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Circuit Breaker Sizing

Circuit Breaker Sizing

Circuit Breaker Sizing

I have a 30Amp 24VDC power supply.  The power input to the power supply is 480VAC and the output from the power supply is 24VDC.  I want to put a 3-phase circuit breaker before the power supply, but I'm having a hard time sizing the circuit breaker.  Is there a formula that I can use for this?  I talked to the support guys at SOLA, but he just guessed and told me that a 10A breaker would do the job, but I'd like to confirm that, plus also learn how this can be determined.  Is there a formula that I can use for this?


RE: Circuit Breaker Sizing

If the power supply is listed by a recognized testing laboratory (e.g. U.L.), they must specify a rating for the input overcurrent protection as part of that rating. I think you need to ask someone else at Sola.

RE: Circuit Breaker Sizing

Sematics alert!
I think that you will likely get the same answer regardless of who you ask at Sola. Sola is based in the US and here, the term "circuit breaker" traditionally is used to describe what everyone else calls an MCCB (Molded Case Circuit Breaker). In that case, there is no size smaller than 10A for a 3 phase device, if you can even find that. Most are 15A.

Elsewhere in the world you have small 3 phase DIN rail mountable "circuit breakers" with much lower ratings, i.e. 1.2A, 1.6A etc., which is what you would need as the only protective device (we would use fuses downstream from the 15A CB). We import those devices here as well, but they are not called "circuit breakers", they are referred to as "supplemental circuit protectors" because the definition for circuit breaker was already well established as an MCCB.

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RE: Circuit Breaker Sizing

The little breakers jraef mentions are known as MCBs or miniature circuit breakers in IEC territory. They have some advantages compared with a fuse but are limited in their breaking capacity compared with a fuse: 6, 10 & 15kA are typical maximums. I haven't seen them rated at less than 2A, but there are other 'circuit breakers' by the likes of E-T-A and Airpax which are really designed to be built in to equipment rather than part of a distribution system.

  Sometimes I only open my mouth to swap feet...

RE: Circuit Breaker Sizing

Perhaps 10A is the correct answer... Still it depends on how the device was tested and listed. There is not a formula to apply for this.

RE: Circuit Breaker Sizing

You say that 30 Amps at 24VDC is the output rating of your power supply.  You also state that the input voltage is 480VAC, 3ph.  Is there an input current rating listed on the unit nameplate?

It sounds like you have a Sola/Hevi-Duty SDN-30-24-480 power supply.  This is a snap-in or screw mounted DIN rail mounted power supply.  If this is the case then the input current is about 2 Amps.  These units are internally fused, so if you put a 10A/3P circuit breaker to protect the input conductors that is fine, but the unit itself is protected by the internal fusing.

Hope this is of help.  I assume you already looked at www.solaheviduty.com ?

RE: Circuit Breaker Sizing

Your circuit breaker must be adequate for the load and must protect the conductors.
In North America, the minimums for field wiring would be #14 AWG and a 15 amp breaker.

RE: Circuit Breaker Sizing

NEC lists standard breaker sizes in 240.6(A).  The smallest breaker listed there is 15A.  NEC would never require a 10A or smaller because there is no such thing per NEC (but note that there are fuses listed down to 1A).

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