Circuits for Motors- how to in the UK?

[Cookie]

How do you size the MCB, wire and protection for a motor in the UK/Europe? How is it typically done?

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If anyone is curious here how its done in the US:


You size the conductor based on 125% of tables 430.247-430.250 determined by the type of motor and duty cycle of course. (When Table 430.249 says 2 phase they don't mean 180* out of phase like in 120/240 but 90* out of phase- this was common 100 years ago during the electrification of the United States but obsolete today)

This will then take you to table 310.15 B 16:

Ambient Temperature

Table 310.15(B)(16) of the National Electrical Code lists the allowable ampacities of insulated general use conductors. This makes it a very useful table, but, we must keep in mind that the conductor...

www.edrenzi.com

60*C or 75*C based on your wire and termination. Typically you use 75*C, and de-rate (more than 3 conductors in a conduit) off the 90*C column as THHN/THWN is rated 90*C. Once the conductor size is known you size the fuse/breaker based on Table 430.52.

For a breaker you can size at 250% of the conductor's ampacity (current carrying capacity), for a none time delay fuse 300% and for a time delay fuse 175%.

Remember there is no loop impedance requirement- meaning you can place #14 (2.08mm2) on a 40 amp breaker and run it as long as you like, ie 500 feet or more.

Next, you size the motor's overload protection based off 115% to 125% of the motor's actual nameplate rating.

If any of you have 3 hours time to burn, here is a crash course on the subject;

Over all idea under the NEC:

View: https://www.youtube.com/watch?v=_7FmwY442dQ



Sizing the breaker/fuse (Short circuit and ground fault protection of the wires):

View: https://www.youtube.com/watch?v=nvJaaWGJMo0



Sizing the conductors:

View: https://www.youtube.com/watch?v=Sic1uoua3og



Sizing of the overload protection for the motor and wires:

View: https://www.youtube.com/watch?v=2cprO8ZdT1U

 

[Lucien Nunes]

90 degree 2 phase is not something we've ever had here and it is always nice to learn something.

I think we have used it here historically in a couple of supply systems, I would have to look it up. IIRC there were two implementations, one used equal sized supplies and symmetrical motors, the other used a lower-rated 'teaser' phase with 1-ph load always on the main phase.

I always cringe when people call a circuit using two line conductors from a 3-phase supply '2-phase' as it's often more trouble than it's worth to explain why it isn't!

 

[Lucien Nunes]

I find it quite odd that you would look up a current rating in the book based on HP rather than using the FLC of a motor.

I believe the logic, and perhaps Cookie can comment, is that the circuit (which is not adjustable after installation) should be sized to allow sufficient ampacity for any standards-compliant motor of that power that might be fitted now or in the future, that might have a marginally different FLC compared to the one specced at installation time. But the overloads are set according to the particular motor in use, based on nameplate FLC.

 

[Cookie]

I believe the logic, and perhaps Cookie can comment, is that the circuit (which is not adjustable after installation) should be sized to allow sufficient ampacity for any standards-compliant motor of that power that might be fitted now or in the future, that might have a marginally different FLC compared to the one specced at installation time. But the overloads are set according to the particular motor in use, based on nameplate FLC.

You got!

The code mandates larger wire in the event the new motor coming in has a higher FLC. Also, HVAC techs, electricians and maintenance guys are always told to re-adjust the motor overloads when replacing an old motor with a new one. Its common to forget.

 

[Cookie]

I think we have used it here historically in a couple of supply systems, I would have to look it up. IIRC there were two implementations, one used equal sized supplies and symmetrical motors, the other used a lower-rated 'teaser' phase with 1-ph load always on the main phase.

I always cringe when people call a circuit using two line conductors from a 3-phase supply '2-phase' as it's often more trouble than it's worth to explain why it isn't!

If you look at old power lines that are still in service in US cities you can even see the remnants of 2 phase power. Often the cable riser pot will have 4 bushings and a 4th insulator (without a conductor in it) on the cross arm or the cross arm with have insulator pins in multiples of 4 instead of 3. The 2 phase was abandoned and changed to 3 phase decades ago but the old stuff remains.

HELCO 4.8kV - East Hartford, CT

By Power Lines. Here's a pretty old riser pole that looks to be a possible remnant of HELCO's long gone four-wire two-phase distribution system (which also ran at 4.4/4.8kV as well). Note the four ter…

www.ipernity.com

[automerge]1575483695[/automerge]
Here is a pic I took of one of those poles 10 years ago:
[automerge]1575483750[/automerge]
Only the pic to the right is mine, the one on the left is AR's from Ipernity.

 

[Cookie]

2 Phase power was mentioned in this thread. Here is post where a US electrician is replacing it in a US installation.

3 phase and 2 phase in the same system

Would this be a viable system? Is this called a Taylor system? Does the scott-T autotransformer do any phase displacement or shifting? Could a neutral be added for the 3 phase?

forums.mikeholt.com

2 phase power does still exist in select locations throughout the US.

 

[Lucien Nunes]

A similar-but-different situation that we sometimes find in the UK is the presence of two lines and a neutral in the service cable to an older building. Because we've never used split phase 120-0-120V in the UK it is not common and usually indicates a legacy configuration, found in one of two kinds of locations.

One is where it was intentional; high load installations (e.g. farms) in rural areas where the only HV nearby was single-phase. For these, we supplied split phase 240-0-240 and ran large motors etc between the outers at 480V. It's the same deal as US 120-0-120 but twice the voltage, however we never added a high-leg to make it into a 3-phase service, i.e. it never formed part of an edge-grounded delta. Some of these installations still exist.

The other reason for two lines, found in towns, was a consequence of the switchover from DC to AC. Where the original distribution and service cables were laid for 3-wire DC, there were two outers and a neutral. Alternate houses would be supplied from the +ve outer and -ve outer, with larger installations receiving both outers to run motors, heating etc at 480V. When these areas were changed over to AC, a new 4-wire distribution cable was needed, but the existing service cables were often retained. 3-wire service cables were connected to two phases and neutral, resulting in 400V between the lines, not 480V. This is never supplied to a customer as a single supply, only as two separate single-phase supplies. I have this in my house, which has its original 1920s 3-wire DC service cable. As they are on different phases both lines can be fully loaded without overloading the neutral, so I can have 2 x 100A 230V supplies for a total of 46kW.

 

[Carl S]

Scott T transformers (2 phase) are still used on the railways for 25kV OH supplies.

 

[Cookie]

Your service reminds me of what they do in very densely populated areas. They will hang a 3 phase transformer and run 120/208Y 4 wires down the street. Business get 4 wires, while homes only 3 (two hots and a neutral).

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