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transformers
- Thread starter K_Mark
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- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Re: transformers
You actually have four major things to consider. And work from the load to the source.
1)Does the panelboard need protection?
2)Does the conductor feeding the panelboard need protection? Can a tap rule be used? Can the main for the panel also protect the conductor?
3)Is their a reason increase the size of the transformer primary protection beyond the normal 125% FLA?
4)If the primary protection to the transformer is increased are the conductors feeding the transformer still protected?
You actually have four major things to consider. And work from the load to the source.
1)Does the panelboard need protection?
2)Does the conductor feeding the panelboard need protection? Can a tap rule be used? Can the main for the panel also protect the conductor?
3)Is their a reason increase the size of the transformer primary protection beyond the normal 125% FLA?
4)If the primary protection to the transformer is increased are the conductors feeding the transformer still protected?
fc
Senior Member
- Location
- New Jersey
Re: transformers
The rule is that the primary MUST have protection within 125 percent of it's rating. After this, you go to article 240 for protection of the secondary conductors. When you look at section 240.21(C) you will see that the seconary MUST have conductor protection when the secondary has "more than two conductors". The protection of the secondary is dependant on the wire size, and has nothing to do with the primary. The primary OC device can be located anywhere, but the secondary must be NO MORE than 25 feet of conductor length. See 240.21(C)(6). Table 450.3 where it says primary and secondary. This does not eliminate the secondary protection required, it means that IF you protect the secondary condcutors at 125 percent, then you protect the primary at up to 250 percent of it's rating. You must install conductors rated at this ampacity also. When you protect the primary at 125 percent, the secondary can have any size overcurrent device that you want to install.
The rule is that the primary MUST have protection within 125 percent of it's rating. After this, you go to article 240 for protection of the secondary conductors. When you look at section 240.21(C) you will see that the seconary MUST have conductor protection when the secondary has "more than two conductors". The protection of the secondary is dependant on the wire size, and has nothing to do with the primary. The primary OC device can be located anywhere, but the secondary must be NO MORE than 25 feet of conductor length. See 240.21(C)(6). Table 450.3 where it says primary and secondary. This does not eliminate the secondary protection required, it means that IF you protect the secondary condcutors at 125 percent, then you protect the primary at up to 250 percent of it's rating. You must install conductors rated at this ampacity also. When you protect the primary at 125 percent, the secondary can have any size overcurrent device that you want to install.
Re: transformers
Thanks. But this brings up another question. The primary protection of a 75kva transformer. Since the primary must be 125%, that means I do not use a 90 amp breaker but instead I need at least 113 amp breaker, but the closes size is a 110 or do I need to go to a 125. I was reading the fpn on table 450.3(B) then if I use the 125 I need my wire sized for that breaker. I am I looking at this correctly?
Thanks. But this brings up another question. The primary protection of a 75kva transformer. Since the primary must be 125%, that means I do not use a 90 amp breaker but instead I need at least 113 amp breaker, but the closes size is a 110 or do I need to go to a 125. I was reading the fpn on table 450.3(B) then if I use the 125 I need my wire sized for that breaker. I am I looking at this correctly?
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Re: transformers
No, the NEC does not require 125% as a minimum. The primary protective device must not exceed 125% unless there is a properly applied secondary protective device.
In reality there are many 75kVA transformers protected by 90A breakers, many of them sometimes nuisance trip on startup. But, because they are rarely turned off, the end user usually doesn't care or notice.
No, the NEC does not require 125% as a minimum. The primary protective device must not exceed 125% unless there is a properly applied secondary protective device.
In reality there are many 75kVA transformers protected by 90A breakers, many of them sometimes nuisance trip on startup. But, because they are rarely turned off, the end user usually doesn't care or notice.
Re: transformers
I had a unique opportunity to have an assignment as an application engineer for molded case circuit breakers (MCCB)and dry type distribution transformers (DTDT) I was given challenge by the DTDT product line manager to study the relationship between MCCBs and DTDTs because of warranty claims due to nuisance tripping. With my breaker background I partnered with a DTDT design engineer who gave be some 'ballpark' inrush values for various DTDT KVAs and designs. Since there has been no reason to actually test each transformer to get accurate values the actual values were not available. My study was not intended to be a "silver bullet" to solve problems but to help determine where there could be a problem with a Pri. OCPD/DTDT combination that may cause nuisance trips of the pri. OCPD.
75kva, 480v-208y/120 transformers are notorious for having the primary OCPD being undersized. One of the reasons in the panelboard that feeds the transformer may be limited to 100a frame breakers. The consultant or contractor concludes it's not going to be that loaded anyway and 100a max is just fine. But, the transformer inrush is overlooked.
The inrush to energize a 150deg 75kva transformer is about 10x the FLA, or 90 x 10= 900a. The inrush is even greater when you go to 115 and 80degC, K4 and K13 transformers. Because of lower losses and less winding resistance that inrush may increase to as much a 60% of the FLA or about 1400a. A 90a breaker's magnetic calibration is normally 10x it?s rating or 900a with a +-20% tolerance. So with a 150degC transformer it places you in a marginal situation to begin with. Yes, you are allowed to go 125% or 113a, which can be rounded to a 125A-rated breaker. Even a 125a MCCB may get a bit marginal when you apply an 80degC or K13 transformer, again 125a x 10=125a (+-20%) and you may be looking at 1400a of inrush.
Again, this is a ball park study to help determine where you could have a design problem and to be cautious or to help determine if there may be an inrush/OCPD conflict which is causing nuisance tripping. If you're going to cut things close it is important to know the risks so there won't be any suprised. I personally would prefer knowing what trouble I may be getting into before hand instead of being surprised and having to get out of deep dew-dew after the fact which can be $$$ and getting people just plain ornery and irritated.
I had a unique opportunity to have an assignment as an application engineer for molded case circuit breakers (MCCB)and dry type distribution transformers (DTDT) I was given challenge by the DTDT product line manager to study the relationship between MCCBs and DTDTs because of warranty claims due to nuisance tripping. With my breaker background I partnered with a DTDT design engineer who gave be some 'ballpark' inrush values for various DTDT KVAs and designs. Since there has been no reason to actually test each transformer to get accurate values the actual values were not available. My study was not intended to be a "silver bullet" to solve problems but to help determine where there could be a problem with a Pri. OCPD/DTDT combination that may cause nuisance trips of the pri. OCPD.
75kva, 480v-208y/120 transformers are notorious for having the primary OCPD being undersized. One of the reasons in the panelboard that feeds the transformer may be limited to 100a frame breakers. The consultant or contractor concludes it's not going to be that loaded anyway and 100a max is just fine. But, the transformer inrush is overlooked.
The inrush to energize a 150deg 75kva transformer is about 10x the FLA, or 90 x 10= 900a. The inrush is even greater when you go to 115 and 80degC, K4 and K13 transformers. Because of lower losses and less winding resistance that inrush may increase to as much a 60% of the FLA or about 1400a. A 90a breaker's magnetic calibration is normally 10x it?s rating or 900a with a +-20% tolerance. So with a 150degC transformer it places you in a marginal situation to begin with. Yes, you are allowed to go 125% or 113a, which can be rounded to a 125A-rated breaker. Even a 125a MCCB may get a bit marginal when you apply an 80degC or K13 transformer, again 125a x 10=125a (+-20%) and you may be looking at 1400a of inrush.
Again, this is a ball park study to help determine where you could have a design problem and to be cautious or to help determine if there may be an inrush/OCPD conflict which is causing nuisance tripping. If you're going to cut things close it is important to know the risks so there won't be any suprised. I personally would prefer knowing what trouble I may be getting into before hand instead of being surprised and having to get out of deep dew-dew after the fact which can be $$$ and getting people just plain ornery and irritated.
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