Current capacity of CT clamp on 240v

[btarb24]

Let's say there's an appliance with a 29 amp MCA. It uses a 30 amp 2-pole breaker that will trip if either pole exceeds 30amp.

If you crossed both wires through a CT clamp would you expect a theorized maximum current of 30 amps or 60 amps?

 

[Dennis Alwon]

You would read 30 amps or close to it

 

[infinity]

Usually anything that lists an MCA has a factor of 125% added to it so the actual current will be lower than the MCA.

 

[winnie]

1) Breakers trip on a time/current curve that is identified by the handle rating. Breakers by design will carry far more than their handle rating for short periods of time.

2) What do you mean by

If you crossed both wires through a CT clamp would you expect a theorized maximum current of 30 amps or 60 amps?

All wires in a single path circuit carry the same current. If 30A is flowing through pole A then 30A should be flowing through pole B.

But the direction of flow should be opposite. If you clamp both conductors at once, you should measure 0A.

If you twist one of the wires around so current flows in the same direction, then you would measure double current. But this is simply the same current passing twice through the clamp. If I took one wire and wrapped it around the sense jaw so it went through twice, you would see the same doubling.

Jon

 

[btarb24]

by crossing both wires through the same CT i meant like this so that each pole passes through the CT in a different direction. This way you that you get a cumulative load of both poles.

I don't think the poles are guaranteed to be equal. It could be an asymmetrical load, or you could just be trying to get a cumulative measurement of two completely separate circuits that are on different phases.

I'm struggling to understand the logic though. For instance, it baffles me as to how a 30 amp 2pole breaker trips at 30 amps on either pole .. but they're used for symmetrical loads of not more than 30 total amps - it seems to me that each circuit should trip at 15 amps to yield 30 total. And, of course, this confusion makes me uncertain how the CT clamp would read -- i'm concerned of saturating the clamp if the current it actually sees is a total of 60a.

 

[winnie]

The current clamp in the picture will read the sum of the two 30A sides, and could read 60A before the breaker starts getting into its trip range.

If a neutral is present in the circuit, the two sides could have different current; but only if current is flowing to the neutral. This would require a 3 wire circuit (two hots and a neutral), not a 2 wire 240V circuit (two hots). Or, as you said, you could have two completely separate circuits on opposite phases, and are looking for the totalized usage of them together. But if you are feeding a pure 240V two wire load, then the current on both legs must match (or something is broken).

Your confusion about the 'maximum 60A' issue is a matter of naming convention. A breaker rating is based on the current per pole, not some concept of 'total current' (total current is not well defined in the industry). A 30A two pole breaker is in many ways two 30A single pole breakers side by side, and can either supply two separate 30A 120V loads _or_ a single 30A 240V load.

-Jon

 

[LarryFine]

For instance, it baffles me as to how a 30 amp 2pole breaker trips at 30 amps on either pole .. but they're used for symmetrical loads of not more than 30 total amps - it seems to me that each circuit should trip at 15 amps to yield 30 total.

Nope. 30a is 30a regardless of the voltage. The current in a single circuit is the same everywhere around the loop. Don't confuse mathematics with actual circuits.

Yes, 30a at120v is the same amount of power as 15a at 240v, but that's not relevant to what's going on here. 30a at 240v is twice as much power as 30a at 120v.

Two 15a circuits at 120v can deliver the same amount of power as one 30a circuit at 120v. A 2-pole breaker is used on 240v because both lines are hot to ground.

 

[btarb24]

Thank you for all the wonderful information. Truly very helpful.
One last question on my mind. If a 240v appliance utilizes a neutral for the return then would the mca be per phase or a total of both legs? I suppose if both hots were returning on a neutral then it'd have to be a cumulative mca or the neutral wire would be undersized?

 

[roger]

You would not have a neutral returning current from a 240 volt circuit, (at least as far as this conversation is concerned) on a MWBC such as a dryer circuit for example a neutral would only be common to one 120 V side.

 

[jim dungar]

Thank you for all the wonderful information. Truly very helpful.
One last question on my mind. If a 240v appliance utilizes a neutral for the return then would the mca be per phase or a total of both legs? I suppose if both hots were returning on a neutral then it'd have to be a cumulative mca or the neutral wire would be undersized?

There is no calculation done using a Total value of current from different legs. Stop trying to think of examples.

 

[LarryFine]

btarb, read this: https://forums.mikeholt.com/threads/understanding-the-neutral-conductor.140537/

 

[btarb24]

Thanks for that great explanation, Larry. I paired it with the visuals on this stack answer and it made a lot of sense. I had previously been under a false impression that each phase was generated separately rather than being part of the same coil. I also clearly didn't understand how the neutral line worked. I appreciate the knowledge share. https://diy.stackexchange.com/a/33603/45146

 

[LarryFine]

I had previously been under a false impression that each phase was generated separately rather than being part of the same coil.

Actually, it's common to have two separate 120v secondary coils that can be connected in either series (to obtain 240v (or 120/240v)) or in parallel (to obtain 120v at full capacity).

Power company transformers are often made this way so the same unit can be used to supply 120/240v 1ph (or as part of a 240v 3ph delta bank), or as part of a 208/120v wye bank.

 

[jim dungar]

Actually, it's common to have two separate 120v secondary coils that can be connected in either series (to obtain 240v (or 120/240v)) or in parallel (to obtain 120v at full capacity).

Power company transformers are often made this way so the same unit can be used to supply 120/240v 1ph (or as part of a 240v 3ph delta bank), or as part of a 208/120v wye bank.

View attachment 2566172

Which is why we call this, 120/240V, a single phase system.