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Topic Title: Recommended Voltage Drop in British Standard
Topic Summary: Voltage Drop
Created On: 16 April 2018 06:29 AM
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 16 April 2018 06:29 AM
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Posts: 27
Joined: 14 January 2018

Hi Everyone,

What is the recommendation for allowable voltage drops for induction motors (3 phase and single phase) in starting and normal running conditions in British Standard (400V phase to phase/50Hz)?

For other instruments, is there any general recommendation for allowable voltage drop in the British Standard at the load end of power supply systems (400V phase to phase/50Hz) (except allowable voltage variations that have been given by instrument manufacturer)?

Thansk & Regards,
 16 April 2018 07:26 AM
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There is nothing for motors in BS7671 over the general guidance in Appendix 12 that the maximum allowable volt drop between 'the origin of the installation and any load point' should not exceed the following:

Lighting 3%
Other uses 5%
However, this can be over-ruled by manufacturer's instructions - which can be important.
That is the drop in the customer's wiring, but if you are designing the motor controller, you should note that that is not all of it, as the supply voltage at the origin (i.e. where supplier's wiring meets the customer's) is already permitted variation not exceeding 10% , by the electricity continuity and supply regulations, which may be voltage drops due to your own loads or other peoples.
So you could be down as much as 15% or up 10% on the nominal 230/400 at the motor terminals in different fully compliant installations in different parts of the country.
That is at normal load, what happens during inrush will require information from the motor manufacturer - if for an extreme example the start current was ten times the running, then during start that 5% drop in the local wiring could become 50%. However, I think few motor manufacturers would endorse that as a good solution, and instead would insist on provision of a higher current supply or an alternate starting method.

regards Mike
 16 April 2018 11:00 AM
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Dear Mike,

Thank you very much for dedicating your valuable time to help me.

Thanks & Regards,
 22 April 2018 05:26 PM
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To add to what mapj1 has said, the acceptable volt drop on start can also depend on the load being started. Something like a compressor is much more demanding than an axial fan.
The volt drop on start calculations are complicated by the power factor on start often being quite extreme but my experience is that if the running volt drop is OK then the starting one usually is as well.

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