Transformer Sizing for Continuous Loads

[PurpleHaze]

I have an installation with a small single phase, single winding transformer that is dedicated to feeding two electric vehicle charging stations. The inspector says that since the chargers are considered continuous loads the transformer is too small because it is not sized at 125% of the full load rating of the chargers. The primary and secondary overcurrent protection is sized at 125% and the primary and secondary conductors are sized for the overcurrent protection. The transformer is UL Listed and rated to carry 100% of its nameplate rating. The transformer rating has not been exceeded by the chargers. I can't find anything in the NEC that supports the inspectors thinking. In fact, it is my understanding that if the breakers were 100% rated, nothing in the circuit would have to be upsized. What am I missing? Any thoughts, comments, similar experiences?

 

[templdl]

I have an installation with a small single phase, single winding transformer that is dedicated to feeding two electric vehicle charging stations. The inspector says that since the chargers are considered continuous loads the transformer is too small because it is not sized at 125% of the full load rating of the chargers. The primary and secondary overcurrent protection is sized at 125% and the primary and secondary conductors are sized for the overcurrent protection. The transformer is UL Listed and rated to carry 100% of its nameplate rating. The transformer rating has not been exceeded by the chargers. I can't find anything in the NEC that supports the inspectors thinking. In fact, it is my understanding that if the breakers were 100% rated, nothing in the circuit would have to be upsized. What am I missing? Any thoughts, comments, similar experiences?

Are you going to win the battle and loose the war? What's your relationship with the inspector? Do you think he is a reasonable person if you were to point it out what you believe to be true?
The only problem that I personally see is that, even though the transformer is designed to carry 100% of its rating, you well reduce the life expectancy of the transformer from what we are accustomed to. Most transformers are not loaded even near to 100% except for a limited period of time maybe. Unless The transformer will run hotter but stay within it's designed rating so running it a 100% isn't prohibited.

For reliability and longevity I would have opted for a larger transformer so it would run cooler.

 

[PurpleHaze]

Are you going to win the battle and loose the war? What's your relationship with the inspector? Do you think he is a reasonable person if you were to point it out what you believe to be true?
The only problem that I personally see is that, even though the transformer is designed to carry 100% of its rating, you well reduce the life expectancy of the transformer from what we are accustomed to. Most transformers are not loaded even near to 100% except for a limited period of time maybe. Unless The transformer will run hotter but stay within it's designed rating so running it a 100% isn't prohibited.

For reliability and longevity I would have opted for a larger transformer so it would run cooler.

I'm sure I'll end up losing the war but the installation is complete and it's a waste of money (that I don't have) to redo a brand new installation if it's not required. I am not worried that the the transformer life will be compromised. There are multiple chargers so there is some diversity. Additionally, they are at a County building that is closed more hours than it is open. If it was a transformer feeding a panelboard, I would definitely have extra capacity for circuits to be added. This transformer is dedicated to the chargers and there are no provisions to add loads.

 

[augie47]

With the wording of 625.21 it will be a difficult battle to win.

 

[PurpleHaze]

With the wording of 625.21 it will be a difficult battle to win.

Why? I have 125% overcurrent protection for feeders and branch circuits. This requirement is to prevent nuisance tripping of typical overcurrent protection that has a continuous rating of 80% unless it is specifically rated for 100% use.

 

[augie47]

I would want to see some documentation that the transformer was designed for 125% continuous rating

 

[PurpleHaze]

I would want to see some documentation that the transformer was designed for 125% continuous rating

I appreciate your input but don't understand why this is relevent. The transformer has been tested by and Listed by UL for continuous use at 100% of it's rating. This installation is under this rating. We are talking about a transformer not the overcurrent protection which has the 125% requirement for the reasons I noted above.

 

[augie47]

Apparently we do not view it the same, but since 625.21 calls for some of the components to be sized for continuous duty I would expect the transformer to be sized likewise.
If you have documentation that the transformer is designed for a 125% load for 3 hours that would be fine.

 

[hurk27]

I would want to see some documentation that the transformer was designed for 125% continuous rating

Gus, There is no requirement in 450 that a transformer has to be rated max 125% of the load, if his calculated load does not exceed the transformer rating its good to go, the only thing the NEC requires is the OCPD rating if below 600 volts table 450.3(B) states 125% max setting, it can be lower if calculated load and inrush will allow.

Since the conductor still have to meet 240.4 since table 240.4(G) does not list primary transformer supply, just Secondary tie
conductors.

I may be wrong but I don't see it in the code?:?

As far as I can see you could have a 25 kva transformer protected by a 50 amp breaker if the calculated load and inrush will allow it.

Look at the heading for table 450.3(B):

Table 450.3(B) Maximum Rating or Setting of Overcurrent Protection for Transformers
600 Volts and Less (as a Percentage of Transformer-Rated Current)

Not minimum

 

[Dennis Alwon]

I had talked to Don - the OP- about this and I wasn't certain. My first response was the same as Gus but then I found nothing in 450 to support the 125% nor the fact that a trany must be rated 125% for continuous duty. My gut says go with Gus on this but my reading tells me otherwise. I will be curious what Ron says--- Don have you contacted Ron. FYI- Ron is the NC state inspector. I am going to guess he will say 125%.

 

[iwire]

I would want to see some documentation that the transformer was designed for 125% continuous rating

I want to see the NEC section that requires a transformer to be rated 125% of the load.

 

[Dennis Alwon]

Has anybody ever seen documentation that a trany was rated 100% for continuous load? Or are they simply rated 100%.

 

[Dennis Alwon]

Here is a comment from another thread

Any xfmr can be operated continuously up to its rated KVA provided a few conditions are met.

1) adequate ventilation is provided (rare in an enclosed room). This includes no accumulation of dust, etc. on the core and coils, and no blocked ventilation openings.

2) Ambient temperature is at or below nameplate rating (usually 40C, 104F).

3) Voltage is not higher than nameplate.

4) on 3PH units, phase balance must be reasonable, though I can't remember what the tolerance is. This is especially true involving delta connections.

On larger units, there's usually 2 (or more) KVA ratings. One is AA (atmospheric aspiration), another is FA (Forced aspiration). The difference can be significant. An example of forced aspiration is a utility switchyard. If you look closely at the xfmrs there, alot of them have fans mounted on the cooling fins. This boosts the KVA rating considerably. Some manufacturers have fan kits you can install in the bottom of smaller dry-type xfmrs. The ones I've installed come with a new label to stick next to the existing one that lists a higher KVA rating for FA cooling.

 

[templdl]

Transformers have never been rated at 125% load as they are designed to carry 100% rating at a given ambient temperature with a specific temperature rise not to exceed the insulation class.
The common dry type distribution transformer is constructed with 220degC insulation.
The normal ambient temperature is 30degC with a 40degC max.
Based upon 40deg max you now add the temperature rise which is normally 150degC max. (other available temperature rises are 115 and 80degC).
Then you have to add 30degC to allow for the hot spot in the transformer windings.
150+40=30= 220degC which is the winding insulation rating. This is at 100% of the transformer rating.
Of course if the ambient temperature is closer to a common 30degC you have more head room to go above the kva rating but still not exceed the 220degC.'
If you order a 115deg C transformer because you still have 220deg insulation, you still have a 40degC rise as well as a 30degC hot spot allowance, you have 350degC temperature difference to work with before exceeding the insulation class. This transformer that has been designed for 115dec at 100% of t transformer rating can be subjected to a15% overload with no sacrifice in transformer if.
An 80degC transformer has even more room by being 70degc bellow the 150degC. 80+40+30=150degC when you have 220degC insulation it i give you 70degC to work with that allows for a 30 overload without getting gigged for transformer life expectancy.

It used to be that transformer life seemed to be endless because transformers were very seldom loaded even close to their rating and when they were it was equalized be an extended period of time at a much lower loading. As such you could easily expect 30 years or even more of transformer life. With budgets the way they are today transformer sizing is not so conservative. The design engineer that I once used as my brain trust advise me that if a transformer was loaded 100% continually expect the to be 3-5 years.
One place where this is true is with oil drilling platforms in the gulf where real-estate is at a premium. They don't have the room for oversized transformers and opt for replacing them on a regular basis. As such they keep spares on hand.

Some things that your mother never taught you.

 

[templdl]

One small added note. Yes, by adding fans an additional capacity can be realized which I think is 15% if I remember correctly. BUT!! IN ANY EVENT when you load a transformer above what its name plate rating the transformer connections and/or bussing on larger must be sized for the additional loads. Simply adding fans and/or using the additional temperature rise on an 80degC or 115degC transformer may not work. This must be confermed twith the transformer manufacturer.
Transforemrs with a name plate that includes AA/FFA allows for the addition os a pan package at a later date. If is does not it most likely will not be engineered for the addional load.

 

[augie47]

I want to see the NEC section that requires a transformer to be rated 125% of the load.

Point made ! Had to think about it .... my ole brain was playing games..I was looking at the load as 125% of the transformer rating.. my bad.

templdl seems to have given a great synopsis.

 

[iwire]

templdl seems to have given a great synopsis.

He has, he is good like that.

 

[roger]

I will be curious what Ron says--- Don have you contacted Ron. FYI- Ron is the NC state inspector. I am going to guess he will say 125%.

I don't know if Ron will say that but, Ron will reply with an answer which is more than other state level inspectors and engineers will do if they are wrong, they just ignore you and hope it goes away.

Roger

 

[ceb58]

I don't know if Ron will say that but, Ron will reply with an answer which is more than other state level inspectors and engineers will do if they are wrong, they just ignore you and hope it goes away.

Roger

Say its not so :angel:

 

[augie47]

"wrong" pasahhhh ! perhaps misunderstood