Primary OCPD for XFMR

[ron]

Table 450-3(b) indicates 250% MAX for primary protection, when both primary and secondary will be provided.
On the other hand, if the load on the secondary is fixed and will be low, how close has anyone set the primary OCPD relative to the FLA.
I don't want to go below 125% of primary FLA to avoid nusiance tripping during in-rush, but I am being asked if there is a code violation to go to 95% to save some $$$.
I realize on large transformers the inrush could be as high as 6x FLA, but is the inrush so severe on small transformers (<75kVA)?

Code references?

 

[bob]

Re: Primary OCPD for XFMR

Ron
You can use a time delay fuse and set it at 100%.
I am curious how much you are saving.
The secondary protection is covered in section
240.21.5c.

[ March 11, 2003, 05:13 PM: Message edited by: bob ]

 

[ron]

Re: Primary OCPD for XFMR

The issue is the primary protection. Using 95% of FLA permits the use of an existing panel. More than that will require a new panelboard with spaces that fit the larger frame breaker.
The primary OCPD will be a thermal magnetic breaker.

[ March 11, 2003, 06:31 PM: Message edited by: ron ]

 

[jim dungar]

Re: Primary OCPD for XFMR

Historically, I have found the inrush on smaller transformers to be higher than large transformers. Also lightly loaded transformers seem to be worse than moderately loaded ones.

In anwer to your question, Yes I often use breakers sized less than 125%. For example:
75KVA Three phase transformer, 480-208Y/120
Primary = 90A -> I often use a 100A breaker
Secondary = 208A -> I normally use a 200A breaker

 

[templdl]

Re: Primary OCPD for XFMR

Ron, As a former applications engineer I have some bench mark figures for what the first asymetrical 1/2 cycle magnetizing current would be.
What is the KVA, pri. voltage, temp. rise, and if it's has a k-factor such as K4,K13, etc of the transformer in question?
Each can have an affect of the inrush. Actually a 150degC rise transformer can have as much as 23x the FLA for transformers below about 7kva to somewhere around 8x for transformers up to about 225kva. Lower temperature rises can increase the inrush as much a 20% and as much as 60-80% for a K13.
I'm not saying that this is an exact science but it can give some direction to help trouble shoot nuisance tripping problems or when you may pushing the edge when selecting an OCPD such as the question you have asked.

 

[ron]

Re: Primary OCPD for XFMR

Single phase 50 kVA, 480-240/120, 150 degree rise, no k-factor rating.
Primary FLA = 104A
The installer wants to use a 100A breaker.
Although the secondary load will be relatively small, the inrush, in my opinion will trip the primary breaker.
I can't find a code to stop the installation, before the client wastes their money.

 

[templdl]

Re: Primary OCPD for XFMR

The inrush for a 50kva 150degC transformer with a 480v should be about 10x the FLA which puts it about 1040a. Since the instantaneous pickup of a 100a breaker is 10x the rating or 1000a with a +-20% calibration allowance or 800-1200a, this may be a marginal application.
I would advise your client that of the posibility of nuisance trips and it's his call.

 

[toddb1963]

Re: Primary OCPD for XFMR

yeah, what he said.