Wiring a three-phase socket.

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  #1  
Old 09-03-05, 04:41 PM
Jason210
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Wiring a three-phase socket.

Hi

I need to add a three-phase socket in my home in order to operate an electric saw I've bought. Don't know much about three-phase, except the stuff I did in school.

The saw is rated at 5.2KW 400v. If this was single phase that would give a current of 13A, so I'd probably put in a 20A breaker in the fuse box and use 2.5 square millimeter wire. Simple enough.

But what do I do for 3-Phase? Clearly I need to put in three breakers, but kind of wire should I use for each phase circuit? 2.5mm? What trip current on each breaker? I don't know how to figure this out. This 5.2Kw, is that, 13A on each phases circuit, or 1/3 of 13 (3.5A) on each phase circuit, totalling 13A?

By the way, I'm in Sweden and we operate single phase AC at 50 cycles at 230v here.
 
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  #2  
Old 09-03-05, 04:46 PM
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I don't know about Sweden, but 99.99% of homes in the U.S. don't even have access to three-phase power. Do you?

You can probably buy a motor/generator that can produce three-phase from single-phase, but that will probably cost more than you paid for the saw.
 
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Old 09-03-05, 06:05 PM
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This is not quite as simple as saying '5.2 kW at this voltage means this current means these breakers...' in the US at least, there are extensive rules requiring things such as overload protection for motors, with rather different wire sizes and breaker sizes than would ordinarily be used for the same kW rating in, for example, a resistive load.

Sweden will almost certainly have _different_ rules for motors. I believe that three phase 400V/230 wye is relatively common in Sweden, but I may be recalling this incorrectly. It may be that case that they use three phase transformers, but only deliver a single phase to each home.

If you don't actually have a three phase panel, then this whole plan is right out. You will only have three phase power available if that is what is actually delivered to your home.

If you get a copy of the appropriate rules, we can probably help you figure them out, but pretty much only by helping you with the background theory.

For example, the power delivered to a three phase load resistive load is A * V * sqrt(3), and if the load is something like a motor with a 'power factor' (inductive current flow) then the power delivered is pf *A * V * sqrt(3). My guess is that this 5.2kW motor will draw about 9 amps per phase when operating.

But as I said, this is only part of the story, because you need to provide for starting current and overloads, and you also need to provide overload protection, and to get this part figured out you will need to go to the appropriate local codes.

-Jon
 
  #4  
Old 09-03-05, 06:38 PM
Jason210
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Thanks - We do have three phase power. I live on a farm that has three phase equipment such as fans and other machinery.

I knew it wouldn't be simple, but the info you gave me helps. I'll see if I can find some rules and translate them.

Just one quick question - for the breakers, is it best to use three seperate breakers, or one where they are all operated togther - tripole?
 
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Old 09-03-05, 09:27 PM
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It will be essential to verify proper phasing with a phase meter when wiring that receptacle. Improper phasing will cause very bad things to happen!
 
  #6  
Old 09-04-05, 01:47 AM
Jason210
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594tough:

>>It will be essential to verify proper phasing with a phase meter when wiring that receptacle. Improper phasing will cause very bad things to happen!

Meaning here that I should be careful to wire it correctly and make sure I don't mix the different phases up? Of course...
 
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Old 09-04-05, 03:37 PM
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In the US you would be required to use a three pole breaker with 'internal common trip', which opens _all_ of the phases to the load in the event of a failure on one of them. Even if this is not required under your code, I would do so.

In regard to phasing, getting the phasing wrong will cause anything from no problem, to very minor problems, to mechanical damage to serious electrical damage. If this is a simple belt driven table saw with a simple mechanical control switch, then getting the phasing wrong will simply cause the motor to run in reverse; not a big deal. If you were connecting a pump that would be damaged by reverse operation, or a more complex control system that could connect phases together if not wired properly, then having a phase meter would be essential.

-Jon
 
  #8  
Old 09-25-05, 11:05 AM
Jason210
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Thanks all for the help.

Winnie: I found some more information out about the motor. It says it's 5.2KW(P1/S6/40%). And its Y. We definitely have 3-phase supply. There are three seperate lines coming in called L1, L2 and L3, and we already have some three-phase motors in operation (water pumps).

The information on the motor I don't get: P1, S6 and 40%.

Winnie, I know the kind of circuit breaker you mean, with the 'internal common trip'. The only problem left for me now is to decide on the trip current of that circuit breaker, and from the cable cross-sectional area. You estimated that it would be around 9A. Given the new info (P1/S6/40%), can you say any more about this?

I don't get the formula power = A * V * sqrt(3). Is the V the single phase 230 AC voltage or the combined phases 400V voltage? Must be the latter otherwise this doesn't make sense:

9 * 230 * SQRT(3) * 0.85 = 3585w

9 * 400 * SQRT(3) * 0.85 = 5300w

Since I don't know this motor's pf or eff, and we're kind of guessing 9A, and I have to choose between a 10A breaker or a 16A breaker, what should I go for? I've a feeling that 10A would probably trip too often, but then 16A seems too high. I know there are graphs and tabels to look at, but without exact data seems a bit pointless doing so.

Winter's coming and I need to get this wood sawn up...
 

Last edited by Jason210; 09-25-05 at 01:34 PM.
  #9  
Old 09-26-05, 05:27 AM
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"I don't get the formula power = A * V * sqrt(3). "

Use the line to line voltage for this formula.
 
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Old 09-26-05, 07:24 AM
Jason210
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Sorry - what do you mean by Line to Line voltage?
 
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Old 09-26-05, 10:24 AM
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The line to line voltage (phase to phase) is 400V in your case. The line to ground (phase to ground) voltage is 230V.
 
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Old 09-26-05, 01:11 PM
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The issue with phases is the motor will run backwards if wrong. Not realy a big deal. Just switch two phases and you will be fine.

The breaker must be a 3 pole breaker. You want to make sure if one leg trips they all trip. If not you will single phase the motor which is bad.
 
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Old 09-26-05, 01:23 PM
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I'm sorry. I don't think that I will be able to help you without access to the appropriate code books for your region. I've not been able to find something that interprets the design numbers that you provided.

I can tell you in general how this would be handled in the US.

1) We would go to a table that says 'at this voltage and this hp rating, this is the design current that you must use for the circuit conductors'. If you actually go through the numbers in these tables, you will find that the design current works out to roughly the calculated full load current of the motor.

2) We would be required to provide motor overload protection. For most motors this means a true motor overload controller, eg. one with 'heater packs' and a contactor. This is because the overload protection requirements for a motor are quite different from what most circuit breakers are designed to deal with. _Some_ motors have built in thermal overload protection, and others are what are known as 'impedance protected', but somewhere in the system there must be overload protection for the motor.

3) With the overload protection dealt with at the motor, the circuit breaker need only provide 'short circuit' protection, and is generally sized quite a bit larger than normal for a given set of circuit conductors.

Net result: (and this is only a rough paraphrase from _US_ code; the codebook section is quite lengthy) a motor which has a full load current of 9A would be required to have matching overload protection, would have supply conductors with perhaps a 20A rating, and would have a circuit breaker of 30-45A rating.

Good Luck
-Jon
 
  #14  
Old 09-26-05, 01:51 PM
Jason210
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Originally Posted by ibpooks
The line to line voltage (phase to phase) is 400V in your case. The line to ground (phase to ground) voltage is 230V.
Thanks. This is all making sense now. The only remaining issue is the power factor of the motor, and the efficiency of the motor. Usually, these seem to be in the range of 0.85 - 0.9, so the motor might require slightly more current than the 9A. Is it a safe bet to say it'll be between 9A and 11A? Doesn't this mean a 16A breaker would be required, since if the current was 11A, a 10A rated breaker would trip after a minute or two.

A 16A breaker would give a cable rating of 25A, and that'd be 4mm2 core. That'd be a safe circuit but the only thing with this is overload - the saw could run at 30A for 3 mins before the breaker tripped....
 
  #15  
Old 09-27-05, 05:42 AM
Jason210
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Winnie -
Thanks. You've been a great help. I can tell you that none of the three-phase circuits on this farm have any kind of special overload protection other than a standard fuse. Am I right in thinking that such fuses could function both a simple overload protection device, as well as circuit fault cut-out?

The saw I have is new and there is a black box on it where the power goes in, and some info sits here (it's in German). I would imagine that this is some form of overload protection.

And while I was looking I found a figure on the saw. It said "cos ω" and then was followed by the number .83

Not sure if this was efficiency or power factor.
 

Last edited by Jason210; 09-27-05 at 05:54 AM.
  #16  
Old 09-27-05, 06:02 AM
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For some motors in the US, the circuit breaker or fuse functions as the required overload protection.

For other motors, the overload protection is built into the motor.

Possibly the information plate on the saw or motor will provide all of the necessary information. In the US, many 'listed assemblies' simply specify the required conductor ampacity and breaker rating.

cos omega is the power factor.

-Jon
 
  #17  
Old 09-27-05, 12:07 PM
Jason210
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Thanks. I've bought the cable and circuit breaker now. I'll let you know when it's up and running.
 
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