Voltage Drop Calculation

Without a load, there will not be a voltage drop. You should remember that from electricity 101.

The minimum wire gauge that you should use is set by your local building standards. The default standard for most wiring in the USA is NFPA70. I recommend that you attend a seminar on how to use this standard because it is a confusing standard. The confusion comes from the vast number of scenarios and exceptions that the standard covers.

Thanks for your reply. In fact, the power sockets that we are using are 63amps. I got confused on how to calculate the voltage drop in different segment. The connection sequence is from MCC to MCCB and connected to power socket finally. I have calculated the wire gauge from MCC to MCCB and then MCCB to sockets. However, one of my colleague said that I should calculate the volt drop from MCC all the way to sockets in order to determine the wire gauge. I'm totally confused.

The voltage drop of the individual cable segment should be within 3%, while the total voltage drop (for both feeders and branch circuits) from the power source (generally transformer secondary bus or main power panel etc.) to the load should be 5%.

See NEC 210.19(A), FPN 4 and 215.2(A)(3) FRN 2.

Hope it helps.

- MS

MSAMAD got one part correct which is the 5% total voltage drop from source to load and the 3 % from branch (MCC) to load. The second part is to calculate what is the voltage drop that will occur with the assessment of the conductor you select for the load. Remember to identify if the load is continuous and if yes, multiply your current (amps) rating by 1.25%, then; if single phase, the formula to apply is ( 2 x length of conductor x resistance of conductor x Amps divided by 1000) this will equal your voltage drop with the selection you made on the conductor. The preferred method is to apply the following formula ( (2 if single phase or 1.73 if 3 phase) x L x (12.9 if copper 0r 21.2 if alum) x amps divided by the percent voltage drop (either 5 percent of supply or 3% of branch) This formula will give you the size of the conductor to use for this load. If this process is still unfamiliar, then suggest you get licensed electrician to assist with table 310.16 and table 8 of NEC/NFPA70.

Just to clarify:

For voltage drop calculation, use 100% current, not 125% (125% is used for cable ampacity and breaker sizing in NEC). For the voltage drop calculation formula see this: http://cr4.globalspec.com/comment/535743/Re-voltage-drop

- MS

normally the size of cable determined based on the load capacity as well as the short circuit rating of the system.

so you may consider the length from service board to socket.

please note that the selected cable should withstand the shortcircuit current for a specified period of time.