Results 1 to 5 of 5

Hybrid View

  1. #1
    Join Date
    Jun 2006
    Location
    New Jersey
    Posts
    4,336

    Proper Transformer Protection

    So a discussion came up today on transformer protection. And it seems everyone has a different opinion on what is proper/code.

    I have always tried to fuse the primary to 150% of rated VA, with current limiting time delay fuses. And not worry about fusing the secondary. If I couldn't fuse the primary, then I would 150% fuse the secondary.

    Well I got talking to few fellow techs today about this. And the opions are all over the place. From the secondary should be fused and not the primary. From you should fuse both. From fast blow fuses and not time delay. To fuses rated at 100% of VA, with no extra current for inrush.

    Is there a NEC code that addresses this?

  2. #2
    Join Date
    Aug 2009
    Location
    Jurupa Valley, CA
    Posts
    1,777
    I'd defer to someone who knows a thing or two about fuses:

    http://www.littelfuse.com/data/en/Wh...Protection.pdf

    In short, the sizing of the primary fuse depends on the rating of the transformer, and whether there is proper secondary fusing as well.

  3. #3
    Join Date
    Jun 2006
    Location
    New Jersey
    Posts
    4,336
    Quote Originally Posted by CraziFuzzy View Post
    I'd defer to someone who knows a thing or two about fuses:

    http://www.littelfuse.com/data/en/Wh...Protection.pdf

    In short, the sizing of the primary fuse depends on the rating of the transformer, and whether there is proper secondary fusing as well.
    Thanks for the read.

    I was primarily referring to 600v and less. It seems pretty open game on protection.

    There is alot of talk about recommended size and NEC allowable size.

    For example....I have a 150VA 480/120v control trans. NEC says 300% above Littelfuse's recommended fuse size?

  4. #4
    Join Date
    Aug 2009
    Location
    Jurupa Valley, CA
    Posts
    1,777
    [QUOTE=ascj;13618311]Thanks for the read.

    I was primarily referring to 600v and less. It seems pretty open game on protection.

    There is alot of talk about recommended size an

  5. #5
    Join Date
    Aug 2009
    Location
    Jurupa Valley, CA
    Posts
    1,777
    Quote Originally Posted by ascj View Post
    Thanks for the read.

    I was primarily referring to 600v and less. It seems pretty open game on protection.

    There is alot of talk about recommended size and NEC allowable size.

    For example....I have a 150VA 480/120v control trans. NEC says 300% above Littelfuse's recommended fuse size?
    It is certainly confusing, because there are so many competing standards for it. If you are talking about inside of a controller, it is different than just a standalone transformer. Also, the use of primary only, secondary only, or both changes the requirements. Also the percentage varried depending on the size. The way I read it (and unfortunately, I don't have my NFPA 70 here with my), since your primary is rated <2 amps (150VA/480V=0.3125A):

    Table 2 lists 300% primary with no secondary, or 250% primary with a properly sized (167%) secondary for general purpose, or 500% in motor control circuits.
    OR, if the controller is rated for 10,000A or more short circuit rating, you have to use Class J protection, which for a 150VA, 460V transformer, is a 0.8A fuse, with or without secondary protection, or Class CC protection, which calls for a 1.5A for control transformer primaries, or 0.8A general transformers.

    Ultimately, I'm all for sizing the fuses just large enough to not blow unless there is a fault. The codes are all for 'maximum rated sizing'. In most cases, that 150VA transformer will be installed in a system that at most will be using about 50VA. This is why littelfuse also lists their 'recommended fuse sizing' in the class CC tables, and those sizes are quite a bit smaller than the recommended (for your 150VA 480 transformer, they recommend a KLDR-1/2A Time Delay fuse). This is large enough with it's time delay to handle any inrush the solenoids might throw at it, while small enough to limit the potential damage caused by a short. There is almost never a reason to put in the maximum allowed fuse size (which, in essence, is based on fire protection, not equipment protection).

    For further reading, cooper-bussman also publish a similar guide: (http://www1.cooperbussmann.com/pdf/3...a9fc439f07.pdf), though they are less conservative, sticking to the max rating of a 1-1/2A fuse... I prefer going smaller if I can. I don't need a minor short somewhere in the control wiring damaging switch contacts because the fuses were sized for fire protection, instead of equipment protection.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
Comfortech Show Promo Image

Related Forums

Plumbing Talks | Contractor Magazine
Forums | Electrical Construction & Maintenance (EC&M) Magazine
Comfortech365 Virtual Event