I have a 1 KVA xformer mounted inside a control enclosure It is 480x120 and the 120 will be used for Enclosure lighting. Do I have to provide OC protection on the primary and secondary or can I get by with primary only. The table 450-3(b)is a little confusing on wether you need both or not under 9 amps.
Transformer Overcurrent protection
- Thread starter wattup
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- Illinois
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- retired electrician
Re: Transformer Overcurrent protection
You can use primary only as long as the rating of that device is 3.479 amps or less. You can't round up, so the maximum fuse size that I can find in a catlog is 3.2 amps. The secondary wires as long as they are sized for at least 13.91 amps. See 240.21(C)(1).
Don
You can use primary only as long as the rating of that device is 3.479 amps or less. You can't round up, so the maximum fuse size that I can find in a catlog is 3.2 amps. The secondary wires as long as they are sized for at least 13.91 amps. See 240.21(C)(1).
Don
Re: Transformer Overcurrent protection
Since the primary if feed L-L, that requires (2) fuses. The secondary only one fuse.
Primary fusing for a 1kva, 480-120v CPT is normally provided with (2)600v 10A KTKR (or equal)fuses with a maximum of 20a if there is a problem with the 10a fuse blowing when the transformer is energized which I have never found it necessary. The secondary is a 250v Max. 15a FNM (or equal) fuse. The line opposite the fuse is normally grounded.
There are CPT manufacturers that offer primary/secondary fuse holder assembly mounted on the top of the transformer for your convenience instead of you having to provide separate fuse blocks.
Since the primary if feed L-L, that requires (2) fuses. The secondary only one fuse.
Primary fusing for a 1kva, 480-120v CPT is normally provided with (2)600v 10A KTKR (or equal)fuses with a maximum of 20a if there is a problem with the 10a fuse blowing when the transformer is energized which I have never found it necessary. The secondary is a 250v Max. 15a FNM (or equal) fuse. The line opposite the fuse is normally grounded.
There are CPT manufacturers that offer primary/secondary fuse holder assembly mounted on the top of the transformer for your convenience instead of you having to provide separate fuse blocks.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Re: Transformer Overcurrent protection
Transformer, with primary =<600V, secondary protection is always optional.
However usually the secondary conductors and almost always the load require separate overcurrent protection.
So for your 1KVA 480-120V single voltage power transformer:
Lighting load protection = ?A
Secondary conductor protection = 15A max for #14AWG, unless a "tap rule" is applicable
Primary protection = 3.2A
Optionally, providing secondary conductor/load protection of not more than 12A would allow the primary protection to be increased to 5A.
There are no provisions in the NEC which would allow the primary protection to be increased in size enough to allow the use on non-time delay fuses, though UL does allow this for manufacturers.
Transformer, with primary =<600V, secondary protection is always optional.
However usually the secondary conductors and almost always the load require separate overcurrent protection.
So for your 1KVA 480-120V single voltage power transformer:
Lighting load protection = ?A
Secondary conductor protection = 15A max for #14AWG, unless a "tap rule" is applicable
Primary protection = 3.2A
Optionally, providing secondary conductor/load protection of not more than 12A would allow the primary protection to be increased to 5A.
There are no provisions in the NEC which would allow the primary protection to be increased in size enough to allow the use on non-time delay fuses, though UL does allow this for manufacturers.
- Location
- Illinois
- Occupation
- retired electrician
Re: Transformer Overcurrent protection
Jim,
This is a two wire to two wire transformer so the primary transformer fuses are also permitted to protect the secondary conductors as long as the ampacity of the secondary conductors is not less then 480/120 times the value of the primary OCPD.
Don
Jim,
This is a two wire to two wire transformer so the primary transformer fuses are also permitted to protect the secondary conductors as long as the ampacity of the secondary conductors is not less then 480/120 times the value of the primary OCPD.
Don
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Re: Transformer Overcurrent protection
Don,
You are correct, I tend to look (somewhat incorrectly) at the 2 wire connection as falling under the generic "tap rules" exceptions.
Don,
You are correct, I tend to look (somewhat incorrectly) at the 2 wire connection as falling under the generic "tap rules" exceptions.
Re: Transformer Overcurrent protection
Technically speaking I agree that pri. protection only is allowed. But when applying primary protect only there may be a problem with blowing the primary fuse(s) beacause of inrush which can be very high with these smaller va transformers.
From my point of view go with the highest rated OCPD as allowed for the primary and use a single fuse on the secondary. The sec. OCPD when sized correcty protects the transformer form overload.
The pri. OCPD protects the distribution system which feeds the transformer from a transformer failure such as if a fault occurs in the transformer.
This is the scheme that's commonly used in MCC buckets and enclosed control.
Technically speaking I agree that pri. protection only is allowed. But when applying primary protect only there may be a problem with blowing the primary fuse(s) beacause of inrush which can be very high with these smaller va transformers.
From my point of view go with the highest rated OCPD as allowed for the primary and use a single fuse on the secondary. The sec. OCPD when sized correcty protects the transformer form overload.
The pri. OCPD protects the distribution system which feeds the transformer from a transformer failure such as if a fault occurs in the transformer.
This is the scheme that's commonly used in MCC buckets and enclosed control.
Re: Transformer Overcurrent protection
Darned, I have a copy of NFPA79, the 1997 edition and I simply forgot about it.
My reference is 8.12 control circuit transformer, 8.12.1 Where a control citcuit transformer is provided, the primary shall be protected in accordance with table 4, and the secondary shall be protected in accordance to table 5.
There are also exceptions which important to consider.
Interesting enough the values that I had included in a previous post were base upon NFPA79 and an exception.
Darned, I have a copy of NFPA79, the 1997 edition and I simply forgot about it.
My reference is 8.12 control circuit transformer, 8.12.1 Where a control citcuit transformer is provided, the primary shall be protected in accordance with table 4, and the secondary shall be protected in accordance to table 5.
There are also exceptions which important to consider.
Interesting enough the values that I had included in a previous post were base upon NFPA79 and an exception.
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