Transformer Protection

[tulum]

Hello all,

Situation: 75kVA 600V-240/120VAC 1P solidly grounded transformer with 200A fusible disconnect c/w TRS150R fuses on the primary, and a 400A 30CCt lighting panel on the secondary. Panel has main lugs only, 3x100A branch circuits, 1x60A branch circuit, 1x15A branch circuit. Items all located in the same lineup and the transformer is a dry type in a NEMA 4x non-ventilated enclosure. We are located in Ontario Canada.

Question: This seems to meet all codes (ESA inspector has confirmed). Does anyone see the need for a main on the secondary?

Thank-you in advance. TULUM.

 

[tulum]

To reword; are there any codes requiring a main over-current device be installed in (or in close proximity to) a panel-board regardless of the system supply?

 

[tulum]

To further explain I have read that in the US .... According to NEC® Article 408.36(D), individual protection for the panel-board must be provided on the secondary side of the transformer. The over-current protection device can be installed either ahead of or in the panel-board. However, I was wondering if this apply to single phase transformers? Does CSA have a similar rule?

 

[deltawhy]

No, according to rule 26-256 of the CEC you do not require protection of the transformer secondaries provided they are rated to 125% of the rated secondary load current. The protection of the panel is provided by the primary overcurrent protection. However, given that you have 200A protection (assumed) on your primary, and that the full load current of a single phase 75kVA Tx at 600V is 125A, this actually does not meet code. Your overcurrent protection on the primary side of the transformer must be a maximum of 125% of the full load current. In this case, that would be 125A x 1.25 = 156.25 rounded up to the nearest fuse / breaker size being 175A. Might want to change that.

 

[waross]

Some safety codes require a disconnect (disconnect, not protection, although they are often the same device) for the panel board. It often depends on the separation distance and sight lines. In some instances the transformer primary disconnect may be acceptable, again depending on the separation distance and sight lines.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[tulum]

Thanks Deltawhy and waross. I do have 150R fuses on the primary (slightly small but seem ok with the inrush points).

However, I have received the damage curve from the manufacturer and the fuses protect the transformer ok mechanically however, the thermal portion is really unprotected. I know this is typical in alot of applications, however in this case the manufacturer plotted the damage curve with points @100% rated current occurring at 10000s through 100000s. I thought transformers could handle 100% rated current for ever - not 10000s (2.76hrs) before damage begins? My fuse curves stop at 1000s so hard to verify what happens after that point....

I know minimum code says primary protection alone is ok as long as it is sized at 125%... but with 375A worth of load breakers, how can the transformer be protected with the above damage curve?

 

[cslater]

Even though your breakers all add up to 375A, they don't necessarily draw that much amperage. For example, it is not uncommon for a 20A receptacle breaker to only have 360 VA applied in the load calculation (3 Amps at 120V) if it a dedicated GFI circuit for example.

If you put in a 125A fuse on the 600V side, that will mean you can pull around 312A on the downstream side before the fuse blows.

I suspect your panel is bigger than it needs to be and that your actual load is below 300A. You may want to turn everything on and put an amp clamp on the feeder to see what you're drawing.