor. When the OCPD is larger than 30 amps, the EGC is not the same size and you do have to do some calculations to find the required size of the EGC if you upsize the ungrounded conductor.
Click to expand...

I know this is in the book but IMO it is the absolutely absurd. There no engineering reason given for this requirement. I keep hoping that the committee
will eventually make a change.
 
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