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Thread: Small Single Phase transformers in MCC or LV Switchgears/Switchboards

  1. #1
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    Small Single Phase transformers in MCC or LV Switchgears/Switchboards

    Hello,

    I have been told that small transformers for single phase loads such as Heating/Lighting should not be mounted inside the MCCs, LV Switchboards/Switchgears (NEMA world). Is it true? If those transformers are allowed to install inside the MCC's, Upto what capacity they are allowed?

    Thanks!

  2. #2
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    What is your load?

    Control power, station service, etc. are all typically mounted in an instrument cabinet in switchgear/MCCs.

    The concern is likely with respect to voiding the UL listing due to excessive heat.

    Get pics of the serial number of the gear and work with the manufacturer.

  3. #3
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    Quote Originally Posted by Charz View Post
    Hello,

    I have been told that small transformers for single phase loads such as Heating/Lighting should not be mounted inside the MCCs, LV Switchboards/Switchgears (NEMA world). Is it true? If those transformers are allowed to install inside the MCC's, Upto what capacity they are allowed?

    Thanks!
    If the transformer is to supply a heating load, it may not be so small.
    Si hoc legere scis nimium eruditionis habes.

  4. #4
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    MCC manufacturers routinely install panelboards and smallish xfmrs in MCCs. I would guess the majority of MCCs we supply are that way. it is just an easy way to get 120V power.

    AB allows up to 50 kVA single phase 240/120 in a bucket or 45 kva 208/120 3 phase. And you can get up to a 42 circuit panelboard with it. Standard options out of the catalog.

    I think you can also get (3) 50 kva single phase transformers wired up 3 phase to get 150 kva of 208/120 but it is an engineered option so we rarely do that because it takes forever. Since the panelboard and the xfmrs have to be in the same section, that is about the most you can get.
    Bob

  5. #5
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    Yes, many are/have been provided that way. As long as the manufacturer does it, they have taken into consideration the heating effect.

    Now, as far as arc flash, I would think that to work on that panel, since it's inside the MCC, you may, and I say may because I don't know for sure, but you may have suit up as if the arc flash level is that of the MCC, which is going to be invariably much higher than a normal 208Y/120V panelboard. So from a strictly safety aspect, the NEMA article recommendation may be a valid approach.

    I personally, will not specify them. I think it's a waste of MCC structure space to add a panel and transformer.
    "Just because you're paranoid, doesn't mean they're not out to get you"

  6. #6
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    Quote Originally Posted by kingpb View Post
    Yes, many are/have been provided that way. As long as the manufacturer does it, they have taken into consideration the heating effect.

    Now, as far as arc flash, I would think that to work on that panel, since it's inside the MCC, you may, and I say may because I don't know for sure, but you may have suit up as if the arc flash level is that of the MCC, which is going to be invariably much higher than a normal 208Y/120V panelboard. So from a strictly safety aspect, the NEMA article recommendation may be a valid approach.

    I personally, will not specify them. I think it's a waste of MCC structure space to add a panel and transformer.
    I used to think that too but it turns out that adding an MCC section is often more cost effective that putting a transformer and PB out on a wall somewhere. Can be factory wired and anything you can do to reduce field work is usually cost effective. Field work has just gotten to be insanely expensive. OTOH if the budget for the MCC is separate from the budget from the install the people selecting the MCC will invariably put the PB and xfmr into the install to make their part of the deal look better.
    Bob

  7. #7
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    As an end-user I just want to say I agree with petersonra. Typical would be M-T-T-M and would include an engineered solution to reduce the incident energy to an acceptable level s.t. the maintainability is a non-issue and can be done safely.

  8. #8
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    Having worked in an industrial plant where the older MCC's had the 120V pnlbd's in the MCC's, I absolutely hated it. Difficult to add circuits, to work on, etc. I eventually replaced dpzend of such MCC's and put 42 ckt pnlbd's on walls where they were easy to service and add to. To us an MCC section was an expensive place for a pnlbd.
    It's my name going on that drawing, not yours. If what you want ain't right, it ain't going on the drawings!

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