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#129395 02/22/05 01:59 AM
Joined: Aug 2001
Posts: 545
A
aldav53 Offline OP
Member
When measuring current on panel phases. Why doesn't the current add for each phase for the total?
Ex:
On a 200 amp panel, (single or 3 phase) the maximum is 200 amps per phase.


The Golden Rule - "The man with the gold makes the rule"
#129396 02/22/05 04:26 AM
Joined: Jul 2002
Posts: 8,407
Member
Al,
Phase currents are never equal.
The main reason being, that all of the loads are not purely resistive.
There are Reactive components of the currents to be taken into account, also.
It also depends upon what sort of meter you use to take the reading, too.
If you are taking a reading on a Commercial or Industrial installation, Harmonics can upset your readings as well, if they are present.

#129397 02/22/05 11:12 AM
Joined: Dec 2002
Posts: 317
S
Member
At times my demented self does not read things right and this may be one of them. As I read the question, it sounds as if the person is asking:

Why isn't the rating on the 200 amp panel such that if phases A, B, and C are loaded to 67 amps, and 3 times 67 is 201, therefore the panel would be fully loaded?

Answer:
A good question to ask, yet a rating based on this would create many problems due to the unbalnced loading that Mike mentioned. If panel ratings were such, what amperage would you build the panel bus to handle? 67 amps? If so, what would happen in a severly unbalanced system where phase A has 125 amps on it and phases B and C each have only 5 amps on them. The total amperage of the panel load is under the 200 amps, yet if this were the case the phase A bus would be carrying twice its rated current. This would be a big fire hazard due to misunderstanding of the rating.

Therefore to avoid misunderstanding, the ratings of each individual bus (phase A, B, and C) within the panel are used for rating the panel. Each bus must be able to handle the current the panel is rated for and keep within safety margins. If every bus can handle 200 amps safely we have a 200 amp panel, or if every bus can handle only 67 amps we would have a 60 amp panel (note: the manufacturer would have to round down to standard size).

#129398 02/22/05 11:57 AM
Joined: Aug 2001
Posts: 545
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aldav53 Offline OP
Member
sabrown,
Yes, that is basically what I was asking, and yes, I understand the reasoning of it.
I was just wondering how or why in theory it works that way in the transformer.
Thanks,

(I use a Fluke amp clamp to measure with which works well)


The Golden Rule - "The man with the gold makes the rule"
#129399 02/25/05 11:49 AM
Joined: Dec 2002
Posts: 317
S
Member
Yes, it does work that we add up the watt ratings of each coil with tranformers. Completely flys in the face of what I just told you for what goes with panels.

Each half of a single phase transformer can connected many different ways sometimes splitting the winding, and sometimes keeping the winding intact by not using the center tap. Even though the center tap may be a physical break in the winding circuit forming the equivilent of two coils, those coils could just as easily have been just one continuous wind. As such, we rate the single phase transformer as a whole.

Beats me why when we take three transformers and install them in the same case, we then add their ratings to create the whole. I suppose someone in marketing looked at the cost of the three phase transformer and noticed that it cost three times as much and demanded that the ratings of each leg be added together giving it a name 3 times as big which matches the price. Yet, if the three phase transformer were listed for parallel connection, you could effectively connect the three phase transformer up to provide the full transformer rating on a single phase.

Let's not even get into autotransformer connections and ratings. Oops, I should have quit already.

#129400 02/25/05 04:26 PM
Joined: Jul 2001
Posts: 599
J
JBD Offline
Member
The laws of physics (not marketing) dictate how currents are added in transformers and thus how they are rated.

#129401 02/28/05 10:58 AM
Joined: Dec 2002
Posts: 317
S
Member
Dry humor is hard to put to writing.

#129402 02/28/05 12:20 PM
Joined: Nov 2004
Posts: 37
J
Member
You're forgetting to work volts into the equation; remember that volts * amps = power.

Single phase power this question is easy.

The panel is rated 240v 200 amp, lets say.

If one leg is loaded 100a and the other is loaded 100a--that is 200a, but it is 200a at 120v. Which is 100a at 240v, so the panel is only half loaded.

Just like if you pulled 200a off of a single leg it is 200a at 120v, which is 100a at 240v, and once again the panel is still just half loaded.

And then (you can see this coming from a mile away) if you pull 200a on each leg, that is 400a at 120v, which is then 200a at 240v.

As for three phase it must be similar math.

[This message has been edited by jdadamo (edited 02-28-2005).]

#129403 03/01/05 01:02 AM
Joined: Aug 2001
Posts: 545
A
aldav53 Offline OP
Member
jdadamo,
That makes sense..


The Golden Rule - "The man with the gold makes the rule"
#129404 03/01/05 05:43 AM
Joined: Jul 2002
Posts: 8,407
Member
Just to insert a little confusion here. [Linked Image]
You cannot get get out of of the Secondary side of the Transformer, what you put in the Primary side.
In a 100% efficient Transformer, you will get out what you put in.
But Transformers have nasty things like Iron Losses, Copper Losses and even things like Hysteresis (Magnetising Current) Losses.
The losses are quite minimal, but they have to be taken into account.


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