Single Phase Motor

Status
Not open for further replies.
A

Alwayslearningelec

Senior Member
Location
NJ
Occupation
Estimator
If you had a single phase motor would a 3 phase disconnect ever be used for the OCPD or it would have to be 2 pole disconnect?
 
Yes a 3 pole disconnect is often used for single phase. For example a 3 pole disconnect can supply any of the three possible circuits, a 3Ø, 208 volt circuit, a 1Ø, 208 volt circuit, or a 1Ø, 120 volt circuit
 
Ok, so you would only use 2 of the poles on the disco for 208v? What about 120v?

I sometime get confused when it comes to voltages, phases and number of wires used in the specific voltage/phase application.
 
Alwayslearningelec said:
Ok, so you would only use 2 of the poles on the disco for 208v? What about 120v?

I sometime get confused when it comes to voltages, phases and number of wires used in the specific voltage/phase application.
Click to expand...
For 120 volts you would use 1 pole. Although you could splice the neutral I would use a disconnect with a neutral kit in it.
 
infinity said:
For 120 volts you would use 1 pole. Although you could splice the neutral I would use a disconnect with a neutral kit in it.
Click to expand...
That would be an installation where you are permitted to switch the neutral.
404.2(B) Grounded Conductors.
Switches or circuit breakers shall not disconnect the grounded conductor of a circuit.
Exception:
A switch or circuit breaker shall be permitted to disconnect a grounded circuit conductor where all circuit conductors are disconnected simultaneously, or where the device is arranged so that the grounded conductor cannot be disconnected until all the ungrounded conductors of the circuit have been disconnected.
Click to expand...
 
augie47 said:
Sure, any two phases off a 208 3 phase system would be 208 single phase.
Click to expand...
I'm so sorry. See you said any two phases? That's what confusing. Two phase conductors but single phase. I understand it's correct but just hard for me to wrap my head around when I hear (SINGLE) phase but TWO phase conductors. Again I understand it's 1P/208v.

So whenever you have single phase you would never have 3 phase conductors obviously but either one or two phase conductors? But when using a disconnect you would never have only one phase conductor?

Finally, if you had 208v circuit would there be any issue using a 600v switch or it would have to be 250v?
 
Last edited:
It is a confusing term.
There is a two phase system but they are rare and would possibly confuse this situation.
As stated, you could have a disconnect with one phase. Best example being a light switch on a 120v circut
 
augie47 said:
It is a confusing term.
There is a two phase system but they are rare and would possibly confuse this situation.
As stated, you could have a disconnect with one phase. Best example being a light switch on a 120v circut
Click to expand...
Thanks. But I don't think you would ever have a disconnect switch(handle/knife type) that would only have one phase connected to it, correct?

1690764995038.png
 
Alwayslearningelec said:
Thanks. But I don't think you would ever have a disconnect switch(handle/knife type) that would only have one phase connected to it, correct?
Click to expand...
Sure you could have a 120v or 277v line to neutral load that needs a disconnecting means. Due to availability, I would likely use a two pole switch with the other pole not used. Actually the situation where I have needed just one pole, just a few months ago in fact, is on a 240/415 system. I needed to take some line to neutral taps off this system. I could have used a 480v panel board and breakers, but I didn't have time to order that equipment. The disconnects I could get immediately.
 
Alwayslearningelec said:
I'm so sorry. See you said any two phases? That's what confusing. Two phase conductors but single phase. I understand it's correct but just hard for me to wrap my head around when I hear (SINGLE) phase but TWO phase conductors. Again I understand it's 1P/208v.
Click to expand...
Try to see it for what it actually is, and not worry about what it's called.

You have two lines with 120v to the neutral, just like with 120/240v 1ph, and 208v instead of 240v between them. You use the same equipment and wire them the exact same way.

Save knowing the correct terms for impressing people during discussions. ;)

So whenever you have single phase you would never have 3 phase conductors obviously but either one or two phase conductors? But when using a disconnect you would never have only one phase conductor?
Click to expand...
As I said above, you treat 120/208 1ph just like 120/240v 1ph.

Only the load cares about the difference in the line-to-line voltage.

Finally, if you had 208v circuit would there be any issue using a 600v switch or it would have to be 250v?
Click to expand...
No, you can always use a higher-rated switch. Voltage rating is a minimum limit.
 
Alwayslearningelec said:
I'm so sorry. See you said any two phases? That's what confusing. Two phase conductors but single phase. I understand it's correct but just hard for me to wrap my head around when I hear (SINGLE) phase but TWO phase conductors. Again I understand it's 1P/208v.

So whenever you have single phase you would never have 3 phase conductors obviously but either one or two phase conductors? But when using a disconnect you would never have only one phase conductor?

Finally, if you had 208v circuit would there be any issue using a 600v switch or it would have to be 250v?
Click to expand...

The two-phase system had two phases that were a quarter cycle apart. It's now very rare (if ever) that you'll encounter such a system. Since the name "two phase" was already taken, we don't use the term for today's system types for which it would seem more logical to call two-phase.

The two kinds of systems that one might think should be called two-phase, are as follows. Both are called "single phase", although I find the following terms more descriptive.
1. Split-phase. A single phase is center-tapped, to produce two live conductors with equal and opposite voltages, and the neutral from the center tap. Usually 120V to neutral for both phases, and 240V between them. Common for 1 and 2 family dwellings.
2. Open-wye (my preferred term for it). Two phases and a neutral are derived from a 3-phase WYE grid. Usually used for apartments, where each dwelling unit gets a staggered pair of phases from their neighbors. 120 V to neutral powers most of the household loads, while 208V powers the big ticket loads like dryers, HVAC and ovens. All the loads that normally get 240V in a 1 or 2 family dwelling with 120/240V, would get 208V instead.

You can always have unused poles on your switch. Quite common when you only need two of them, and there don't exist any 2-pole switches with the specs that apply to you. What you cannot do, is fuse and disconnect the neutral, unless the NEC specifically requires otherwise. The neutral generally just gets a simple splice. If there are multiple phase conductors on any given circuit, you always fuse and disconnect all of them.

600V switches, fuses, and breakers are all compatible with 240V, 208V and anything less than that, by design. As long as it's still AC.
 
Alwayslearningelec said:
I'm so sorry. See you said any two phases? That's what confusing. Two phase conductors but single phase.
Click to expand...
To add, the term "phase" is being used here to refer to "line" when you hear that. Some even use phase when referring to the two lines of a 120/240v 1ph supply, which we know is single phase.
 
Alwayslearningelec said:
Thanks. But I don't think you would ever have a disconnect switch(handle/knife type) that would only have one phase connected to it, correct?

View attachment 2566616
Click to expand...
When I was a kid, these were everywhere, Service to small out buildings, Pump houses for shallow wellsdc4.jpgdc1.jpgdc2.jpgdc3.jpg, disconnects for small machine tools
 
That one is definitely a rarity... most have 30 amp fuses regardless of the load and wire size :) :)
 
Status
Not open for further replies.
Top