Transformer Secondary OCPD Sizing

[fifty60]

According to NEC 450, if my transformer secondary current is less than 9A, if I size an OCPD at 1.67 times or less the secondary current, then I can protect my primary size at 250% the primary current. Or I can use 250% for the primary if the secondary amps are below 9A, and protected with 125% rounded up. I'm having trouble with making sense of how to calculate the secondary fuse size on a transformer that has a center tapped secondary.

My transformer is 2KVA. My secondary is 110 and 220V. If received some help on this forum about this, and began taking the 2KVA divided by the 220V, and then multiplying that by 1.25 to get my max secondary OCPD size. Is this correct? I can get a lot more current out of my transformer using the center tap, then going across the entire secondary.

My problem is that I want to find 1 circuit breaker size on my primary that will work between 380V and 480V (it is a multi tap primary, always 110-220 secondary). For 480V, using the 1.67 times I get a 7A circuit breaker (6.95 actually always so close on these calculations). For 380V using 1.67 I get an 8A breaker. I don't want to drive myself too crazy over a single Amp, but I do not want to have nuisance trips in the field by rounding down to the 7A breaker. I also want to be in accordance with the NEC and not size the primary too large (even if by 1 amp).

My secondary has a 10A circuit breaker between the secondary line and center tap (110V) and a 6A circuit breaker on a motor that is connected across the 220V of the secondary. The 1.67 times calculation on the secondary gives me a maximum of 15A combined OCPD's on the secondary to be able to size the primary up to 250% of the primary current. I'm one amp over here as well.

Am I adding the OCPD's on the secondary correctly? Should I only count the OCPD's across the entire secondary, or do I have to count the center tap as well? If so, then do am I limited to the amps across the entire secondary in my max OCPD sizing for the secondary?

My options are to either go with the 7Amp breaker and hope that it holds up well at 380V, or go with an 8Amp breaker and try to decrease my secondary OCPD's. The problem is that the particular UL489 breakers I am using goes 8A, 10A, 13A....no 9A. If I could use a 9A breaker I would have 15A total on the secondary and could size the primary larger. These are D-Curve breakers, would this hold up on a 2KVA general purpose transformer at 380V, in a 7A rating?

 

[fifty60]

For the 2KVA, 2000/110 equals 18.8A. I am sizing the circuit breaker between the centertap and live leg of the transformer at 10A. This is well below the 18.8A current possible. Across the entire secondary I have 2000/220= 9.09A. My circuit breaker on the motor load (3.5A FLA) across is 6A. This is also well below the full load current of the entire secondary.

If I only use the current drawing across the entire secondary, then I would have to use a 15A of total fusing between the centertap and live leg, and across the entire secondary combined. Is this correct? Or is there a better way of looking at this?

 

[Smart $]

Your rated secondary current is 2kVA/220V, or 9.1A... and doesn't change for any circuit powered through the center tap. Your rated power output from line-to-neutral is only half the transformer rating, i.e. 1kVA. And that is still the same rated current because it is only 1kVA/110V is 9.1A.

BTW, standard voltage is 120/240, not 110/220.

 

[fifty60]

If I used 9A between my line and centertap, then I would not be able connect any loads across the entire 220V secondary? Or would I have the other 1KVA available, at 1000/220 = 4.5A?

 

[Smart $]

If I used 9A between my line and centertap, then I would not be able connect any loads across the entire 220V secondary? Or would I have the other 1KVA available, at 1000/220 = 4.5A?

You are limited to 2kVA on full winding or 1kVA on either or both half-windings. The max current off either or both of the two Lines is 9.1A, no matter where the current returns. Center tap is required to be grounded and does not get fused.

 

[fifty60]

It makes sense then that the max OCPD for the secondary would be sized for each half winding. For my 110V winding, I would have 1000/110= 9.1A total. Then for the 220V winding I would have 1000/220 = 4.55A. Am I then able to 9.1A from the 110 winding and 4.55A from the 220V winding for a total of 13.64A for the entire secondary?

Or would it be 9.1A for each winding so 18.2A total? Or is it just 9.1A split betwee 2 windings so 9.1A total?

 

[Smart $]

It makes sense then that the max OCPD for the secondary would be sized for each half winding. For my 110V winding, I would have 1000/110= 9.1A total. Then for the 220V winding I would have 1000/220 = 4.55A. Am I then able to 9.1A from the 110 winding and 4.55A from the 220V winding for a total of 13.64A for the entire secondary?

Or would it be 9.1A for each winding so 18.2A total? Or is it just 9.1A split betwee 2 windings so 9.1A total?

Let me try to put it another way...

You have L1, N, and L2. N (neutral) is not fused.

You can have up to 9.1A on L1 and up to 9.1A on L2... period. Whether the 9.1A goes to a 110V or 220V load does not matter.

If you have a 110V 9.1A load connected to L1. That is all that can be powered by L1... but you can power a 110V 9.1A load with L2.

If you have a 220V 9.1A load connected L1 to L2, that is all that can be powered.

If you have a mix of 110V and 220V loads, you have to track which line the 110V loads are connected to so as not to go over 1kVA per half winding. And that still amounts to not more than 9.1A per Line.

 

[fifty60]

Thanks a lot for trying to clarify. So if I have a 2KVA center tap secondary (110/220V), If I have 4A between L1 and N, then I am limited to a 5A load betwee L1 and L2, so that the total load on L1 is 9.1A? Is that correct?

 

[GoldDigger]

Thanks a lot for trying to clarify. So if I have a 2KVA center tap secondary (110/220V), If I have 4A between L1 and N, then I am limited to a 5A load betwee L1 and L2, so that the total load on L1 is 9.1A? Is that correct?

Yes, (give or take .1A)


Tapatalk!

 

[fifty60]

Do the same rules apply when it comes to not having to use overcurrent protection on the secondary when there is a center tapped secondary? If the primary OCPD meets the minimum requirements so that no secondary protection is required, does this carry over to secondarys that are split into two windings from a grounded center tap?

 

[augie47]

Partly because of that "balancing act" required when you have a center tap you will note in 240.21(C) that allowing the primary to protect the secondary conductors is only allowed in two-wire to two wire and delta-delta connections.
Keep in mind with transformers you have to keep both Article 450 and 240 into account.

 

[fifty60]

Looking at UL508A, it gives the exception for multi-winding secondary's. "A transformer with multiple secondary windings shall be provided with a set of branch circuit or an inverse time circuit breaker for each secondary sized in accordance with table 35.2" Table 35.2 calls out for the primary sized up to 250% with the secondary sized at below 167%.

From this, referencing UL508A, I can size my primary (more than 2A less than 9A) at 250% or less. For a 3KVA single phase dry type transformer that will operate at 380V or 480V, wanting to use a single size circuit breaker, referencing UL508A I can size my primary at 13A. 13A will hold for both 3KVA at 380V and 480V.

Anyone see anything wrong with this approach?

 

[GoldDigger]

Just so you protect the secondary at no more than the limit current and protect both poles, preferably with a common trip.

Tapatalk!