Xfmr temp rise & conductors

[SteveFehr]

What is the impact to conductors and lugs when terminated to an encapsulated dry-type transformer in a high-ambient environment? Must the transformer be derated to prevent temperature rise from exceeding the temperature/current rating of the conductors and lugs? I'd hate to spec xfmr and cabling 3x too large, but I'd hate even more to burn it all up.

 

[dpc]

The transformer itself must be de-rated if the ambient is high. Transformer ratings are generally based on an average ambient of 30 deg C. In addition, the conductors must be de-rated for high ambient even without the extra heat from the transformer.

I'd get manufacturer's installation instructions for the transformer and follow their guidelines/requirements. The installation instructions should deal with the extra heat coming from the transformer.

If you have to de-rate the conductors, you can start from the 90 deg C ampacity.

Normally the primary conductors end up being oversized anyway because the primary breaker or fuse has to be sized to handle the transformer inrush.

 

[davidbeach]

If you have to de-rate the conductors, you can start from the 90 deg C ampacity.

ONLY if the conductor has a 90 deg C ampacity for the conditions. If, for instance, type THW is used which does not have a 90 deg C rating your derating would have to start with the 70 deg C ampacity.

 

[dpc]

That is correct - thanks for the clarification.

 

[SteveFehr]

Yeah, temp derating is a #####- even worse with solar loading on the conduit. What concerns me is that if I put a 115C rise enclosed transformer in, say, a 60C environment, the core will be 175C, which is WAY beyond anything I can derate for with THWN. Don't think I've seen THHHHHHHHHWN in the supply house I've never used Z rated cable; is that an option? I haven't spoken to any xfmr supplier yet, btw, I'm still writing the spec- and this was my main reservation from recommending a dry-type enclosed transformer.

Does this mean I have to derate the transformer to 20% in order to keep the enclosure temp to 80C? I can't imagine manufacturers would sell enclosed transformers with 115C or 150C rise if this were the case. Is there something I'm missing?

 

[SteveFehr]

Clarification: I meant dual rated THHN/THWN (90C). This will be in a hot wet location.

 

[dpc]

The building conductors will not really be at the same temperature as the windings. It will be somewhere in between ambient and the transformer winding temp. This is why you need to rely on the manufacturer's recommendations.

You can get high-temperature wire. It can be run some distance away from the transformer and then spliced onto normal building wire. This is routinely done around burner fronts on boilers.

Can you find another location for the transformer?

 

[SteveFehr]

I recieved data from the manufacturer that ambient rise is 65°C for an 80°C rise enclosed transformer! If 115° and 150°C rise and hotter are common ratings for transformers, how in the world is this reconciled with 90°C rated cables?

Am I worried over nothing, will the cables wick away the heat fast enough that it's not an issue? I don't mind derating cabling, but it's difficult to derate a 90C cable for 128C ambient conditions. What's the proper way to terminate to a NEMA 4 transformer?

 

[DRWeig]

Hi Steve,

Not sure about your termination issues -- I'd think you would want a bit of liquidtight flex, though.

Here's some cable you can use to get yourself outside the hot area:

http://www.awcwire.com/ProductSpec.aspx?id=FLAME-1000-Power-Cable

Let us know what you decide on!

Good on ya,

Goober Dave

 

[davidbeach]

Cable generally aren't inside the core of the transformer. If the cable termination area of the transformer enclosure gets to 128[°]C you certainly can't use 90[°]C wire.

 

[resqcapt19]

I have never seen any markings on a transformer to indicate that you cannot use standard building wire to make the connections. It was my thought that as long as you kept your field wire below the line (the one that says "do not install conductors above this point") you would not have heat problems. I don't know anyone who uses high temperature wire for transformer field connections.

 

[davidbeach]

I don't either, but then I've never seen the insides of a totally enclosed transformer case.