circuit breaker

Hi all,
I am learning to design some basic control systems, amd need some help with the sizing of circuit breakers, fuses and transformers.
i.e.
3 Phase 480v feed with a fused disconnect(How do I determine what size fuses do I need here?)
I will be coming off two of the phases to give me 240v to a step down transformer 240v to 110vac which will be my control voltage. (how do I determine the fuse size for my control voltage.) here is an example of what I am trying to do

480vac DISCONNECT IIIIIIIIIII IIIIIIIIII
-------------fuse------------I MCC I------------I MTR 1HPI
-------------fuse------+-----I I------------I 2.5FLA I
-------------fuse---+--I-----I I------------I 480VAC I
I I IIIIIIIIIII IIIIIIIIII
I I
I I
TRANSFORMER 240VAC=>110VAC
====== How do determine what size in KVA trans. I need I I
+------------------+ +------------------------------------------+
+ +
fuse (how do I find the size I need) +
+ +
+ +
+ how do I size the CB below +
+-------CIRCUIT BREAKER---------------------plc pwr supply-------+
+ +
+ +
+-------CIRCUIT BREAKER---------------------plc OUTPUT MODULE----+

Also where should a person like myself look to find more information on this subject for load calculation and rules/guidlines for this type of stuff
thanks

Let's first fix your post so others can see:

480vac        DISCONNECT     IIIIIIIIIII            IIIIIIIIII

-------------fuse------------I MCC     I------------I MTR 1HPI

-------------fuse------+-----I         I------------I 2.5FLA I

-------------fuse---+--I-----I         I------------I 480VAC I

                    I  I     IIIIIIIIIII            IIIIIIIIII

                    I  I

                    I  I

                   TRANSFORMER 240VAC=>110VAC 

                   ====== How do determine what size in KVA trans. I need

                    I  I

 +------------------+  +------------------------------------------+

 +                                                                +

fuse (how do I find the size I need)                              +

 +                                                                +

 +                                                                +

 +      how do I size the CB below                                +

 +-------CIRCUIT BREAKER---------------------plc pwr supply-------+ 

 +                                                                +

 +                                                                +

 +-------CIRCUIT BREAKER---------------------plc OUTPUT MODULE----+

Here's a quick answer:

The transformer will be sized according to the total load on the secondary. Your total load in amps multiplied by 110V will give you the required minimum VA of your transformer (using a larger transformer is okay). Note that KVA is 1000 VA.

The fuse size will be based on the transformer VA rating. This is to protect the transformer from overload. Since we don't know where you're from (please register), I won't guess as to the code requirements in your location. There are tables/formulas to determine the proper size.

Circuit breaker type and size will be based on the load (and type of load) that the circuit breaker is protecting (in this case, larger is NOT better!).

Others will expand on this (and correct any errors I made)...

beerchug

-Eric

most of my cases= instead of circuit breaker for plc and out put module, i'm using circuit protector or 1.5(min load) ampere fuses only

Re: Re: circuit breaker

Eric Nelson said:

The transformer will be sized according to the total load on the secondary. Your total load in amps multiplied by 110V will give you the required minimum VA of your transformer (using a larger transformer is okay). Note that KVA is 1000 VA.

Click to expand...

I would also get the primary voltage of the transformer correct!

You have drawn a 240v/110v transformer connected across two phases of a 480v three phase supply!

Question - What is the voltage across these two phases?

Hint - It is NOT 240v!

Eric, although I used your reply in the quote, my comments are directed towards the original poster.

Paul

Re: Re: circuit breaker

Eric Nelson said:

Circuit breaker type and size will be based on the load (and type of load) that the circuit breaker is protecting (in this case, larger is NOT better!).

Click to expand...

That is true, but don't forget the most basic reason for circuit protection- the ampacity of the conductors. I have often seen 16AWG wires with 20A fuses on them.

Sometimes fuses can be expensive...

so... what's wrong with using a #16 fuse-link to protect the fuse?

(109)

Rick Densing said:

I have often seen 16AWG wires with 20A fuses on them.

Click to expand...

And I have seen even SMALLER gauge wire with 30A fuses on it! Why? Simply because a 30A 1/4 x 1-1/4" fuse fit JUST AS WELL as the 1A version!...

I use circuit breakers rather than fuses for 2 main reasons:

1.) The customer doesn't have to keep 100 different sizes and types of fuses on hand in case one blows.

2.) They won't stick a 30A fuse in place of a 1A fuse because they're out of 1A fuses. Remember, bayonet fuseholders make it all too easy for the 'average Joe' to play electrician. You think he actually reads the type and rating on the fuse????

Breakers cost a little more up front, but they pay for themselves many times over in the long run... :nodi:

beerchug

-Eric

P.S. to Terry. Ever piggybacked a 'regular' fuse to a blown non-standard one? I have been guilty of doing that in a pinch...

Eric Nelson said:

I use circuit breakers rather than fuses for 2 main reasons:

Click to expand...

Unfortunatly, breakers are incredibly less safe to use than fuses.
There is the potential for uninterupted short circuit currents (hundreds of thousands of amps) for a relativly long time (a couple of cycles) before your breaker trips.

They have gone so far as to require fuses for most applications in Canada.

I used to use breakers all the time for convience, but now I stick to fuses whenever I have the option.

Re: Re: Re: circuit breaker

PLucas said:

Question - What is the voltage across these two phases?

Hint - It is NOT 240v!

Click to expand...

I want to repeat what PLucas said, because it is key here, and I wouldn't want you to miss it.

You CAN NOT use a 240VAC ->120V transformer with 2 phases of a 3 phase 480VAC feed.

The voltage between any 2 legs of the 480V Feeder is 480VAC RMS.
You need a single phase 480->120 transformer.


As to the fusing on the transformer, If you are designing to the U.S. NEC code, the secondary needs to be fused at < 125% of the rated secondary current of the XFMR.
It needs to protect the wires as well, so you should consider the wire sizing too.
The primary needs to be fused as well. If the secondary is fused as I stated above, the primary can be fused at up to 250% of the primary rated current. It is unclear if your feeder fuses will meet this requirement. If not, then you will need to add 2 additional fuses (one on each leg of the feeders to the transformer)

Unfortunatly, breakers are incredibly less safe to use than fuses.There is the potential for uninterupted short circuit currents (hundreds of thousands of amps) for a relativly long time (a couple of cycles) before your breaker trips.

Click to expand...

I have always considered it best practise to install current limiting fuses rated to the fault current on the incomer to the cabinet, then circuit breakers for all internal distribution. A fault current of

hundreds of thousands of amps

Click to expand...

would be a massive transformer and a short run of BIG conductors.

Unfortunatly, breakers are incredibly less safe to use than fuses.There is the potential for uninterupted short circuit currents (hundreds of thousands of amps) for a relativly long time (a couple of cycles) before your breaker trips.

Click to expand...

Huh ..??

Its one of the jobs of the person in charge of the electrical design:
Establish what is the shortcircuit current level.
And make certain that all CBs (and fuses !) has enough breaking capacity to match that level.
If a short-circuit occurs, a CB must break the current within the first half-cycle. And it will if it is rated for the shortcircuit current level.

Both CBs and fuses can have current-limiting capabilities.

Terry Woods said:

Sometimes fuses can be expensive...

so... what's wrong with using a #16 fuse-link to protect the fuse?

(109)

Click to expand...