Buck Boost for Spa equipment

[cbishop7]

I am running into some confusion, here is my situation:

I have a residential hot tub that operates at 240V being installed in a commercial application where only 208V is available.

The hot tub states a 60amp, 240V, 1 phase feed for a 6kw heater and 5hp pump. Manufacturer told me pump has to have 240V.

The thought is to feed a buck boost transformer from a 208V/1phase breaker, which will boost to 236 volts ,close enough. The buck boost will be a 120/240V, 2kVA transformer based on its selection guide.

What size overcurrent protection do I need at my 208V panel board? I am thinking a 60amp will not be enough due to a lower voltage of 208V. Do I need secondary protection after my buck boost? In reading 450.4 I think not. Would a 60amp, GFCI, non-fused SPA disconnect on the secondary side of the buck boost be sufficient? If the hot tub equipment needs a 60amp feed to protect its equipment, then maybe I do need secondary protection after the buck boost?

Thanks for any input.

 

[kevin]

A buck-boost transformer is an autotransformer, and the conductor amps on the low voltage side are identical to the conductor amps on the high voltage side for your application. A 60 amp CB at the panel will work. Carefully read Part IV of Art. 680 and follow the requirements as applicable to your situation. Secondary overcurrent protection for the buck-boost transformer is not required, but for other-than-single-family dwellings, 680.41 requires installation of an Emergency Switch not less than 5 ft from the hot tub that is readily accessible to the users and is within sight.

 

[hurk27]

I am running into some confusion, here is my situation:

I have a residential hot tub that operates at 240V being installed in a commercial application where only 208V is available.

The hot tub states a 60amp, 240V, 1 phase feed for a 6kw heater and 5hp pump. Manufacturer told me pump has to have 240V.

The thought is to feed a buck boost transformer from a 208V/1phase breaker, which will boost to 236 volts ,close enough. The buck boost will be a 120/240V, 2kVA transformer based on its selection guide.

What size overcurrent protection do I need at my 208V panel board? I am thinking a 60amp will not be enough due to a lower voltage of 208V. Do I need secondary protection after my buck boost? In reading 450.4 I think not. Would a 60amp, GFCI, non-fused SPA disconnect on the secondary side of the buck boost be sufficient? If the hot tub equipment needs a 60amp feed to protect its equipment, then maybe I do need secondary protection after the buck boost?

Thanks for any input.

cost wise it will be cheaper to use the GFCI spa disconnect that can serve both emergency disconnect and GFCI protection. $125.00 verses $165.00 for just a two pole GFCI for the panel, and you will need the disconnect anyway.

a 6kw heater and a 5hp pump is about 48 amps, now add 3 amps for a air pump, and maybe a couple more for controls, so your looking at about 53-54 amps on the 208 side, but this is all for nothing, if the manufacture requires a 60 amp feed at 240 volts which would require about 70 amps on 208 volts, if you only use a 60 amp breaker on the 208 then you will only be supplying 52 amps on the 240 volt side which would be a violation of the manufactures instructions, and 110.3, and the fact you will void the manufactures warranty

 

[cbishop7]

Thanks for the responses, great insight.

Hurk27, my question for you is if I then go with a 70A breaker in my 208V panel, would it be recommended to put a 60A overcurrent protection on the secondary side of the buck boost to provide protection for the spa equipment? Its literature is asking for a 60A breaker. Not sure the 70A breaker would suffice because in all actuality the feeder can actually draw about 61.6amps on the 240V side (the buck boosts it to closer to 236 volts, hence the 61.6 amps).

Thoughts?

Thanks!

 

[winnie]

A big issue with using a buck/boost transformer in this situation is leg to neutral voltage.

If the hot tub requires 240V, then this is not a problem. But if the hot tub requires 120/240V, meaning if it has any 120V loads along with the 240V loads, then you have to be very careful applying the buck/boost. Your 208V system has two legs, each 120V from the common 'neutral' conductor. After the boost, one leg is about 145V from the common neutral conductor.

Of concern is that GFCI, which probably requires a neutral connection, even if the hot tub is straight 240V.

Also, you say 'The buck boost will be a 120/240V, 2kVA transformer based on its selection guide. ' I presume you mean a 120/240V to 16/32V, 2kVA transformer.

-Jon