Exceeding Transformer Capacity

[zog]

A common thumb rule for life expectancy of transformers is as follows:

Class ?A? insulation - 8?C above max temp rating= ? life expectancy
Class ?B? insulation - 12?C above max temp rating= ? life expectancy

 

[steve66]

What's the "current capacity" of a 30kVA transformer? If its tested at 200% of rated current per UL 1561, doesn't it have the "capacity" to carry twice its rated current?

For all I know, that could be a nice little safety margin UL feels is necessary. Maybe they have had som bad experiences with transformers being overloaded.

 

[steve66]

Seems clear cut when stated like that...

However, when we make the distinction the 38kVA calculated load is noncontinuous and the transformer 30kVA rating is continuous, the judgement is not so clear cut. If we take into consideration transformer protection rating is permitted up to 125%* of rated current seems to indicate a noncontinuous load is permitted up to 125% of the transformer rating.

*Where 125% of rated current does not correspond with a standard OCPD rating, we are permitted to round up to the next standard rating. We could feasibly have a protection rating in excess of 150% of rated current.

i don't see any reason to bring continuous vs non continuous loads into the equation. I don't think you will find those terms used anywhere in the nec with regard to transformer load calcs.

And breaker size doesn't always match the allowed load calc. For example, I can use a 400 amp breaker on 500 Kcm wire, but only when the load calc is less than the wires rating. I believe the extra 25% may be to help prevent nuisance trips when first energizing a xformer.

i don't have the nec with me, but I think some of the tables in 450 allow really large OCP. Maybe even 600% of the former rating. does that mean I can put 240kva of load on a 30kva xformer? I hope not.

 

[Smart $]

i don't see any reason to bring continuous vs non continuous loads into the equation. I don't think you will find those terms used anywhere in the nec with regard to transformer load calcs.

Of course not. Your position is enforced by exclusion...

...and there's no such thing as transformer loads calc's. There's only service/feeder and branch circuit load calc's.

And breaker size doesn't always match the allowed load calc. For example, I can use a 400 amp breaker on 500 Kcm wire, but only when the load calc is less than the wires rating.

I agree... but there's nothing preventing one from installing a conductor with an ampacity equal or exceeding the OCPD rating

I believe the extra 25% may be to help prevent nuisance trips when first energizing a xformer.

Perhaps.

i don't have the nec with me, but I think some of the tables in 450 allow really large OCP. Maybe even 600% of the former rating. does that mean I can put 240kva of load on a 30kva xformer? I hope not.

When you go larger than 125% (*with "allowance"), it's regarding a primary and secondary protection scheme, under which the secondary OCPD will be limited to 125%*. And since we're talking about a 30kVA transformer, either scheme will limit the protection to 125%* on one side or the other.

Keep in mind throughout this discussion, I am not recommending my view as a standard practice.

 

[steve66]

Keep in mind throughout this discussion, I am not recommending my view as a standard practice.

Yes, I do realize that. And on the other hand, NEC load calcs. tend to be so conservative, I'm not sure 38KVA of calculated load on a 30KVA transformer would ever be a real problem.

When you go larger than 125% (*with "allowance"), it's regarding a primary and secondary protection scheme, under which the secondary OCPD will be limited to 125%*. And since we're talking about a 30kVA transformer, either scheme will limit the protection to 125%* on one side or the other.

I see one row, that for higher voltages allows 600% on the primary and 300% on the secondary. My point is that if you can use table 450.3 to justify a 125% load on a transformer, you can also use it to justify a 300% load on a transformer. I find it hard to believe the intent of the NEC was to allow that.

I think the argument basically boils down to "How do we determine the current rating for a transformer?"

My reply is that we do this every time we use table 450. Notice the title is "maximum rating or setting..... as a percentage of transformer rated current." I think everyone just uses the labeled KVA and voltage to determine the current rating.

 

[Smart $]

Yes, I do realize that. And on the other hand, NEC load calcs. tend to be so conservative, I'm not sure 38KVA of calculated load on a 30KVA transformer would ever be a real problem.

Not bound by NEC compiance, POCO's consistently take advantage of that.

I see one row, that for higher voltages allows 600% on the primary and 300% on the secondary. My point is that if you can use table 450.3 to justify a 125% load on a transformer, you can also use it to justify a 300% load on a transformer. I find it hard to believe the intent of the NEC was to allow that.

Not stated but I was only discussing LV transformers here... but point taken. For this application, no matter which protection scheme is used, one of the OCPD's is limited to 125%*.

 

[templdl]

Regarding the subject of transformer overload capacity, Per ANSI standards basically a distribution transformer wil deliver 200% NP load for 1/2 hr, 150% for 1hr, and 125% for 4hr w/o damage provided that 50% load precedes and follows the overload. This is not deliberately designed into the transformer but is the result of the winding temperature rise allowed with NP rating.
In addition, knowing that the winding insulation is rated 220degC a transformer with a 115degC rise can carry 115% of its NP rating and a transformer with an 80degC rating w/o a reduction of life expectancy.

 

[templdl]

Regarding the subject of transformer overload capacity, Per ANSI standards basically a distribution transformer wil deliver 200% NP load for 1/2 hr, 150% for 1hr, and 125% for 4hr w/o damage provided that 50% load precedes and follows the overload. This is not deliberately designed into the transformer but is the result of the winding temperature rise allowed with NP rating.
In addition, knowing that the winding insulation is rated 220degC a transformer with a 115degC rise can carry 115% of its NP rating and a transformer with an 80degC rating w/o a reduction of life expectancy.

I left out that the 80degC transformer has the ability to carry 130%? of its NP rating w/o reduction of life expectancy. These are at an anticipated 30degC ambient.